Chapter 5 Chemical Kinetics

Question 1

What does the change in Gibbs free energy determine?

Answer 1

It determines whether or not a reaction will occur by itself or with outside help assistance

Question 2

What is an intermediate?

Answer 2

it is found within a reaction mechanism, but it does not appear in the overall reaction

Question 3

What is the rate determining step?

Answer 3

The rate determining step is the slowest step in any proposed mechanism; it prevents the overall reaction from proceeding any faster than that slowest step

Question 4

What does the collision theory of chemical kinetics state?

Answer 4

It states that the rate of the reaction is proportional to the number of collisions per second between reacting molecules. It also suggests that not all collisions result in a chemical reaction.

Question 5

When does an effective collision (one that leads to the formation of products) occur?

Answer 5

It occurs only if the molecules collide with each other in the correct orientation and with sufficient energy to break their existing bonds and form new ones.

Question 6

What is the activation energy?

Answer 6

The activation energy, E_a, (or energy barrier) is the minimum energy of collision necessary for a reaction to take place

Question 7

How is a transition state formed? What bonds are present in the transition state?

Answer 7

It is formed when molecules collide with energy greater than or equal to the activation energy.

In the transition state, the old bonds are weakened and the new bonds begin to form.

Question 8

Describe the energy of the transition state/activated complex.

Answer 8

The transition state (also called the activated complex) has greater energy than both the reactants and products.

The energy required to reach this transition state is the activation energy.

Question 9

What two things can happen once the activated complex is formed?

Answer 9

The activated complex can either dissociate into the products or it can revert to reactants without any additional energy input.

Question 10

How are transition states distinguished from reaction intermediates?

Answer 10

Transition states are theoretical constructs that exist at the point of maximum energy, rather than distinct identities with finite lifetimes.

Question 11

What is the free energy change of the reaction?

Answer 11

The free energy change of the reaction (ΔG_rxn) is the difference between the free energy of the products and the free energy of the reactants.

Question 12

What does a negative free energy indicate?

Answer 12

A negative delta G indicates an exergonic reaction, meaning energy is given off.

Question 13

What does a positive free energy indicate?

Answer 13

A positive delta G indicates an endergonic reaction, meaning energy is absorbed.

Question 14

Where does the transition state exist on an energy diagram?

Answer 14

It exists at the peak of the energy diagram

Question 15

What is the activation energy of the forward reaction?

Answer 15

The difference in free energy between the transition state and the reactants is the activation energy of the forward reaction.

Question 16

What is the activation energy of the reverse reaction?

Answer 16

The activation energy of the reverse reaction is the difference in free energy between the transition state and the products.

Question 17

What are the factors that affect reaction rate?

Answer 17

1. reaction concentrations
2. temperature
3. medium
4. catalysts

Question 18

How do reaction concentrations impact reaction rate?

Answer 18

The greater the concentrations of the reactants, the greater the number of effective collisions per unit time, thus the reaction rate will increase for all but zero order reactions.

Question 19

How does temperature impact reaction rates?

Answer 19

for nearly all reactions, the reaction rate will increase as the temperature increases.

This is because increasing the temperature increases the average kinetic energy of the molecules.

Thus, the proportion of reactants gaining enough energy to surpass E_a increases with higher temperature

However, once the temperature gets too high, catalyst can denature and the reaction rate will plummet.

Question 20

How does a medium impact the reaction rate?

Answer 20

Some molecules are more likely to react with each other in aqueous environments, while some prefer non-aqueous solvents.

Question 21

What are catalysts, and how do catalysts impact reaction rate?

Answer 21

Catalysts are substances that increase the reaction rate without themselves being consumed in the reaction. They help reduce the activation energy necessary for the reaction to proceed.

They may:

increase the frequency of collisions between the reactants;

change the relative orientation of the reactants, making a higher percentage of the collisions effective;

donate electron density to reactants;

or reduce intramolecular bonding within reactant molecules.

Question 22

What is a homogenous catalyst? A heterogenous catalyst?

Answer 22

A homogenous catalyst is a catalyst that is in the same phase as the reactants.

A heterogenous catalyst is a catalyst that is in a different phase than the reactants.

Question 23

How do catalyst impact the free energies of the products or reactants?

Answer 23

Catalysts have no impact on the free energies of the reactants or products.

Catalyst only change the rate of reactions!

Question 24

What is the difference between the equilibrium constant expression (K_eq) and the rate law?

Answer 24

K_eq includes the concentrations of all the species in the reaction (both reactants and species). K_eq indicates where the reaction’s equilibrium position lies.

Rate law only includes the reactants. The rate indicates how quickly the reaction will get to equilibrium.

Question 25

Explain zero order reactions. What is the rate law? What are the units of k?

Answer 25

A zero order reaction is one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B. These reactions have a constant reaction rate equal to the rate constant, k.

The rate law for a zero order reaction is:

rate = k[A]⁰[B]⁰ =k

the units of k are M/s

Question 26

What are two ways the rate of a zero order reaction can change?

Answer 26

The rate constant itself is dependent on temperature, thus is it possible to change the rate for a zero-order reaction by changing the temperature.

The other way the rate of a zero order reaction can change is by the addition of a catalyst, which lowers the activation energy, thus increasing the value of k.

Question 27

How does the “concentration vs time” graph of a zero order reaction look like?

Answer 27

the line shows that the rate of formation of product is independent of the concentration of reactant. The slope of such line is opposite of the rate constant, k.

k = -slope

Question 28

Explain first order reactions. What is the rate law? What are the units of k?

Answer 28

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

The rate law for a first order reaction is:

rate = k[A]¹ or rate =k[B]¹

The units of k are: s^-1

Question 29

What does the “ln[A] v time” graph look like for first order reactions?

Answer 29

Question 30

Explain second order reactions. What is the rate law and the units of k?

Answer 30

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

The rate law for a second order reaction is:

rate = k[A]¹[B]¹ or rate = k[A]²

The units of k are: M^-1 s^-1

Question 31

What does the “1/[A] v time” graph look like for a second order reaction?

Answer 31

Question 32

what are mixed order reactions? what is described as broken order?

Answer 32

Mixed-ordered reactions sometimes refer to non-integer orders (fractions), and in other cases to reactions with rate orders that vary over the course of the reaction (reactions that change over time).

Fractions are more specifically describe as broken order.