Topic 9 + 16: Kinetics I, II

Question 1

What is rate of reaction?

Answer 1

Change in concentration / time

Question 2

What is activation energy

Answer 2

This minimum amount of energy required for the reaction to take place

Question 3

What is collision theory

Answer 3

Particles must collide with sufficient energy and in the correct orientation in order for a successful reaction to occur

Question 4

What factors affect the rate of reaction

Answer 4

• Nature of reactants - particle size
• Temperature
• Concentration
• Presence of a catalyst

Question 5

Describe the effect of particle size on rate of reaction

Answer 5

In a solid molecules can only react at the surface (where molecules are exposed). If there is a higher surface area (i.e. smaller particle size) then more molecules are able to react and the frequency of collisions increases.

Question 6

Describe the affect of temperature on reaction rates

Answer 6

Increasing the temperature means more particles have a higher kinetic energy. Therefore more collisions will have sufficient energy and there will be more frequent collisions. Therefore there are more frequent successful collisions and a faster rate of reaction.

The reverse would be true for decreasing

Question 7

Describe the affect of concentration on rate of reaction

Answer 7

If the solution is more concentrated then there are more molecules in the same volume. This means there are more molecules closer together. Therefore there are more frequent collisions. Also more particles with the required minimum energy are added to the system.

Question 8

Describe the affect of catalysts on rate of reaction.

Answer 8

Decrease the activation energy so that a higher proportion of molecules have sufficient energy for successful collisions. Therefore faster rate of reaction (more frequent successful collisions).

Question 9

What is the most probable energy on a Maxwell-Bolztmann distribution?

Answer 9

The energy at the peak (maximum point) of the graph.

Question 10

What is the average energy on a Maxwell-Bolztmann distribution

Answer 10

The average energy lies slightly to the right of the most probable energy (the peak).

Question 11

What does the area under the curve of a Maxwell-Bolztmann distribution represent?

Answer 11

The number of molecules

Question 12

Explain the shape of a Maxwell-Bolztmann Distribution

Answer 12

Due to many collisions taking place between molecules, there is a spread of molecular energies and velocities.
* No particles have zero energy/velocity
* Some have very low and some have very high energies
* Most have intermediate energies

Question 13

How does increasing the temperature change the Maxwell-Bolztmann Distribution?

Answer 13

• Shift to higher energies
• Curve gets broader and flatter due to greater spread of values
• Area under the curve stays constant

The opposite is true for decreasing the temperature

Question 14

What is the affect on the Maxwell-Bolztmann distribution of increasing concentration

Answer 14

Area under the curve increases. Distribution of energies stays roughly the same (it almost looks like another curve the same just copied slightly above the original).

Question 15

Homogeneous catalyst

Answer 15

Catalyst is in the same phase as the reactants

Question 16

Heterogeneous catalyst

Answer 16

The reactants and the catalyst are in different phases

Question 17

Give examples of heterogeneous catalysts

Answer 17

• Zeolite in cracking
• Iron in the Haber process
• Ni/Pt/Pd in hydrogenation of alkenes

Question 18

Give the 3 stages of heterogeneous catalysis

Answer 18

1. Adsorption (molecules bond to active sites on catalyst)
2. Reaction (weakened bonds allow for molecules to react with each other forming new bonds and breaking old ones)
3. Desorption (products released from surface of the catalyst).

Question 19

What are the units of rate of reaction

Answer 19

moldm^-3s^-1

Question 20

What method would you use to measure the rate of reaction of a reaction which produces a gas

Answer 20

• measure volume of gas produced using a gas syringe
• weigh reaction mixture on a balance at time intervals -> look for change in mass

Question 21

What method would you use to measure the rate of reaction of a reaction which has an acidic or basic reactant or product

Answer 21

• Titration; sample reaction mixture at time intervals, quench and titrate samples
• measure the pH using a pH probe meter

Question 22

What method would you use to measure the rate of reaction of a reaction which has a coloured reactant or product

Answer 22

Colorimetry -> spectroscopic method
Coloured compounds frequently include transtition metal compounds, aqueous bromine/iodine, etc.

Question 23

Why is the initial reaction rate often quoted

Answer 23

As the reaction proceeds,
* the concentration of reactants decreases and it can be difficult to measure the changes accurately
* reverse reaction may occur introducing error

Question 24

What is initial rate of reaction

Answer 24

Rate of reaction when reactants are first mixed

Question 25

What is instantaneous rate

Answer 25

Rate of reaction at a specific time

Question 26

What is the general rule for the average rate of the reaction:
aA + bB -> cC + dD

Answer 26

Rate = -1/x * dX/dt

where x = a,b,c or d

Question 27

What can be said about the rate of reaction with respect to A and B in:
2A -> B

Answer 27

The rate of disappearance of A is twice as fast as the appearance of B

Question 28

What is the general method for determining rate law

Answer 28

For a reaction involving several reactants;
1. Keep concentration of one reactant constant
2. See how the rate changes as the concentration of the other reactant changes

Question 29

what is the general rate equation?
A is 2nd order
B is 1st order

Answer 29

rate = k [A]^2[B]

Question 30

What is the effect of temperature on the rate constant?

Answer 30

Experimentally find that rates of most chemical reactions increase with increasing temperature.
Rate constant approximately doubles for every 10K rise in temp.

Question 31

Describe conc-time graph for zero-order reaction

Answer 31

linear, negative gradient

Question 32

describe conc-time graph for first order

Answer 32

non-linear, convex curve (negative gradient increasing)

Question 33

Describe conc-time graph for second order

Answer 33

non-linear, convex curve (negative gradient increasing)
Steeper gradient initially than first order and flattens out faster

Question 34

what does a rate-conc graph look like for a zero order reaction

Answer 34

y = constant (i.e horizontal line)

Question 35

what does a rate-conc graph look like for a first order reaction

Answer 35

y = x (linear relationship)

Question 36

what does a rate-conc graph look like for a second order reaction

Answer 36

starts at 0, convex curve with increasing gradient (“exponential”)

Question 37

How doe half lives change for a 0 order reactant

Answer 37

Successive half lives decrease with time

Question 38

How do half lives of a first order reactant change

Answer 38

Half lives remain constant

Question 39

How do half lives of a second order reactant change

Answer 39

They increase with time

Question 40

What is half life

Answer 40

The time required for the concentration of a reactant to decrease to half its original value (can identify from concentration time graph)

Question 41

What is the relationship between the rate equation and the rate determining step

Answer 41

The rate determining step is the slowest step of the reaction. The reactants involved (or needed to produce the reactants in previous steps) in the slow step appear in the rate equation.