2. Reaction Kinetics

Question 1

What is the basic way of working out the rate of a reaction?

Answer 1

Change in concentration/time for the change to occur

Delta[X]
————
Delta t

Question 2

What is the sign for rate of reaction?

Answer 2

Rate of reaction is always a positive number

Question 3

What is the integrated rate law for zeroeth order?

Answer 3

[A]=[A]o -it

Question 4

What is the integrated rate law for first order?

Answer 4

[A]=[A]o e^-kt

Or

Ln[A]= -kt + ln[A]o

Question 5

What is the integrated rate law for second order?

Answer 5

1/[A] = 1/[A]o + kt

Question 6

What do you need to plot for a linear relationship for zero order?

Answer 6

[A] vs t

Question 7

What do you need to plot for a linear relationship in first order?

Answer 7

Ln[A] vs t

Question 8

What do you need to plot for a linear relationship in second order?

Answer 8

1/[A] vs t

Question 9

What does the slope indicate for zero order?

Answer 9

-k

Question 10

What does the slope indicate for first order?

Answer 10

-k

Question 11

What does the slope indicate for second order?

Answer 11

K

Question 12

What are the units for k in a zero order?

Answer 12

Moles per litre per second

Question 13

What are the units for k in first order?

Answer 13

Per second

Question 14

What are the units for k in second order?

Answer 14

Per moles L per second

Question 15

What is the formula for half life in a zero order reaction?

Answer 15

t(1/2) = [A]o
——-
2k

Question 16

What is the formula for half life for first order?

Answer 16

t(1/2)=ln2/k

Question 17

What is the formula for half life in a second order reaction?

Answer 17

t(1/2) = 1
——-
K[A]o

Question 18

What is collision theory?

Answer 18

• The rate of a reaction is proportional to the number of effective collisions per second among the reactant molecules
• An effective collision is one that actually gives product molecules, only a small fraction of collisions lead to a net change
• Concentration and temperature can influence the number of effective collisions per second

Question 19

What is activation energy?

Answer 19

The threshold energy which must be overcome for a reaction to occur

Question 20

What does the size of the activation energy barrier suggest about the speed of the reaction?

Answer 20

High barrier gives a slow reaction

Low barrier gives a fast reaction

Question 21

How does temperature affect the rate of reaction?

Answer 21

• The rate constants (rates) of most reactions increase with temperature
• Increase in T increases velocity of the molecules and hence increases the collision frequency

Question 22

What is the Arrhenius equation?

Answer 22

K = Ae^(Ea/RT)

Question 23

What does A represent in the Arrhenius equation?

Answer 23

Z (collision frequency or number of collisions per second) x p (steric factor, fraction of collisions with correct orientation)

Question 24

What does the e in the Arrhenius equation represent?

Answer 24

The fraction of collisions of energy Ea or greater at temperature T

Question 25

What is the graphical way of evaluating activation energy?

Answer 25

Plot of ln(k) vs 1/T has the slope -Ea/R where T is in kelvin

Question 26

What is the second way of evaluating activation energy?

Answer 26

If you have measured k at 2 temperatures, you can obtain the equation:

Ln(k2/k1) = -Ea/R (1/T1 - 1/T2)

Ea = -R ln(k1/k2)
——————
(1/T1 - 1/T2)

Question 27

What is the mechanism of a reaction?

Answer 27

The sequence of actual events that take place as reactant molecules are converted into products

Question 28

In what two ways do elementary reactions differ from ordinary net chemical reactions?

Answer 28

• The rate law of an elementary reaction can be written by inspection of the stoichiometry
• Elementary steps often involve unstable or reactive species that do not appear in the net reaction equation

Question 29

What is the steady state approximation?

Answer 29

In a reaction mechanism involving an intermediate species, the concentration of the intermediate remains constant during the course of the reaction

Rate of change of the intermediate concentration = 0

Question 30

What is a catalyst?

Answer 30

A catalyst speeds up a reaction by providing a route with lower Ea, without being consumed itself in the reaction

Question 31

How do you work out how many times faster a reaction is with a catalyst than without?

Answer 31

Work out k with and without catalyst using the Arrhenius equation and then divide k (cat) by k (no cat)