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Chemistry Unit 5
Terms in this set (36)
Average speed or average rate =
Total distance traveled
The rate of a chemical reaction or the reaction rate =
the changes in amount of reactant or product per unit of time.
Δ reactant or Δ product
What does the slope of the line on a rxn graph indicate about the rate of the rxn?
The steeper the slope, the faster the rxn rate.
Observable properties for studying rxn rates
1. Volume of gas produced
2. Pressure of gas produced
3. Color change of solution (spectrophotometer)
4. Mass of product produced (solid phase is easiest to measure)
5. Mass of reactant used up (solid phase is easiest to measure)
What 4 factors influence the rates of a chemical reaction? For each factor, explain how this factor can be used to increase the rate of the reaction.
Increase in temp = increase in avg KE = more collisions with sufficient energy to react = faster reaction rate.
Examples from class:
Burning of charcoal: Carbon and oxygen molecules collide with higher energy by greater frequency at higher temperatures
More particles = more collisions = faster reaction rate
Examples from class:
In a crowded room, more people are moving higher chances of collisions than with fewer people. Increased collision frequency leads to a higher reaction rate.
3. Particle Size
Smaller particles = more surface area = more collisions = faster reaction rate
Examples from class:
If you put a bundle of sticks on fire they burn quicker than a log with the same mass. This is because the bundle of sticks have a greater surface area
A catalyst is a substance that can increase the rate of the rxn (by lowering the activation energy) without being consumed in the rxn.
Examples from class:
at 37 degrees reactions in the body would be too slow without catalysts that increase rates of biological reactions are called enzymes when you eat a meal containing potential enzymes in digestive tract help break down protein molecules in a few hours.
5. Nature of Reactants
Different substances react differently.
More bonds to break & make = slower reaction rate
The collision theory states:
Atoms, ions, and molecules can react to form products when they collide if the particles have enough kinetic energy.
The minimum energy that colliding particles must have in order to react.
An unstable arrangement of atoms that forms for a moment at the peak of the activation-energy barrier (transition state between reactants and products).
A catalyst ______ the activation energy for a reaction.
change in energy during a chemical reaction.
energy is absorbed, + ΔH
energy is released, - ΔH
A reaction in which reactants are converted to products in a single step.
Most chemical reactions consist of
two or more elementary reactions.
The series of elementary reactions or steps that take place during the course of a complex reaction.
Rxn mechanisms happen because
the probability of 2 or 3 molecules colliding (at the exact same time) is much higher than 4, 5 or more.
A product of one step in a reaction mechanism and a reactant in the next step. They do NOT appear in the overall chemical equation for a reaction.
Rate determining step is the
slowest step in the rxn mechanism.
You can control the rate (speed) of the reaction by
controlling the rate of the rate determining step.
A reaction where the conversion of reactants to products and the conversion of products to reactants occur at the same time.
A reaction has reached chemical equilibrium if the rates of the forward reaction and reverse reactions are equal.
Le Chatelier's Principle:
When a system at equilibrium is subjected to a stress or change, the equilibrium will shift in the direction that tends to counteract the effect of the stress.
What 3 stresses can cause a change in the equilibrium position of a chemical system? For each factor, explain how this factor can be used to change the equilibrium position.
1. Changes in concentration
*add a reactant = D shift to products
*add a product = D shift to reactants
The rxn tries to use up the excess, so the D will shift the opposite direction.
*remove a reactant = D shift to reactants
*remove a product = D shift to products
The reaction tries to make more of what was removed.
2. Changes in temperature
*Temp (add energy) = D shift endothermic (opposite the energy)
The reaction tries to use up the added energy.
*¯Temp (remove energy) = D shift exothermic (toward the energy)
The reaction tries to replace the energy that was removed.
3. Changes in pressure and volume
*P¯V = D shift favors the side that takes up the least space.
If all are gases, smallest # of moles.
*¯PV = D shift favors the side that takes up the most space.
If all are gases, largest # of moles.
decrease the time it takes to establish equilibrium but they do NOT affect the amounts of reactants and product present at equilibrium.
Equilibrium Constant, Keq is
the ratio of product concentrations to reactant concentrations at equilibrium.
* Where [ ] is the concentration of gases or aqueous solutions at equilibrium (usually measured in Molarity).
* Do NOT use solids or pure liquids, since their density (concentration) remains constant no matter the amount.
* Coefficients from the balanced equation become exponents.
The size of the equilibrium constant indicates
whether reactants of products are more common (favored) at equilibrium.
What will be the value of the Keq if equilibrium favors products?
Greater than 1 (or large).
(Makes a lot of products before the rxn reaches D)
What will be the value of the Keq if equilibrium favors reactants?
Less than 1 (or small).
(Makes a lot of reactants before the rxn reaches D)
potential energy graph for endothermic reaction (first end is lower than last end)
activated complex is on the top. Activation energy is activated complex to reactants and for the reverse reaction its activated complex to products. Underneath reactant or first end label P reverse and underneath Products or last end R reverse. H is distance from activation energy to last end
reading potential energy graph for endothermic reaction
activation energy for forward reaction or current reaction is activation complex-reactants or first end level should have positive number the negative of that number is activation energy for reverse reaction and it will be in kj. the H should be positive for forward products or last end level-first end level negative for H of reverse reaction first label whether graph is endothermic or exothermic
potential energy graph for exothermic reaction
activated complex is the peak it is exothermic if last end is lower and first end is higher up first label that activation energy for this would be from first end level to a.c. from last end level to a.c is a.e reverse. label first end level product r for reverse and and last end level r and p for reverse.
reading potential energy graph for exothermic reaction
activation energy for forward reaction is activated complex-first end level and a.c-last end level for reverse h for forward should be a negative number last end level-first end level should be negative for forward and positive for reverse of it
kinetic energy graphs
k.e in kj represents temperature for drawing a new line on graph representing reaction at a higher temperature is a smaller hill but end of hill goes farther than original (has less molecules) for drawing new line on graph representing reaction at a lower temperature is a larger hill but ends earlier than the original (has more molecules). draw a dotted line on the graph to represent the effect of a catalyst draw a vertical dotted line on the left of the original dotted line because catalysts lower activation energy
Equilibrium direction when reactant/product being added/removed is solid
no effect if it was a liquid or a gas or a soln then it would shift to the opposite/toward
THIS SET IS OFTEN IN FOLDERS WITH...
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