271 terms

Ch 14 Chemical Kinetics

Book, lecture notes, and Powerpoints Pg. 576-613
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reaction rate (in terms of products and reactants)
-how fast a given amount of reactants are converted to products.
-speed at which chemical reactions occur
what can the collision model tells you about a reaction?
-it tells me whether a reaction will occur or not
-what needs to happen to be able to have a reaction
state the 3 basics of the collision model
Molecules must collide to react
Must have sufficient energy
Must be in correct orientation
What does it take to have a reaction to take place?
Molecules must collide to react
Must have sufficient energy
Must be in correct orientation
to have sufficient energy to have a reaction to occur, what speed would the reactants have? what can you do to make this happen?
-they would move faster since speed is directly proportional to energy
-increase the temperature
chemical kinetics
the area of chemistry concerned with the speeds, or rates, of reactions
what are the 4 factors that affect the reaction rate?
-The physical state/surface area of reactants
-The concentration of reactants
-The temperature of reaction
-The presence (or absence) of a catalyst
the effect of concentration on rate is expressed quantitatively by_
rate laws
activation energy
minimum input of energy required to make a reaction to happen
reaction mechanism
-is a step-by-step, molecular-level view of the pathway from reactants to products
explain the circumstances at which a reaction will be faster: physical state of the reactants
-the more readily the reactant molecules collide with one another, the more rapidly they react
what are the two classifications of the reaction depending on the reaction phases?
-homogeneous
-heterogeneous
homogeneous reaction (possible phases of the reactants) (2)
-reactants being either all gases or all liquids
heterogeneous reaction
-reactants are in different phases
under heterogeneous conditions, a reaction is limited by_
the area of contact of the reactants
how heterogeneous reactions involving a solid happen faster?
-heterogeneous reactions that involve solids tend to proceed more rapidly if the surface area of the solid is increased
why increasing surface area would increase rate of reaction?
Having a lot of surface area is almost the same as increasing concentration because increases reaction sites
which state has more chances that a reaction happens faster? (liquid vs solid)
liquid
how "reactant concentration" would affect the rate of the reaction by increasing it? why? (molecular explanation)
-most chemical reactions proceed more quickly if the concentration of one or more reactants is increased
-as reactant concentration increases, the frequency with which the reactant molecules collide increases, leading to increased rates
how "temperature" affects reaction rate? explain why in the molecular perspective
-reaction rates increase as temperature is increased
-increasing temperature increases the kinetic energies of molecules. As molecules move more rapidly, they collide more frequently and with higher energy, leading to increased reaction rates
catalysts (what they are and what they do)
-are agents that increase reaction rates without themselves being used up
-they affect the kinds of collisions (and therefore alter the mechanism) that lead to reaction
on molecular level, reaction rates depend on_
the frequency of collisions between molecules
what is the relation between reaction rate and collision of molecules?
-the greater the frequency of collisions, the higher the reaction rate
in reaction involving reactants in the gas state, how does increasing the partial pressures of the gases affect the reaction rate?
increasing the partial pressure increases the number of collisions between molecules. For any reaction that depends on collisions, we would expect the rate to increase with increasing partila pressure
define speed
-the change that occurs in a given time interval (m/s)
-Change in distance over time
what is the rate of flow?
Change in volume of fluid per unit time
what is the rate of a reaction?
Change in quantity of reaction species over time (reactants or products)
what are the units of reactions that are in aqueous or gaseous phases?
Molarity (moles per liter) per second (M/s or M∙s⁻¹)
what are the units that can be used with reactions in gaseous phase?
Pressure per second (atm/s) Or: bar/hr, mmHg/ms, etc.
when we measure the rate of a reaction we measure what? (usually)
concentration of reactants
how to write rate in terms of molarity?
Rate = ∆[X] / ∆t
"The change in Molarity of X over change in time"
how to write rate in terms of pressure?
Rate = ∆Px / ∆t
"The change in partial pressure of X over change in time"
what are the two ways to express the rate of a reaction? (think about reactants vs products)
-rate of disappearance of reactant A
-rate of appearance of product B
what is the the formula for the appearance of B (product)?
average rate of appearance of B=change in concentration of B/change in time
∆[B]/∆t
what is the meaning of the brackets in the rate formula?
-we use brackets in a chemical formula to indicate morality
what is the meaning of ∆?
-it means change in
-it always equals to a final value minus an initial value
what is the the formula for the disappearance of A (reactants)?
-average rate of disappearance of A=-(change in concentration of A)/change in time
-∆[A]/∆t
why we use the negative sign in front of the rate disappearance of reactants equation?
-we use it to indicate that the concentration of A (reactant) decreases
-rates are always expressed as positive quantities
-the minus sign we put in the equation converts the negative ∆[A] to a positive rate of disappearance
how the average rate of disappearance of reactants relates to the average rate of appearance of products?
-because one molecule of reactant is consumed for every molecules of product that forms, the average rate of disappearance of reactant equals the average rate of appearance of B
how the reaction rate changes as a time passes? why?
-it is typical for rates to decrease as a reaction proceeds because the concentration of reactants decrease
how does the instantaneous rate of reaction change as the reaction proceeds?
the instantaneous rate decreases as the reaction proceeds.
Initial rate (other names and definition)
Either an instantaneous rate or an average rate taken during a short time period starting at time = zero
How initial rates are used? why?
-Initial rates are usually used compare rates of different reactions
-We know concentration values much better at the beginning of the reaction than we do later.
average rate
Average between any two times
how can you determine the instantaneous rate from a graph?
-Determined by the slope of a tangent line when it intersects the curve at the time of interest
-the slope of the tangent line is the ratio of the height of the vertical side (M) to the length of the horizontal side
why a reaction rate is faster at the beginning of a reaction vs later on?
because we have the greatest concentration of reactants
explain what happens at the molecular level that makes the reaction rate to decrease as time passes
with time, it is more difficult for particles to find others because there is a decrease in the encounter of other particles
instantaneous rate
-rate at a particular instant during the reaction
for a reaction: aA+bB→cC+dD, how do you write the rate equation?
rate=⁻1/a ∆[A]/∆t= -1/b ∆[B]/∆t= 1/c ∆[C]/∆t= 1/d ∆[D]/∆t
-The average rate calculation (∆[X] / ∆t) cannot describe the rate of a reaction unless the _.
reaction coefficients are all one
what happens when the coefficients are not 1:1 in the rate of a reaction? (how substances appear vs disappear)
When the coefficients are not 1:1, one substance appears or disappears faster than another
how is the rate of a reaction defined in terms of its value?
The rate of a reaction is defined as the same value regardless of which variable is being studied
what is the general formula for rate expression?
if we have the format: aA + bB → cC + dD
reaction rate= -1/a ∆[A]/∆t= -1/b ∆[B]/∆t= 1/c ∆[C]/∆t= 1/d ∆[D]/∆t
If in an reaction all the reactants and products are aqueous and we have the presence of water, what we do with it when writing the rate expression? explain why
-we do not include water in the rate expression only when it is in liquid state (include when it is in gas state)
-Can't measure change in H₂O because its concentration is so high, it stays constant.
What is the formula to determine pressure of a gas?
PV=nRT
when writing a rate expression, why we can leave water out of it?
-Pure liquid is pretty much constant because it is most probably to be media (l)
-do not change at much, do not worry about them. They are found in the greater concentration comparing to the aqueous reactants and products
describe what we measure in a experiment when trying to obtain the rate (focus on concentration)? Include two methods
-Measure change in concentration of at least one reaction species with time
-by measuring absorbance or pressure
describe the experiment at which we are trying to determine rate of reaction by obtaining observace (include name of the law)
-Put sample in cube bag, you have light source
-every substance absorbs energy but a particular wavelength
-what does not get absorbed, pass through and go to detector. Machine measures the light that was not absorbed and compare with the original amount of light
-BEER'S LAW applies in this experiment
describe Beer's Law (include what describes, formula, variable meaning, how graph looks)
-a spectroscopic method that can be used to monitor reactant and product concentration during a reaction by relying on the ability of substances to absorb (or emit) light
-we use a spectrometer, an instrument that measures the amount of light transmitter or absorbed by a sample at different wavelengths. For kinetic studies, the spectometer is set to measure the light absorbed at a wavelength characteristic of one of the reactants or products
-the spectometer measures the amount of light absorbed by the sample by comparing the intensity of the light emitted from the light source with the intensity of the light transmitted through the sample, for various wavelengths
-Beer's law: relation between absorbance and concentration (directly proportional= absorbance is directly proportional to concentration)
-Formula: A=εBC
-B= path length
-the ε is a constant but unique to every substance, extinction coefficient
-only variables, concentration and absorbance
-the more concentration, the less light passes so the more is absorbed
-directly proportionality relation
-straight line graph (straight relationship) and goes through zero
Describe how we can determine rate of a reaction by measuring pressure (include formula and how pressure and concentration change if one increase)
-Use pressure to measure concentration because they are directly proportional (pressure is direclty proportional to concentration)
-PV=nRT
-as pressure goes up, more concentration
if a table of values, which include times and molarity of a reactant, which values you should use and why?
-Don't get the initial values because it is too steep or the end values because they are too shallow
-get something in the middle (values in the middle)
if we put brackets in the reaction expression, do we need to include units? why?
-we do not need to include units because brackets are indicating that it is molarity
if we calculate the rate at an interval, is it going to be the same at another interval? why?
No, they are not going to be the same because it is an average of those intervals and, as time progresses, the rate decreases because reactants are decreasing
how the rate of the reaction changes as the reaction progresses? (be specific as how it changes)
The rate usually changes as the reaction proceeds. Usually it gets smaller with time.
in a graph that shows time vs concentration of rate reaction, the rates calculated are _rates for the particular time interval.
average
Why does reaction rate generally decrease as the reaction proceeds?
concentration of reactants decreases, therefore there are fewer molecules to collide, so collision frequency decreases.
which values in the reaction rate are the best when determining reaction rate?
-Cannot use initial because initial is very steep
-Use something in the middle
how concentration and rate of reaction are related? (think mainly the two ways in which reaction rate is dependable of concentration)(2)
-Rates usually vary with initial concentrations of some or all reactants, possibly products.
-For solids, surface area has the same effect. An increase in surface area increases the quantity of accessible reactant.
The relationship between the rate of reaction and the concentration of the reactants at the beginning of a reaction (initial rate) is defined by the _ for that reaction.
-Rate Law
In the rate law, k, m, and n must be determined _. Explain what each means
-experimentally
-k is the rate constant
-The exponents m & n are the reaction orders for the individual species. They are NOT stoichiometric coefficients, and are most likely 0, 1, or 2 (very rarely 3)
how do you determine the overall reaction order of a reaction?
-sum the reaction orders.
-sum of the orders form each reactant
m+n+o... = overall reaction order
rate law only uses_in its equation
reactants
the rate law relies on what?
-Rely on Initial rate
-so use initial rates always for the calculations of rate law
reaction order refers to _
molecularity
molecularity
refers to how many molecules of a reactant are involved in the elementary steps that, when added together, yield the balanced equation.
explain what order one means
An order of 1 means the rate of a reaction depends on the collision/reaction of one molecule of that reactant.
explain what order two means
An order of 2 means the rate of reaction depends on the collision/reaction of two molecules of that reactant.
how the order is related to the rate of reaction?
The greater the order, the more the rate of reaction depends on the concentration of that reactant.
In first order, if double concentration of reactant and keep everything the same. The rate of reaction would _. If triple concentration->_ rate
-double
-triple
In second order, If double concentration of reactant and keep everything the same->the reaction rate_. If triple the concentration, the reaction rate. In summary, to figure out this you need to do what to rate?
-is 4 times faster
-is 9 times faster
-square the rate
How zero order affects reaction rate? does it mean it is reacting?
-it does not affect reaction rate
-it indicates that the concentration of this reactant does not affect the rate of reaction
-In zero order, it does not affect the reaction rate but it is reacting
rate constant (what it is, relates to what, will remain constant as long as_)
-k
-is an important experimental value specific to a given reaction that relates reactant concentration too the rate of the reaction. the rate constant will remain constant regardless of reactant concentration as long as temperature remains constant
what does order of one indicates?
indicates a direct proportionality
what does order of two indicates?
it indicates that the rate of reaction is directly related to the square of that reactant's concentration
In third order, if double concentration, get _ times as fast
-To figure out this you need to_
-8
-raise it to the three (x^3)
what you should conclude if you change the concentration of one reactant and the rate reaction does not change? )think about the order you would assign to it)
if change concentration and it did not change rate, get zero order
How do we determine rate laws?
-Because rate laws cannot be written directly from the balanced equation, experimental data must be collected (or provided to you).
-Multiple experiments would be performed to determine the relationship between initial conc. and rate.: When the data was collected, each experiment differs by initial conc. of one of the reactants.
-To solve, compare two experiments where only one variable changed. Do this for each reactant.
-Because the general form of the rate law is: Rate = k [A]m[B]n
-Study how the ratio of the change in concentration affects the rate.
how concentration raised to the order power is related to reaction rate?
-Concentration raised to the power of its order is proportional to Rate:
Rate ∝ [A]^order
how can you determine m and n in the rate law?
-Compare the ratio of rates and the ratio of concentrations for two sets of data where all reactant concentrations except for the one of interest are held constant. [No choose the ones that both variables changed! Only chose the ones in which one variable changed]
(Rate₁/Rate₂ ) = ([A]₁/[A]₂)^order
-Determine the exponent that makes the equation true. Order = 0, 1, 2 or
what you should do after you are done of determining m and n in the rate law?
-After determining the orders, use the data from the table to plug in and calculate k
-do not forget the units
what are the units of K in rate law? Include the units that you are going to use at zero, first, second, third, and fourth order
-The units on k depend on the overall order for the reaction. Determine the units using dimensional analysis.
-zero order: M/s or M∙s⁻¹
-first order: s⁻¹
-second order: M⁻¹∙s⁻¹ or 1/Ms
-third order: M⁻²∙s⁻¹
-fourth order: M⁻³∙s⁻¹
one way of studying the effect of concentration on reaction rate is to determine the way in which the initial rate of reaction depends on the _
initial concentrations
rate law
An equation that relates the reaction rate to the concentrations of reactants
how do reaction rate, rate law, and rate constant differ?
-reaction rate is the change in the concentration of a reactant (disappearance) or product (appearance) over a time interval
-the rate law governs how the reaction rate depends on the concentrations of the reactants
-the rate constant is part of the rate law
once we have both the rate law and the k value for a reaction, we can calculate the reaction rate for _
-any set of concentrations
the exponents in the rate law, which are called_, indicate how the rate is _
-reaction orders
-affected by each reactant concentration
suppose the reactions A→B and X→Y have the same values of k. when [A]=[X], will the two reactions necessarily have the same rate?
No. The two reactions could have different rate laws. Only if they have the same rate law and the same value of k will they have the same rate
to determine the units of rate constant, what you must do?
divide: units of rate (M/s)/units of concentration (M^n) [where n is the reaction order]
in rate laws, the rate of the reaction depends on _(1)and the rate constant depends on_(2)
-concentration
-does not depend on concentration. rate constant depends on temperature and the presence of catalyst
what does the rate expression tell us?
tells how the concentration of reaction components (reactant or product) vary over time.
what does the rate law tell us?
-tell us about the dependence of initial concentration on initial rate.
-tells how the rate of a chemical reaction depends on the concentration of one or more reactant.
what are the limitations of the rate law? what laws can help us with these limitations?
-This form of the rate law does not tell us information about time, such as
-How long it will take for a specific concentration to react completely?
-How long will it take to react half way?
-How much will be left after a certain time has elapsed?
-Time issues that can be solved by using Integrated rate laws.
how do we obtain the integrated rate laws?
Combine the rate expression and rate laws to get what we need.
In integrated rate laws, state what each symbol mean
-[A]₀ is initial concentration
-t=time
-K=rate constant
-[A]t=concentration at some point in time (t)
Within the three integrated rate laws, which order equation has positive sign?
2nd order
How the integrated rate laws are arranged? (think about shape)
These equations are arranged to fit the general equation for a straight line!
In integrated rate laws, what represents order of reactant? what represents rate constant (k)?
-The graph that is linear will represent the order of that reactant. [try each of the three formulas]
-The absolute value of the slope of that line is the rate constant, k.
If the y-axis of the graph that is a straight line is [A]t, what order of reaction it is?
zero order
If the y-axis of the graph that is a straight line is ln[A]t, what order of reaction it is?
1st order
If the y-axis of the graph that is a straight line is 1/[A]t, what order of reaction it is?
2nd order
Two ways to use experimental data to determine the order of each reactant and the rate constant for the reaction. Explain each (rate law (include disadvantage) and integrated rate law (include advantage))
*Rate Laws:
-Multiple experiments changing conc. & measuring rate to get order
-Solve for the rate constant, k
-Does not allow for effect of time on concentration
*Integrated Rate Laws:
-Graph of variations of conc. data with time. The relationship that is linear gives that reactant's order.
-Slope of linear relationship is the rate constant, k
-Allows for effect of time on concentration
Only zero order reactions have a straight line relationship for _ plots.
[A] vs time
Only first order reactions have a straight line relationship for _ plots.
ln[A] vs time
Only second order reactions have a straight line relationship for _ plots.
1/[A] vs time
First order reaction
-reaction whose rate depends on the concentration of a single reactant raised to the first power
an integrated rate law of first order, relates its initial concentration [A]₀ to_
its concentrations at any other time t, [A]t
given any of the three following quantities when using integrated rate laws, we can solve for_(include variables)
-fourth
-k, t, [A]₀, and [A]t
using integrated rate laws you can determine_(3)
1. the concentration of a reactant remaining at any time after the reaction has started
2. the time interval required for a given fraction of a sample to react
3. the time interval required for a reactant concentration to fall to certain level
second order reaction
is the reaction for which the rate depends either on a reactant concentration raised to the second power or on the concentrations of two reactants each raised to the first power
zero-order reaction
the reaction in which the rate of disappearance of A is independent of [A]
A zero-order reaction proceeds _ over time. In other words, the rate _as the reactant concentration changes.
-uniformly
-does not change
half life
the time to reduce a reactant's concentration to ½ of its original value.
At t½: [A]t = ½ [A]₀
what dating based on half life is the most famous?
carbon-14 dating
formula to determine half life of zero order?
[A]t=[A]₀/2^n
in which n is the number of half lives
what difference of time there is as you increase number of half lives in first order reaction?
-as you increase number of half lives, the time increases at the same value
-if first half life happened at 1 min, 2nd half live will happen at 2 min, 3rd half life will happen at 3 min etc (you add 1 minute to time)
2nd reaction half life: describe time between half lives
-Note that each half-life is twice as long as the preceding one because t1/2 = 1/k[A]0 and the concentration of A at the beginning of each successive half-life is smaller by a factor of 2.
-The difference of 1st and 2nd order, it is the amount of time of half life
-Double every time the time
explain how half lives happen in first order vs 2nd order
the length of time of half life to occur
1st order: same amount of time in all halflife
2nd order: the time is double with increasing half lives
A fast reaction has_half life (length of time)
short half life
in first reaction order, how half life varies?
it remains constant throughout the reaction
in a first order reaction, the concentration of the reactant decreases by_in each of a series of regularly spaced time intervals, each interval equal to_
-one half
- t1/2
If a solution containing 10.0g of a substance reacts by first order kinetics, how many grams remain after three half lives?
-After 3 half lives, the concentration will be 1/8 of its original value, so 1.25g of the substance remains
how half life is for second order and other reactions vs first order? (think about duration)
-depends on reactant concentrations and therefore changes as the reaction progresses
what is the formula for the half life of 1st life? 2nd life? (with respect to k)
first order: t1/2=0.693/k
second order: t1/2=1/(K[A]₀)
the lower the initial concentration of half life of second order, the_the half life
longer
why can we report the half life of the first order reaction without knowing the initial concentration, but not for a second order reaction?
-the half life of first order reaction is independent of initial concentration. By contrast, the half life of a second order reaction depends on initial concentration
how temperature affects average kinetic energy? This results in a_(low vs high) percentage of molecules posses the required_(what), and the reaction goes (slow vs fast). This relationship is shown by_equation
As temperature rises, the average kinetic energy of molecules increases. In a chemical reaction, this means that a higher percentage of the molecules possess the required activation energy, and the reaction goes faster. This relationship is shown by the Arrhenius equation
what is the gas constant value?
8.3145J/(K⋅mol)
what temperature measurement is used in arrhenius equation? (unit)
kelvin
what is the formula to convert kelvin into celsius? farenheti to celsius?
K=C+273.15
F=(9/5)C+ 32
having large activation energy, how increase the temperature affect the rate of the reaction? how this compares to reactions with small energy barrier (activation energy)?
This large Ea value is the reason why a minor increase in temperature caused the reaction to proceed about 28 times faster. Reactions with a small energy barrier do not see such a drastic increase in rate as temperature increases.
what a potential energy diagrams shows?
They show the potential energy of a system as it changes from reactants to products.
how a potential energy diagram that is not catalyzed compares with one that is catalyzed?
the activation energy is lowered by the catalyst, so the peak decreases at the catalyzed potential energy diagram compared to the graph that is not catalyzed
activation energy (what it is and positive vs negative)
The activation energy of a reaction is the difference in energy between the reactants and the activated complex. Activation energy is always a positive number.
internal energy change (what it ism symbol, positive vs negative, formula)
-The internal energy change, ΔE, of a reaction is the difference in energy between the reactants and the products. ΔE may be positive or negative depending upon whether the reaction is endothermic or exothermic. To ensure that you always get the correct sign for ΔE, use the following equation:
ΔE=Eproducts−Ereactants
enthalpy change (symbol, how it relates to internal energy change and at what conditions)
The enthalpy change, ΔH, of a reaction is often nearly equal to the internal energy change, ΔE, for a reaction because at constant pressure the pressure-volume work is small.
How does the presence of a catalyst affect the activation energy of a reaction? Explain how it does it
-A catalyst decreases the activation energy of a reaction.
-The activation energy is lowered because the catalyst provides an alternate path for the reactants to get to products.
How does the presence of a catalyst affect the enthalpy change of a reaction? explain why
-A catalyst does not affect the enthalpy change of a reaction.
-This question illustrates the meaning of a state function. The enthalpy change of a reaction is the same whether it takes the catalyzed or uncatalyzed path. ΔH, like all state functions, only depends on the starting and ending points of the system.
A multistep reaction can only occur as fast as its _step. In based on this, what is the rate law of the overall reaction?
-slowest
Therefore, it is the rate law of the slow step that determines the rate law for the overall reaction.
what is a catalyst? (what do, how found in the reaction)
Catalysts are substances that increase the rate of reaction but can be recovered unchanged at the end of the reaction.
what are the two classifications of a catalyst? explain each
Catalysts can be classified as either homogeneous (same state as reactants) or heterogeneous (different state than reactants).
how rates are affected by the temperature? why?
-Rates generally increase with increasing T
-Increase temperature=molecules move faster
-If decrease, slow down molecules= have less chances of collision
what else besides the rate is affected by the temperature? how? how this affects the rate of reaction?
-rate constant (k)
-The rate constant k also increases with temperature, causing an increase in rate
how K often increases?
Often k increases
~ x 2 (double) for every 10°C
temperature increase
how the increase of temperature reflected in the rate law?
-the faster the rate at higher temperature is due to an increase in the rate constant with increasing temperature
We have determined that the rate of a reaction increases when concentration or temperature increases. what theory explains this?
collision theory
what are the 3 statements of the collision theory?
-In order for molecules to react they must collide
-The colliding molecules must be oriented properly
-The colliding molecules must have sufficient energy
explain: "In order for molecules to react they must collide" in the collision theory (2: how affect rate, and how achieved it?)
-The greater the number of collisions the faster the rate.
-Often achieved by increasing the concentration or surface area
explain "The colliding molecules must be oriented properly" in the collision theory (related to rate)
-The more molecules present, the greater the probability of collision and the faster the rate.
explain ""The colliding molecules must have sufficient energy" in the collision theory (how related to rate)
-The higher the temperature, the more energy available to the molecules (they "hit harder") and the faster the rate.
how increasing concentration or surface area affect rate? explain using collision theory
Increasing the concentration or surface area of one or more reactants increases the number of effective collisions by increasing the total number of collisions (fraction remains the same at constant Temperature).
explain how temperature affects rate of reaction with the collision theory
-increasing temperature increases molecular speeds
-as molecules move faster, they collide more forcefully (with more energy) and more frequently, increasing reaction rates
explain "orientation" in collision theory
-collisions between molecules result in a chemical reaction only if the molecules are oriented in a certain way when they collide -the relative orientations of the molecules during collision determine whether the atoms are suitably positioned to form new bonds
Why don't all properly-oriented collisions lead to products? think about collision theory (include term for this)
-There must be sufficient energy to break reactant bonds.
-energy of activation
energy of activation (Ea)
it is the minimum energy required to initiate a chemical reaction.
Ea is often referred to as the _ for a reaction
Ea is often referred to as the energy barrier for a reaction
the minimum amount of energy that molecules require to react comes from_(give explanation in terms of collision theory)
-this energy comes from the kinetic energies of colliding molecules
-upon collision, the kinetic energy of molecules can be used to stretch, bend, and ultimately break bonds, leading to chemical reactions
-kinetic energy is used to change the potential energy of the molecule
if the energy of activation was lowered, how the kinetic energy required would change?
-the kinetic energy of molecules required for the reaction would be lower
in a chemical reaction, what happens in the middle of the reaction? (name of the state, energy content of this step, what the energy activation barrier represents in this case)
-intermediate state
-high energy
-the barrier to formation of the products represents the energy necessary to force the molecule through the relatively unstable intermediate state
how calculate energy of activation? (reference to graph)
-the difference between the energy of the starting molecule (reactants) and the highest energy along the reaction pathway (peak of bell shaped graph)
transition state (another name)
-the molecule having the arrangement of atoms shown at the top of the barrier
-the particular arrangement of reactant and product molecules at the point of maximum energy in the rate-determining step of a reaction
where is energy released/absorbed found in a bell-shaped graph that shows energy vs reactants and products? (energy profile)
-the energy released/absorbed (∆E) is found subtracting Eproducts-Ereactants
-found in between products and reactants in the graph
how energy of activation affects rate?
-the rate depends on the magnitude of activation energy; generally, the lower the value of energy activation is, the faster the reaction
how kinetic energy of molecules and temperature are related? Does all molecules have the same energy?
-The number of molecules with sufficient kinetic energy to react increases as the temperature increases.
-Not all molecules have the same
kinetic energy
how activation energy relates to temperature?
-At low temperatures, the barrier will be higher
-Higher temperature, low activation energy
-Higher temperatures, increase rate and more chances to have reaction and arrive to activation energy
how energy compares between reactants and products in exothermic reaction?
reactants have higher energy than products
how energy compares between reactants and products in endothermic reaction?
products have higher energy than reactants
how the energy of activation of forward reaction compares with reverse reaction at equillibrium? compare amount of energy required for both reactions
-if forward reaction is exothermic, reverse reaction is endothermic
-while the value of activation energy of forward reaction is Ea, the activation energy for the reverse reaction is equal to the energy that must be overcome if approaching barrier from the right: ∆E+Ea
-Thus, to reach the activated complex or transition state for the reverse reaction requires more energy than for the forward reaction
-for this reaction, there is a larger barrier to overcome going from right to left (reverse reaction) than from left to right (forward reaction)
suppose you could measure the rates for both the forward reaction and the reverse reactions of the processes in which the forward reaction is exothermic and the reverse reaction is endothermic, in which direction would the rate be larger? why?
the energy barrier is lower in the forward direction than in the reverse. Thus, more molecules will have energy sufficient to cross the barrier in the forward direction. The forward rate will be greater
how the energy of molecules distribute? does all molecules have the same energy values or vary?
-molecules are distributed in energy over wide range, in which most are at the middle ranges and least are at extremes (high vs low energy)
relate temperature, energy of activation and kinetic energy of molecules with rate of reaction
-at higher temperatures a much greater fraction of the molecules have kinetic energy greater than Ea, which leads to a greater rate of reaction
Suppose we have 2 reactions, A→B and B→C, you can isolate B, and it is stable. is B the transition state for the reaction A→C?
No. If B can be isolated, it cannot correspond to the top of the energy barrier. There would be transition states for each of the individual reactions
in exothermic reactions, does energy is needed?
low amount of energy to go forward is requires at the beginning but the energy released in greater
what kind of reactions are more uncommon: exothermic vs endothermic? why? (in terms of activation energy)
-endothermic because the activation energy is Ea+∆E, which is greater the activation of energy of exothermic (Ea)
in terms of energy, how exothermic and endothermic reactions are compared?
-Exothermic reaction->release energy than required to take in
-Endothermic reaction->took more energy than released
-all reactions absorb energy
The energy barrier is called the _
activation energy
how determine activation energy? (equation)
This is the difference between the transition state energy and the average reactant energy.
what is the formula for energy released in terms of forward and reverse reactions? (∆E)
Related to the energy change of the reaction:
∆E = Ea,f - Ea,r
what arrhenius discovered? (include formula and what each variable is)
-Arrhenius discovered reaction-rate data obeyed the Arrhenius equation:
f=e^(-Ea/RT)
where f= is the fraction of molecules that have kinetic energy equal to or greater than Ea
where R is the gas constant=0.8314 J/mol-k T is the absolute temperature in kelvin
how k (reaction constant) relates with f? (formula)
K=Af
arrhenius equation with variables? state what each variable mean
k=Ae^(-Ea/RT)
-f is the fraction of molecules with the energy to overcome the activation energy, Ea. This term is part of the Arrhenius Equation.
-k is the rate constant, R is the gas constant (8.314 J/K-mol) and T is the temperature in K.
-A is called the frequency factor. It is a measure of the probability of a favorable collision.
-Both A and Ea are specific to a given reaction.
in base on arrhenius equation, how k is affected by changing activation energy and temperature? how reaction rate changes?
-As Ea (the hill) increases, k decreases, i.e. the reaction rate decreases.
-As T increases, k increases (rate increases / more reactants get over the hill peak).
Rate ∝ k [reactant]^order
With regard of the 2nd order chemical reaction, which of the following statements about the collision theory is false? why?
a. A successful collision must have energy equal to greater than the activation energy
b. Most collisions do not result in a reaction
c. All collisions with energy greater than the activation energy will result in products
d. Reactant molecules must collide in order to react and form products
e. Reactants must be oriented properly for a successful collision
c. All collisions with energy greater than the activation energy will result in products
BECAUSE it can have enough energy but not correct orientation.
Increase concentration=_ A (how affect it)
?increase
how determine activation energy using arrhenius equation?
-If we measure the rate constant, k, for a range of temperatures, we can determine Ea and A graphically by rearranging the Arrhenius equation:
lnk=(-Ea/RT)+lnA, which will be formated in straight line format y=mx+b
From the above equation, a plot of ln k versus 1/T will be a straight line with a slope of -Ea/R and an intercept of ln A.
where lnk is Y-axis and 1/T is X-axis
what varies with temperature: Ea vs k?
k varies with temperature, Ea does not vary with temperature
Ea units and temperature units in arrhenius equation?
Ea-J/mol and make sure T is in Kelvin
what three factors are incorporated into arrhenius equation?
most reaction rates obeys an equation (arrhenius equation) based on:
1. the fraction of molecules possessing energy Ea or greater
2. the number of collisions per second
3. the fraction of collisions that have the appropriate orientation
what is the value of the gas constant?
R=8.314 J/mol
most reactions occur as_(1 step vs many) how this is called?
-They occur as a series of molecular events/collisions, the sum of which is the balanced equation provided.
-reaction mechanism
reaction mechanism
-A reaction's mechanism is this detailed pathway for a reaction
-multiple steps to get a reaction
-is the steps that helps us obtain the overall reaction
Mechanisms are proposed for reactions to explain the sequence of events that must occur, but they must be consistent with _(2)
the rate law and other experimental evidence
Each of the molecular events (collisions) that must occur in a mechanism are defined as _(name)
elementary step
elementary steps
one-step reactions
how elementary steps interact with each other to get overall reaction? what law describes this?
-Elementary steps must add up to give the balanced chemical equation.
-Hesse's law=Elementary steps must add up to give the balanced chemical equation
molecularity
-number of reactant molecules in an elementary reaction
-the number of molecules that participate as reactants in an elementary reaction
what are 3 classifications of elementary steps regarding their molecularity? define each, which one is rare?
-Unimolecular: elementary step with one molecule
-Bimolecular: elementary step with two molecules
-Termolecular (rare): elementary step with three molecules
elementary step cannot be_
broken down because it is the simplest form of a reaction
why tetrmolecular elementary step is weird?
Termolecular is weird because we need 3 molecules that must collide with right energy, right orientation and right time
what a reaction mechanism can describe?
the order in which bonds are broken and formed and the changes in relative positions of the atoms in the course of the reaction
what is the molecularity of the elementary reaction:
NO(g) + Cl₂(g)→NOCl(g) + Cl(g)
bimolecular
intermediate (what it is and where used and where found with respect to reactions)
-Produced in one step and consumed in another.
-Most multistep reactions involve intermediates.
-In the first reaction found at products and in the second reaction found as reactant
intermediate will be seen in_but not in_
-seen in elementary steps
-not seen in overall reaction
the chemical equations for the elementary reactions in a multistep mechanism must_
add to give the chemical equation of the overall process
intermediates are not the same as_
transition states
how intermediates are different from transition states?
-intermediates can be stable and can therefore sometimes be identified and even isolated
-transition states are always inherently unstable and as such can never be isolated
where in a graph is the transition vs intermediate state found?
-at the cusps, the transition state is found
-at the throughs in the middle is found the intermediates (for example a in a "u" the intermediate would be in the middle)
to fund molecularity of each elementary step, what you need to look?
molecularity of each elementary reaction depends on the number of reactant molecules in the equation for that reaction (look how many molecules in reactants)
how the rate laws of elementary steps can be determined?
-The rate law for an elementary reaction CAN be written directly from the balanced chemical equation
-The exponent(s) in the rate law is (are) the same as the coefficient(s) in the balanced chemical equation.
The exponents (orders) for ANY rate law relate to the _ of the reaction that must occur on which the_
The exponents (orders) for ANY rate law relate to the molecularity of the reaction that must occur on which the rate of reaction depends.
what applies only to elementary reactions in this chapter?
The overall reaction order equals the molecularity of the reaction. i.e., the coefficients = orders
how different are each elementary step in a multistep mechanism from each other? (3)
Each step in a multistep mechanism occurs at a different rate, has a different rate constant, and therefore a different rate law.
what is the rate determining step?
The rate-determining step is the slowest step in the sequence of steps leading to product formation.
the rate law for the overall reaction in a multistep mechanism is?
The rate law for the reaction is determined based off of the rate determining step. (the slowest one)
what happens when first step is the slow step?
When the first step of a multistep mechanism is slow, that step limits the reaction rate. Therefore the rate law is dependent only on the first step
what happens if the first step is fast and second or third step is slow?
When the first step(s) of a multistep mechanism is(are) fast, the slowest step still limits the reaction rate, but the rate law still depends on the first step(s) (need it to eliminate intermediate from rate law)
what if a reactant does not appear in rate law? (think about order)
we will assume it has zero order of reaction
When the rate law for an overall reaction does not correspond to the known rate-determining (slow) step, it usually means _
one or more fast steps precede it in the reaction mechanism.
what we do not want to have in the overall rate law?
an intermediate
what if the rate law matches the first step of a multistep mechanism?
the first step is the slowest step
what molecularity has the following equation and state if it is likely to happpen in a single step mechanism:
2NO(g)+Br₂(g)→2NOBr(g)
-termolecular because it contains 3 molecules (2 of NO)
-it is not likey because requiers 3 molecules to collide with enough energy and good orientation
why the slow step is the rate-determining step?
-the overall rate of a reaction cannot exceed the rate of the slower elementary step
_governs the rate law of the overall reaction(which step?)
-rate-determining step
why cannot the rate law for a reaction generally be deduced from the balanced equation for the reaction?
To determine the rate, we need to know the elementary reactions that add up to give the balanced equation and the rate-determining step (slow rate step)
if K2>>K1, which has greater rate?
K2 has greater rate (it is faster) because it is directly proportional to rate by rate law (rate=k[A])
why are termolecular elementary steps rare in gas-phase reactions?
the likelihood of three molecules colliding at exactly the same time is vanishingly small
whenever a fast step precedes a slow one, what do with intermediate?
-we can solve for the concentration of an intermediate by assuming that an equilibrium is established in the fast step
catalyst
is a substance that increases the rate of a chemical reaction without itself being consumed.
how a catalyst is different from an intermediate regarding how found in elementary steps?
- In general, a catalyst is consumed in one step of a reaction an is re-generated in a subsequent step, whereas an intermediate is formed in one step and is consumed in a subsequent step.
where found catalyst regarding elementary steps?
-it is found as a reactant in the first reaction and as a product in second or last reaction
catalysts increases rates of what kind of reactions?
increases rates of forward AND reverse reactions.
how a catalysts speeds up a reaction? (method and what does)
A catalyzed reaction usually occurs through an alternative mechanism, which lowers the activation energy required for the reaction to proceed.
how a catalysts affects energy of activation? rate constant (k)?
-decreases activation energy
-increases k
how the rate of a catalyzed reaction compares with an uncatalyzed reaction? what about enegies of activation?
rate(catalyzed)>rate(uncatalyzed)
Ea(catalyzed)<Ea(uncatalyzed)
For heterogeneous catalysis: The _ is increased.
frequency factor (A)
does the rate law depend on a catalyst?
The rate law for a catalyzed reaction will often contain the concentration of the catalyst even though the catalyst is not shown in the overall reaction.
to overcome the energy of activation we can use a_
catalyst, which will decrease its value
which one will you have in a rate law: catalysts vs intermediate?
catalyst
A catalyzed reaction usually occurs through an _. explain
-alternative mechanism
-an uncatalyzed reaction may happen in a single step in a slow way. with the catalyst, the reaction happens in two steps in a fast way
why catalyst use alternative mechanism?
this mechanism decreases energy of activation
what are two types of catalysts?
-homogeneous catalyst
-heterogenenous catalyst
homogeneous catalysis
-the reactants and the catalysts are dispersed in a single phase, usually liquid.
-All the reactants are in the same phase along with the catalyst
homogeneous catalysts may happen in which phase?
gas phase
solution phase (aqueous)
heterogeneous catalysis
-the reactants and the catalysts are in different phases. The catalyst still remains unchanged after the reaction.
where a catalyst vs intermediate is introduced to a mechanism?
-catalyst is there at the start of the reaction, whereas the intermediate is formed during the course of the reaction
how a catalyst affects the rate of reaction? (think about it in which variables changes)
-catalyst must affect the numerical value of k, the rate constant, which ,by arrhenius equation, k is determined by the activation energy (Ea) and the frequency factor (A)
-A catalyst may affect the rate of a reaction by altering the value of either Ea or A
how catalyst changes activation energy? (Ea) (process)
-the catalyst could provide a new mechanism for the reaction that has an Ea value lower than the Ea value for the uncatalyzed reaction
How the catalyst affects frequency factor? (A) (process)
-catalyst could assists in the orientation of reactants and so increase A
what is the most dramatic catalytic affect: Ea vs A?
lowering Ea
a catalyst lowers the overall activation energy for a chemical reaction
how compare the elementary steps of the mechanism of catalyst to the elementary steps of the uncatalyzed reaction?
-all the elementary steps must have significantly lower activation energies than the uncatalyzed reaction steps
what phase usually the catalyst is found in an heterogeneous catalysis? (include phases of reactants)
-solid in contact with either gaseous reactants or reactants in a liquid solution
heterogeneous catalysts are often composed of_(specific examples (2))
-metals
-metal oxides
what is the initial step in heterogeneous catalysis?
-adsorption
adsorption (what it is and how different to absorption)
-refers to binding of molecules to a surface
-absorption refers to the uptake of molecules into the interior of a substance
why adsorption occurs? (chemical explanation, include phases of reactants)
-because the atoms or ions on the surface of a solid are extremely reactive
-unlike their counterparts in the interior of the substance, surface atoms and ions have unused bonding capacity that can be used to bond molecules from the gas or solution phase to the surface of the solid
because of adsorption, what characteristic catalysts must have?
they must have a large surface area
what happens while the reactant is adsorbed on the surface of the catalyst?
bonds are break and formed to get product
how does the homogeneous catalyst compare with a heterogeneous catalyst regarding the ease of recovery of the catalyst form the reaction mixture?
-a heterogeneous catalyst in different phase than the reactants and is therefore fairly easy to remove from the mixture
-the removal of an homogeneous catalyst can be much more difficult as it exists in the same phase as the reactants
how k in kinetics (this chapter) compare with K in equilibrium (chapter 15) in regards of units and what states?
-kinetics=units and how fast?
-equilibrium=no units and direction of reaction
the equilibrium expression depends on_
stoichiometry
what are the factors that determine if a reaction will occur?
-they are stated in collision theory
1. they must collide
2. with proper orientations
3. mus have sufficient energy to overcome activation energy
what are the 4 factors that determine how fast a reaction will occur?
1. temperature
2. concentration
3. presence of a catalyst
4. physical state (surface area)
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