Past Exam Questions

Question 1

Define Rate of Reaction

Answer 1

The change in concentration per unit of any one reactant or product

Question 2

Explain the term heterogeneous catalysis

Answer 2

Catalysis in which the reactants and the catalyst are in different phases

Question 3

Describe the surface adsorption theory of catalysis

3 main points

Answer 3

1. Reactant molecules diffuse towards and then settle on the surface of a catalyst
2. The reactant molecules are held on the surface of the catalyst by temporary bonds formed between these molecules and the catalyst
3. The products leave the surface of the catalyst and the process begins again

Question 4

Explain what is meant by activation energy

Answer 4

The minimum energy that colliding particles must have for a reaction to occur

Question 5

State and explain two ways of increasing the rate of reaction, other than by using a catalyst

Answer 5

1. Increase concentration of reactants:
   - Reactants are closer together so there is a greater frequency of collisions between reactants
2. Increase temperature
   - Reactants have more energy and therefore move faster which means there is a greater frequency of collisions between reactants

Question 6

Suggest a reason why NO is not formed in every collision between a pair of NO2 molecules

Answer 6

Colliding NO2 molecules did not reach activation energy

Question 7

What is a catalyst?

Answer 7

A substance that alters the rate of a chemical reaction but is not consumed in the reaction

Question 8

Explain how the type of bonding in the reactants influences the rate of a chemical reaction in aqueous solution.

Answer 8

Ionic compounds react quickly in solution as bonds are already broken
Covalent compounds react slowly as bonds need to be broken first

Question 9

How could you reduce the rate of a reaction that takes place in solution?

Answer 9

Lowering the temperature

Question 10

The diagram shows the oxidation of methanol using platinum wire as catalyst.
State one observation made during this experiment.

Answer 10

wire glows and dims
Smell of methanol
Popping sounds
Flame

Question 11

The diagram shows the oxidation of methanol using platinum wire as catalyst.
Identify two major products of the oxidation.

Answer 11

Methanal
Water (H2O)
Hydrogen (H2)

Question 12

What term is usually used to describe the attachment

of liquid or gaseous molecules to a solid surface?

Answer 12

Adsorption

Question 13

Explain how this type of interaction between the methanol and the platinum catalyst affects the rate
of oxidation.

Answer 13

1. Methanol bonds weaken
2. Higher methanol concentration on surface
3. Methanol forms temporary bonds on surface
4. There are more effective collisions
   between reactants on the surface of the catalyst

Question 14

Explain how a catalyst poison like sulfur interferes with a catalyst.

Answer 14

The surface of the catalyst is blocked by sulfur

Question 15

Name two metals used in catalytic converters

Answer 15

Platinum

Palladium

Question 16

What type of catalysis occurs in a catalytic converter?

Answer 16

Heterogeneous

Question 17

Name a substance that could poison the catalyst of a catalytic converter

Answer 17

Lead

Question 18

Explain clearly why there is an almost instantaneous reaction between aqueous solutions of sodium
chloride and silver nitrate

Answer 18

They are ionic compounds which means their bonds are already broken

Question 19

Explain effective collision.

Answer 19

A collision that results in the formation of products

Question 20

Write a balanced equation for the decomposition of hydrogen peroxide

Answer 20

2H2O2 -> 2H2O + O2

Question 21

Why does the rate of chemical reactions generally decrease with time?

Answer 21

Reactants are gradually used up

Question 22

Give 3 conditions necessary for a dust explosion to occur

Answer 22

Ignition 
Oxygen 
Enclosed space 
Particles must be dry 
Particles must be combustible

Question 23

Name two substances that enter a catalytic converter and name what substances they are converted into

Answer 23

Carbon monoxide enters and is converted to carbon dioxide

Hydrocarbons enter and are converted into carbon dioxide and water

Question 24

What type of catalyst is used during the decomposition of hydrogen peroxide

Answer 24

Heterogeneous

Question 25

Why would a graph showing the decomposition of hydrogen peroxide be steepest at the beginning?

Answer 25

This is when most of the collisions occur