Chapter 5: Chemical Kinetics

Question 1

Reactions can be spontaneous or nonspontaneous; the change in ________ ________ ____________ (∆G) determines whether or not a reaction will occur by itself without outside assistance.

Answer 1

Gibbs free energy

Question 2

Many reactions proceed by more than one step, the series of which is known as the ________________ of a reaction, and the sum of which gives the overall reaction.

Answer 2

mechanism

Question 3

The slowest step in any proposed mechanism is called the ________-____________ step.

Answer 3

rate-determining

Question 4

The ____________ theory of ____________ ____________ states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules.

Answer 4

collision theory of chemical kinetics

Question 5

The minimum energy of collision necessary for a reaction to take place is called the ____________ ____________.

Answer 5

activation energy

Question 6

Rate of reaction can be expressed with which formula?

Answer 6

rate = Z x f

Z = # of collisions per second
f = fraction of effective collisions

Question 7

What equation more accurately represents collision theory? What is it?

Answer 7

Arrhenius equation
k = Ae^-Ea/RT

k = rate constant
A = frequency factor
R = ideal gas constant
Ea = activation energy
T = temp

Question 8

The ____________ ____________ is a measure of how often molecules in a certain reaction collide, with the units s^-1

Answer 8

frequency factor

Question 9

As the exponent in the Arrhenius equation becomes less negative (more positive), the rate constant (decreases/increases?)

Answer 9

increases

Question 10

How can the frequency factor of a reaction be increased?

Answer 10

By increasing the number of molecules in a vessel

Question 11

When molecules collide with energy equal to or greater than the activation energy, they form a ________________ ________ in which the old bonds are weakened and the new bonds begin to form.

Answer 11

transition state

Question 12

How much energy does the transition state have compared to reactants and products?

Answer 12

Greater energy than both of them

Question 13

The energy required to reach the transition state is called ?

Answer 13

activation energy

Question 14

Are transition states and intermediates the same thing?

Answer 14

No, TS are theoretical constructs that exist at the point of max energy, rather than distinct identities with finite lifetimes.

Question 15

A negative free energy indicates that a ____________ reaction, meaning energy is ____________.

Answer 15

exergonic, given off

Question 16

A positive free energy change indicates an ________________ reaction, where energy is ____________.

Answer 16

endergonic, absorbed

Question 17

The difference in free energy between the transition state and reactants is the ____________ ____________.

Answer 17

activation energy

Question 18

How does reactant concentration affect the number of effective collisions?

Answer 18

Increases!

Question 19

An increase in the # of effective collisions per unit time (increases/decreases?) the frequency factor (A) of the Arrhenius equation.

Answer 19

increases

Question 20

For an increase in [reactant], reaction rate will increase for all except which reactions?

Answer 20

Zero-order

Question 21

For reactions in the gaseous state, what serves as a measure of concentration?

Answer 21

partial pressure

Question 22

How does temperature affect reaction rate?

Answer 22

Increases it

Question 23

Which type of solvents will increase reaction rate? Why?

Answer 23

polar solvents, their dipole tends to polarize the bonds of reactants, which lengthens and weakens them

Question 24

In ____________ catalysis, the catalyst is in the same phase as the reactants.

Answer 24

homogeneous

Question 25

Catalysts only change what?

Answer 25

Rate of reaction; no effect on free energy

Question 26

By convention, we place a (+/-?) sign in front of the rate expression for reactants, and a (+/-?) sign for the products.

Answer 26

-, +

Question 27

For nearly all forward, irreversible reactions, the rate is proportional to what?

Answer 27

the concentration of reactants, with each concentration raised to some experimentally determined exponenet

Question 28

The stoichiometric coefficients and orders of a reaction usually don’t match on the MCAT. What are the 2 cases where they would?

Answer 28

1. When the reaction mechanism is a single step and the balanced overall reaction is the entire chemical process
2. When the complete reaction mechanism is given and the rate-determining step is indicated

Question 29

The stoichiometric coefficients on the reactant side of the ________-________________ step are equal to the orders of reaction.

Answer 29

rate determining

Question 30

If the rate-determining step involves an intermediate as a reactant, one must derive the intermediate molecule’s concentration by the ________ of ________ ________ for the step that produced the intermediate.

Answer 30

law of mass action

Question 31

For a reversible reaction, the K_{eq is equal to what?}

Answer 31

The ratio of the rate constant for the forward reaction, k, divided by the rate constant for the reverse reaction, k_-1

Question 32

A ________-________ reactio nis one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants.

Their reaction rate is contant and equal to ____, the rate constant.

Answer 32

zero order; rate constant

Question 33

How can you speed up a zero order rxn?

Answer 33

Increase rate or add catalyst

Question 34

In a zero order rxn’s graph, k is equal to what?

Answer 34

-slope

Question 35

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 35

first-order

Question 36

The unit for k for a first-order reaction is?

Answer 36

1/s

Question 37

A first-order rate law with a single reactant suggests that the reaction begins when what?

Answer 37

The molecule undergoes a chemical change all by itself, without a chemical interaction (and usually without any other molecule_

Question 38

In a first order reaction, plotting what will reveal a straight line?

Answer 38

ln[A] vs. time

Question 39

In a first order reaction, what is the rate constant k equal to?

Answer 39

-slope in a graph of ln[A] vs. time

Question 40

A ____________-________ reaction has a rate that is proportional to either the concentrations of 2 reactants or to the square of the concentration of a single reactant

Answer 40

second-order

Question 41

How can you plot a second-order rxn to yield a linear curve?

Answer 41

1/[A] vs. time

Question 42

In a second order reaction, what is the rate constant k equal to?

Answer 42

the slope of 1/[A] vs. time