Ch 6 practice questions

Question 1

Which of the following factors that affect rate of reaction applies only to heterogeneous systems?

a) surface area
b) concentration
c) temperature
d) catalysis

Answer 1

a) surface area

Question 2

The catalytic converter on a vehicle is

a) biological catalyst
b) heterogeneous catalyst
c) homozygous catalyst
d) reactant

Answer 2

b) heterogeneous catalyst

Question 3

the activation energy of a chemical reaction is affected by

a) the kinetic energy of the reactants
b) the potential energy of the products
c) the presence of a catalyst
d) none of the above

Answer 3

c) the presence of a catalyst

Question 4

Which of the following statements is incorrect?

a) The rxn rate for a zero-order reaction is independent of concentrations
b) The rate constant for a second-order rxn is dependent on temperature
c) The rate law expression relates rate and concentration
d) The rate constant for a first-order rxn never varies with temp or pressure

Answer 4

d) The rate constant for a first-order rxn never varies with temp or pressure

Question 5

True or false:

If one reactant molecule is involved in the rate-determining step, the reaction is a first-order reaction.

Answer 5

True

Question 6

Which of the following conditions will change the rate of a reaction?

(a) chewing food
(b) heating in an oven
(c) exposure to high oxygen concentration (d) all of the above

Answer 6

(d) all of the above

Question 7

Enzymes are

a) highly porous substances that activate acids and bases
b) substances synthesized by chemists to decrease the rxn rate
c) extremely poor in catalytic activity
d) catalysts found in organisms

Answer 7

d) catalysts found in organisms

Question 8

True or false:

It is not necessary to break existing bonds in molecules in order to form new bonds

Answer 8

false

Question 9

True or false:

The frequency of collisions and the proportion of those collisions that are effective determine the rate of a given rxn

Answer 9

true

Question 10

True or false:

the rate constant for a zero-order rxn has no units

Answer 10

false (mol/L * s)

Question 11

True or false:

The rate law for a chemical rxn is always expressed as a product of the initial concentrations of the REACTANTS of the rxn

Answer 11

true

Question 12

Why do the average and instantaneous rates of a reaction usually differ?

Answer 12

The average and instantaneous rates of a reaction usually differ because the
instantaneous reaction rate changes continually as the concentration of reactants changes during the chemical reaction.

Question 13

Reaction rate

Answer 13

Reaction rate is the rate of change in concentration of a reactant or product during a chemical reaction.

Question 14

How do catalysts increase reaction rate?

Answer 14

A catalyst increases reaction rate by providing an alternative pathway for the
reaction that has a lower activation energy. Thus, a much greater fraction of the collisions are effective at a given temperature.

Question 15

How does an increase in temperature increase reaction rate?

Answer 15

An increase in temperature raises the average kinetic energy levels of the molecules involved in the reaction. This increases reaction rate because the molecules move faster and have more energetic collisions, which increases the probability of effective collisions that can overcome the activation energy required for a chemical reaction.

Question 16

How does an increase in concentration of the reactants increase reaction rate?

Answer 16

An increase in concentration of the reactants increases reaction rate by increasing the probability of collisions between reactant molecules. There are more reacting particles in a given volume, so there are likely to be more collisions and therefore more effective
collisions.

Question 17

Homogeneous vs. heterogeneous catalysts

Answer 17

A homogeneous catalyst, such as many enzymes in aqueous solutions in cells, has the same phase as the reactants.

A heterogeneous catalyst, such as the metal in a catalytic converter, has a different phase than the reactants.

Question 18

effect of concentration on the rate of a rxn

Answer 18

Increasing the concentration of the reactants means there are more reacting particles in a given volume, so there are likely to be more collisions between the reactants and therefore more effective collisions.

Question 19

effect of a catalyst on the rate of a rxn

Answer 19

Catalysts speed up chemical reactions by providing an alternative pathway for the reaction—a pathway that has a lower activation energy.

A much greater fraction of the collisions are successful at a given temp for (b) the catalyzed pathway than for (a) the uncatalyzed pathway.

This allows reactants to become products at a much higher rate, even if temp is not increased.

Question 20

Why do reactions of substances containing covalent bonds take longer than reactions of ions in solution?

Answer 20

Reactions of substances containing covalent bonds take longer than reactions of ions in solution because covalent bonds must be broken before a reaction can occur, while in ionic solutions, the bonds are already broken.

Question 21

collision theory

Answer 21

The collision theory states that a chemical
reaction occurs when reactants collide in the right orientation and with enough energy to break chemical bonds and make new ones.

Question 22

Maxwell–Boltzmann distribution

Answer 22

The Maxwell–Boltzmann distribution is the relationship between the numbers of
entities of a gas phase reactant and their kinetic energy.

Question 23

Activation energy

Answer 23

Activation energy is the minimum energy required during a collision in order for a
chemical reaction to occur.

Question 24

Why do all chemical reactions have activation energy?

Answer 24

All chemical reactions have activation energy because all colliding entities must
overcome their mutual repulsions before an effective collision to occur.

Question 25

According to the collision theory, what are two reasons why the rates of most reactions increase with increasing temperature?

Answer 25

According to the collision theory, two reasons why the rates of most reactions
increase with increasing temperature are that a greater proportion of reactant entities will have a quantity of kinetic energy that equals or exceeds the activation energy, so more effective collisions occur, and that the increase in kinetic energy increases the rate and
force of collisions between reactants, and this increases the probability that the collisions will be effective.

Question 26

Why do average rates of reaction decrease with time?

Answer 26

Average rates of reaction decrease with time because the concentration of
reactants decreases as the reaction proceeds.

Question 27

rate of a chemical rxn vs. rate law constant

Answer 27

The rate of a chemical reaction is the change in concentration of a reactant over time, and has units of mol/(L*s).

The rate constant, k, is the proportionality constant in the rate law equation, and it is related to the concentrations of the reactants.