Chemical Kinetics

Question 1

(HIGH YIELD) Reactions can be spontaneous or nonspontaneous. What quantitative measurement determines whether or not a reaction will occur by itself without additional input of energy?

Answer 1

Gibbs free energy (delta G)

Question 2

(HIGH YIELD) Many reactions proceed by more than one step, the series of which is known as the ______ of a reaction.

Answer 2

Mechanism

Question 3

(HIGH YIELD) The slowest step in any proposed mechanism is called the _____ _____ step because it acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step.

Answer 3

Rate-determining

Question 4

(HIGH YIELD) For a reaction to occur, molecules must collide with each other. The _____ theory of chemical kinetics states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules.

Answer 4

Collision

Question 5

(HIGH YIELD) The minimum energy of collision necessary for a reaction to take place is called the _____ _____.

Answer 5

Activation energy (E sub a)

Question 6

(HIGH YIELD) The rate of a chemical reaction can be expressed with what equation? (Hint: rate = ___ x ___)

Answer 6

Rate = Z x f, where Z is the total number of collisions occurring per second and f is the fraction of collisions that are effective.

Question 7

(HIGH YIELD) The energy required to reach the transition state is ______ ______.

Answer 7

Activation energy

Question 8

(HIGH YIELD) What is the name of the difference between the free energy of the products and the free energy of the reactants?

Answer 8

The free energy change of the reaction (delta G of the reaction)

Question 9

(HIGH YIELD) A NEGATIVE free energy change indicates what type of reaction?

Answer 9

Exergonic (energy is given off)

Question 10

(HIGH YIELD) A POSITIVE free energy change indicates what type of reaction?

Answer 10

Endergonic (energy is absorbed)

Question 11

(HIGH YIELD) List some factors that can INCREASE the rate of a reaction.

Answer 11

1. Increasing reaction concentrations (for all but zero-order reactions)
2. Increasing temperature (exception: catalysts may denature if the temperature is too high, causing the reaction rate to plummet)
3. Choosing a specific reaction medium (some molecules are more likely to react in aqueous environments, while others are not)
4. Adding a catalyst

Question 12

(HIGH YIELD) In ______ catalysis, the catalyst is in the same phase (solid, liquid, gas) as the reactants. In ______ catalysis, the catalyst is in a distinct phase.

Answer 12

Homogeneous; heterogeneous

Question 13

(HIGH YIELD) True or false: Catalysts change only the rates of reactions and do not impact the free energies of the reactants or the products.

Answer 13

True. Catalysts change the forward rate and reverse rate by the same factor.

Question 14

For nearly all forward, irreversible reactions, the rate is proportional to the concentrations of the reactants, with each concentration raised to some exponent. How is this exponent determined?

Answer 14

The exponent can only be determined experimentally. It is NOT determined by the stoichiometric coefficient for the relevant reactant.

Question 15

Give an expression for the rate law of a chemical reaction with only two reactants, A and B.

Answer 15

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

where k is the rate constant and the overall order of the reaction is the sum of x and y

Question 16

True or false: PRODUCT concentrations never appear in a rate law.

Answer 16

True. Only the concentrations of REACTANTS are relevant in a rate law.

Question 17

A ____ ____ reaction is one in which the rate of formation of product is independent of changes in concentrations of any of the reactants.

Answer 17

Zero-order. These reactions have a constant reaction rate equal to the rate constant, k, which has units of M/s.

Question 18

What are two ways to increase the rate of a zero-order reaction?

Answer 18

1. Raise the temperature (remember that k is temperature-dependent)
2. Add a catalyst

Question 19

Plotting a zero-order reaction on a concentration vs. time curve results in what type of graph?

Answer 19

Linear, with a negative slope where k = -slope

Question 20

A ____ ____ reaction has a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of product.

Answer 20

First-order, where k has units of s^-1

Question 21

Plotting a first-order reaction on a concentration vs. time curve results in what type of graph?

Answer 21

Nonlinear. If the natural log of the concentration (ln[A]) is plotted vs time, the graph will be linear with a negative slope.

Question 22

A ____ ____ reaction has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant.

Answer 22

Second-order, where k has units of M^-1 s^-1

Question 23

What type of reaction refers to non-integer orders (fractions) or, in other cases, to a reaction with rate orders that vary over the course of the reaction?

Answer 23

Mixed-order reaction

Question 24

What name is given to reactions whose orders are fractions rather than integers?

Answer 24

Broken-order reactions