Kinetics

Front 1

Activation energy (Ea)

Back 1

Energy required to initiate a reaction

-the difference between the energy level of the reactant and that of the transition state

*Can only be affected by a catalyst, NOT by temperature

Front 2

Transition state/activated complex

Back 2

High energy, unstable state between reactants and products (the peak(s) in a reaction coordinate diagram)

-each step of a reaction has a separate transition state

Front 3

Intermediate

Back 3

Lower-energy valley between two transition states on a reaction coordinate diagram

-Produced as a product of one step of a reaction and consumed in a later step of the reaction so not present in the overall reaction equation

Front 4

Delta H

Back 4

Difference in energy between reactants and products

Front 5

Endothermic reaction

Back 5

Products have a higher energy than reactants

-delta H is positive

Front 6

Exothermic reaction

Back 6

Reactants have a higher energy than products

-delta H is negative

Front 7

Activation energy for reverse reaction

Back 7

Endothermic reaction: Ea - delta H

Exothermic reaction: Ea + delta H

Front 8

Rate-determining step

Back 8

Usually the slow step of a reaction, i.e. the step with the larger Ea and therefore, the lower rate constant (k)

-Exception: the step with the highest-energy transition state (the highest peak) is ALWAYS the rate-determining step since it requires more energy to get over the peak than to go backward so the amount of time required to get over the peak determines the amount of time the overall reaction takes

*Gives the overall rate law

Front 9

Catalyst

Back 9

Speeds up the rate of a reaction by providing an alternative mechanism (pathway) with a lower activation energy

-Is not used up by a reaction so it is present as both a reactant and a product but does not appear in the overall reaction equation

-Enzymes are biological catalysts

Front 10

Rate Law

Back 10

Indicates how fast a reaction will proceed given certain conditions

Rate = k [A]^x[B]^y

Front 11

0 order reaction

Back 11

Rate = k [A]^0 = k

Rate is not affected by the concentration of the reactant

-Not common

Front 12

1st order reaction

Back 12

Rate = k[A]

Rate is directly proportional to the concentration of the reactant

Front 13

2nd order reaction

Back 13

Rate = k[A]^2

Rate is directly proportional to the square of the concentration of the reactant

Front 14

Overall reaction order

Back 14

Sum of the orders of the individual reactants

Front 15

Rate order

Back 15

Indicates how the concentration of a particular reactant will affect the rate of the reaction

-the exponents in a rate law indicate the order of the reaction with respect to a certain reactant

-rate orders can be integers, or they can be decimals or fractions

-most reactions slow down over time because their rate is proportional to the concentration of reactant(s), which decreases over time

Front 16

Elementary reaction

Back 16

Reaction that cannot be broken down any further

-Can determine rate order from the coefficients of a balanced reaction only if the reaction is elementary (otherwise, experimental data is needed)

-Mechanism(s) of a reaction are always elementary

Front 17

Molecularity of reaction

Back 17

How many molecules are involved in a reaction

Unimolecular: 1 molecule

Bimolecular: 2 molecules

Termolecular: 3 molecules (very rare for an elementary reaction)

Front 18

Kinetics

Back 18

Can tell you how how fast a reaction proceeds or will proceed (whereas thermodynamics tells you whether a reaction will proceed, i.e. whether it is spontaneous)

Front 19

Rate expression

Back 19

Indicates how fast a reaction is proceeding

Can be measured as:

1. negative change in concentration of reactant/change in time x coefficient of reactant

-Reactants have a negative rate of change (but positive rate of consumption) so that the rate expression will always be positive

2. change in concentration of product/change in time x coefficient of product

Front 20

Calculating rate law

Back 20

1. Find trial in which the concentration of one reactant has changed while the other has stayed constant.

2. Compare the change in concentration of the reactant with the change in the initial reaction rate.

-if the reaction rate is unchanged, the reactant is 0 order.

-if the reaction rate has changed by the same amount as the reactant concentration, the reactant is first order.

-if the reaction rate has changed by the square of the change in the concentration amount, the reactant is second order.

***If the concentrations of both reactants have changed, isolate the effect of each concentration change on the rate by comparing the change in rate to the change in rate caused by changing the concentration of just one reactant.

Front 21

Units of rate constant (k)

Back 21

The absolute value of the sum of the exponents is the same as the overall rate order

-k always has units of s^-1 and M^x

Front 22

Collision Theory

Back 22

Elucidates what must happen in order for a chemical reaction to take place

1. Molecules must collide

2. Proper orientation between colliding molecules (so that the parts of the molecule that will bind are in contact)

3. Sufficient energy during collision to overcome the Ea barrier

Front 23

Effect of temperature on reaction rate

Back 23

Higher temperature = higher average kinetic energy = higher collision frequency and higher percentage of collisions with sufficient energy (high-energy collisions)

Front 24

Arrhenius equation

Back 24

K = Ae^(-Ea/RT)

Ae is the Arrhenius constant and is constant for a particular reaction)

Front 25

Factors affecting the rate of a reaction

Back 25

1. Concentration of reactants (higher concentration = faster reaction)

2. Temperature (higher temperature = faster reaction)

3. Activation energy (lower Ea = faster reaction)

Front 26

Factors affecting the rate constant of a reaction

Back 26

1. Temperature (higher temperature = higher rate constant)

2. Activation energy (higher Ea = lower rate constant)