Reaction rates

Question 1

Explain the term ‘activation energy’.

Answer 1

The minimum amount of kinetic energy needed for the particles to react.

Question 2

Nitrogen monoxide (NO) and ozone (O₃) sometimes react to produce nitrogen dioxide (NO₂) and oxygen (O₂). A collision between the two molecules does not always lead to a reaction. Explain why.
[2 marks]

Answer 2

• The molecules don’t always have enough energy

- Collisions don’t always happen in the right orientation.

Question 3

Explain why an increase in temperature results in an increase in the rate of a reaction.
[2 marks]

Answer 3

• At higher temperatures, more particles will have the activation energy and will be able to react.
• So collisions that result in a reaction (or have the activation energy) will happen more frequently.

Question 4

Name four factors that affect the rate of a reaction.

Answer 4

• Temperature
• Concentration
• Pressure
• Catalysts

Question 5

Why does decreasing concentration decrease the rate of a reaction?

Answer 5

• Decreasing the concentration of reactants in a solution means the particles are further away from each other.
• If they’re distant, they’ll collide less often.
• Fewer collisions mean fewer chances to react.

Question 6

Name two methods for measuring reaction rates.

Answer 6

Any two of these five:

• Measuring the volume of gas produced.
• Measuring the loss of mass as a gas is produced.
• Measuring the change in pH during a reaction.
• Measuring a temperature change
• Taking samples at regular intervals and analysing them by titration.

Question 7

Enzymes are homogeneous catalysts. Explain what a homogeneous catalyst is.

Answer 7

A homogeneous catalyst is in the same state as the reactants.

Question 8

Why are there two humps in the enthalpy profile of a homogeneously catalysed reaction?

Answer 8

Two reactions occur:

• An intermediate compound is being formed with the reactants.
• The products are then formed from the intermediate compounds.

Question 9

Explain how homogeneous catalysts speed up chemical reactions.
[2 marks]

Answer 9

• The homogeneous catalyst forms intermediate compounds.
• The activation enthalpy needed to form the intermediates (and to form the products from the intermediates) is lower than that needed to make the products directly from the reactants.