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GAMSAT Physical Chemistry: Reaction Rates and Equilibrium
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Rate of reaction (Definition and Formula)
the change in concentration of substance X over the time it took for substance X to undergo this change
Rate = change in [X]/change
moles/L/s
Is Rate concentration dependent?
yes
How does concentration effect the rate of a reaction?
Change in concentration is linked to change in reaction rate therefore as the reaction progresses the rate of reaction varies
If molar concentration of substance X is plotted against time how can the instantaneous reaction rate be determined using the graph?
The tangent to the curve at a particular time point gives the instantaneous reaction rate
What is the rate determining step?
The rate determining step (RDS) is the slowest step in a multi-step reaction.
What is the formula for the RDS?
Reaction:
aA + bB equilibrium sign cC + dD
Equation for RDS:
RDS = k[A]^a[B]^b
k = rate constant, independent of concentration, dependent on reaction and temperature
Can rate law (k[reaction]) be calculate with certainty for the overall reaction?
No - it has to be determined experimentally.
Formula for 1st order rate (rate law)
rate = k[A]
Formula for 2nd order rate (rate law)
k = [A]^2
k = [A][B]
k = [B]^2
Formula for 0th order rate (rate law)
rate = k
If [A] is the concentration of substance A at any time and [A]0 is the initial concentration and t is the time that has passed since the reaction started, find k for 1st order reactions.
ln[A] - ln[A]0 = -kt
If ln[A] - ln[A]0 = -kt, then calculate the half life of a first order reaction.
t=(ln2)/k
This is true for 1st order reactions were the initial concentration is independent of half life.
If [A] is the concentration of substance A at any time and [A]0 is the initial concentration and t is the time that has passed since the reaction started, find k for 2nd order reactions.
kt = 1/[A] - 1/[A]0
If ln[A] - ln[A]0 = -kt, then calculate the half life of a 2nd order reaction.
t = 1/(k[A]0)
This is dependent on initial concentration, 1/[A] = straight line
Arrhenius behavior
Empirical relationship - things happen faster at higher temperatures esp. if the activation energy of a substance is low
Which formula shows that k varies with temperature?
k = A/(e^(Ea/RT))
lnk = lnA - Ea/RT
A= frequency of collisions aka frequency factor
Ea = activation energy
R = ideal gas constant
T = temperature
how does Ea effect k?
Small Ea: lnk approx. = lnA ... reaction rate = similar for all temperatures
High Ea: high temperature dependency ... reaction rate = variable across temperatures
k = temperature dependant
When can we assume that lnA for two reactions at two different temperatures is approximately equal?
When the two temperatures are close together
Define catalysts
partake but are not consumed by the reaction, provides alternate pathways with lower activation energies
What is the function of a catalyst?
1. Weaken the bonds in the reactant molecule by adhering to it
2. Effect both the forward and backwards reaction but Ea still remains different for the two (Usually)
note: catalysts are like enzymes and can effect the RDS
Define Dynamic Equilibrium.
When reactions continue without a net result and the forward and backward rates of reaction are equal.
Define Equilibrium constant
1. The ratio of the fwd and bwd rate constants
2. Is temperature dependent
3. Can be calculated using either concentration or partial pressures.
4. Is usually quoted at 25 degrees C
5. Solids do no contribute to the calculation
6. If more than 1000 - products are favored
7. If less than 1000 - reactants are favored
Define Le Chateliers (include formula).
When a stress is applied to a system in dynamic equilibrium the system adjusts to minimize the effect of the stress.
Q = [C]^c[D]^d/[A]^a[B]^b
Q = quotient
if Q is > Keq more left
if Q is < Keq more right
Which process producing NH3 draws on Le Chateliers?
Harber - Bosch: NH3 is drawn off, there by decreasing the concentration of NH3 and making the system adjust so that the products are favored
As temperature increases for an exothermic reaction the ------ are favored
Reactants
As temperature decreases for an endothermic reaction the ------ are favored.
Reactants
Define the Common Ion effect
1. The effect n equilibrium when a substance with an ion partaking in the reaction is added to the system.
2. Adding a common ion prevents the weak acid/base from ionizing to the same extent as it would had the ion not been introduced.
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