Kinetics and Equilibrium

Question 1

What is the reaction rate defined as?

Answer 1

The reaction rate is defined as the rate of change in the concentration of reactants or products. ie. how fast a reactant gets used up, and how fast a product gets produced.

Question 2

If this reaction:

aA + bB → cC + dD

was the rate-determining step, then what is the overall rate order of the reaction?

Answer 2

If the above reaction is the rate-determining step of a multi-step reaction, then the rate of the multi-step reaction = k[A]^a[B]^b

Question 3

Given this data, determine the rate law:

Answer 3

From this table, a 2x increase in [A] corresponds to a 4x increase in the rate. 2x = 4, so x = 2.

A 2x increase in [B] corresponds to a 2x increase in the rate. 2y = 2, so y = 1.

A 2x increase in [C] corresponds to 1x (no change) in rate. 2z = 1, so z = 0.

r = k[A]²[B]¹[C]⁰

Question 4

Given reaction speeds, how do you determine the rate determining step of a reaction?

Answer 4

The slowest step of a multi-step reaction is the rate determining step.

Question 5

What is the activation energy?

Answer 5

The activation energy is the energy it takes to push the reactants up to the transition state.

Question 6

Draw a reaction coordinate diagram, indicating where the transition state is, the activation energy, and the change in enthalpy for the forward and reverse reaction.

Question 7

What does the Arrhenius equation (k = Ae^-Ea/RT) tell us?

Answer 7

Low Ea, High T → large k → faster reaction.

Question 8

When activation energy approaches zero….

Answer 8

When activation energy approaches zero, the reaction proceeds as fast as the molecules can move and collide

Question 9

When temperature approaches absolute zero…

Answer 9

When temperature approaches absolute zero, reaction rate approaches zero because molecular motion approaches zero.

Question 10

What is the difference between a kinetic and thermodynamic product?

Answer 10

• Kinetic product = lower activation energy, formed preferentially at lower temperature.
• Thermodynamic product = lower (more favorable/negative) ΔG, formed preferentially at higher temperature.

Question 11

What factors favor the spontaneinty of a reaction?

Answer 11

Being exothermic (-ΔH) and increasing in entropy (+ΔS)

Question 12

What do catalysts do?

Answer 12

Catalysts speed up a reaction without being used up. They act by lowering the activation energy to speed up the forward and reverse reaction. They increase “k” but do not affect “Keq”

Question 13

How can you obtain Keq from a chemical equation?

Answer 13

Keq = [C]^c[D]^d/[A]^a[B]^b

Question 14

Using thermodynamics, how can you obtain Keq?

Answer 14

ΔG° = -RT ln (Keq)

Question 15

If Keq > 1, what does this mean?

Answer 15

If Keq is much greater than 1 (For example if Keq = 10³), then the position of equilibrium is to the right; more products are present at equilibrium.

Question 16

If Keq = 1, what does this mean?

Answer 16

If Keq = 1, then the position of equilibrium is in the center, the amount of products is roughly equal to the amount of reactants at equilibrium.

Question 17

If Keq < 1, what does this mean?

Answer 17

If Keq is much smaller than 1 (For example if Keq = 10^-3), then the position of equilibrium is to the left; more reactants are present at equilibrium.

Question 18

If Q < Keq, what does this mean?

Answer 18

If Q < Keq, then the reaction is at a point where it is still moving to the right in order to reach equilibrium.

Question 19

If Q > Keq, what does this mean?

Answer 19

If Q > Keq, then the reaction is too far right, and is moving back left in order to reach equilibrium.

Question 20

When REACTANTS are added, where does the reaction go?

Answer 20

FORWARD

Question 21

When PRODUCTS are added, where do the reactants go?

Answer 21

REVERSE

Question 22

DECREASING pressure moves the reaction where?

Answer 22

FAVORS THE SIDE WITH MORE MOLES

Question 23

DECREASING volume moves the reaction where?

Answer 23

FAVORS THE SIDE WITH THE FEWEST MOLES