Reaction Kinetics

Question 1

What is the rate equation?

Answer 1

Rate = k[A]^a[B]^b , determined experimentally

Question 2

Relationship between concentration and pressure?

Answer 2

Concentration directly proportional to pressure

Question 3

What is zero-order reaction? Units of k?

Answer 3

Rate= k[A]^0 , mol dm-3 s-1

Question 4

What is first-order reaction? Units of k?

Answer 4

Rate = k[A]^1 , s-1

Question 5

Charactersitic of first-order reaction? Give two equations and an observation relating to this characteristic

Answer 5

Constant half-life

t1/2 = ln2/k
c/co = (1/2)^n 

*Half-life is independent of concentration of reactants

Question 6

What is second-order reaction? Units of k?

Answer 6

Rate = k [A]^2 , mol-1 dm3 s-1

Question 7

Recall Concentration - Time graphs

Answer 7

Refer to Image 2

Question 8

Recall Rate - Concentration graphs

Answer 8

Refer to Image 1

Question 9

How to differ zero,first and second-order reactions?

Answer 9

Check shape concentration - time graphs.
Straight line –> Zero Order
Curve –> First or second –> Check the half life

Question 10

Three conditions where pseudo-first-order reaction occurs. Common characteristic between them.

Answer 10

1. Presence of large excess of reactants
2. Reactant is a solvent (1)
3. Presence of catalyst

[ ] approximately constant.

Question 11

How to determine order from continuous method?

Answer 11

Shape of concentration time graph, check the half life

Question 12

How to determine Rate Equation of first order reaction from continuous method?

Answer 12

Use t1/2 = ln2/k to find k

Question 13

Given 2 sets of data, how to find order of reaction w.r.t to a reactant?

Answer 13

Initial rates method.

Initial rate = Gradient of graph at t=0min

Question 14

How to determine order from discontinuous method?

Answer 14

Rate1/Rate2 = k[A]^a[B]b/ k[A]^a[B]b –> Solve

Compare 2 experiment where only 1 [reactant] changes

Question 15

One characteristic about the slow step/ rate determing step

Answer 15

Has the highest activation energy

Question 16

Three criteria to meet when proposing a reaction mechanism

Answer 16

1. For slow step, aA + bB –> C ===> rate = [A]^a[B]^b
2. All the elementary steps must add up to the overall balanced equation
3. Rate equation should involve all the reactants in the overall balanced equation (Excluding intermediates)

Question 17

5 things to note when drawing Energy Profile diagram

Answer 17

1. Correct axes (Energy,Reaction pathway)
2. ___ Step Mechanism = ___ Peaks
3. Slow step –> Highest Ea
4. Endothermic vs Exothermic
5. Label species (Start,intermediate,end)

Question 18

Maxwell Boltzmann Curve for increased temperature

Answer 18

Refer to image 3

1. Start at origin
2. Indicate T1 > T2
3. Maximum curve shifts right and lowered
4. Key
5. Correct Axes

Question 19

Explain how increase in temperature increases rate of reaction.

Answer 19

• Average KE of molecules increases
• No. of reactant molecules with energy ≥ Ea increases
• Frequency of effective collision increases
• Rate constant K + Rate increases

Question 20

Maxwell Boltzmann Curve for presence of catalyst

Answer 20

Refer to image 4

1. Start at origin
2. Ea and Ea’
3. Key
4. Correct axes

Question 21

Explain how presence of catalyst increases rate of reaction.

Answer 21

• Catalyst provides alternate pathway of lower Ea.
• More molecules with energy ≥ lowered Ea (Ea’)
• Frequency of effective collision increases
• Rate constant K + Rate increase

Question 22

Define activation energy

Answer 22

Minimum energy needed by reactants for reaction to take place

Question 23

What are the 3 types of catalysis?

Answer 23

1. Hetrogeneous
2. Homogenous
3. Autocatalysis

Question 24

How does hetrogeneous catalyst increase rate?

2 ways

Answer 24

• Adsorption increases concentration of reactant

- Adsorption weakens exisiting bonds within reactant molecules, lowers Ea

Question 25

Kinda explain homogeneous catalysis

Answer 25

Converted into reactive intermediate to take part in reaction, regenerated afterwards

Question 26

What is autocatalysis?

Explain the charactersitics of autocatalysed reaction. (4 points)

Answer 26

Products formed acts as catalyst

1. Initially slow
2. Rate increases as product (autocatalyst) formed
3. Rate reaches max
4. Rate decreases towards end as concentration of reaction decrease. Reaction stops when reactant used up.

Question 27

Condition for maximum rate of enzyme-substrate reaction

Answer 27

All active sites of enzyme occupied by substrate

Question 28

When and why is rate first order wrt substrate?

Rate - [substrate] graph

Answer 28

Initially, when active sites of enzyme not fully occupied

Question 29

When and why is rate zero order wrt substrate?

Rate - [substrate] graph

Answer 29

After certain concentration, when active sites of enzyme occupied

Question 30

Using the inital rates method when given volume

One thing to note when changing total volume

Answer 30

Make sure total volume of solution is same, volume directly proportional to concentration

• Rate does not change when changing total volume*
• -> Concentration remains the same