Unit 1 - Rates and Collision Theory

Question 1

What does collision theory state?

Answer 1

For a reaction to occur, particles must collide. In order to be successful, collisions must…
1. Have energy greater than Ea
2. Have the correct alignment / geometry

Question 2

Explain the effect of particle size on reaction rate.

Answer 2

• Decreasing particle size increases the surface area.
• On solids, this results in an increased area available where collisions could occur.
• More collisions mean that there will be more successful collisions.
• Therefore the rate of reaction is increased.

Question 3

Explain the effect of concentration on reaction rate.

Answer 3

• Increasing concentration increases the rate of reaction.
• Increasing concentration increases the number of particles in a reaction vessel.
• More particles mean more collisions.
• More collisions mean that there will be more successful collisions.
• Therefore the rate of reaction is increased.

Question 4

Explain the effect of temperature on reaction rate.

Answer 4

• Temperature is a measure of the average kinetic energy of a substance (Ek).
• Increasing temperature increases the rate of reaction.
• Increasing temperature increases the kinetic energy of particles so they move faster.
• Faster particles mean more collisions.
• More collisions mean that there will be more successful collisions.
• Therefore the rate of reaction is increased.

Question 5

Explain the effect of pressure on reaction rate.

Answer 5

• Increasing pressure increases the rate of reaction for reactions involving gases.
• Increasing pressure increases the number of reactants in a given volume.
• More particles in a volume of space mean more collisions.
• More collisions mean that there will be more successful collisions.
• Therefore the rate of reaction is increased.

Question 6

How do you calculate relative rate of reaction?

Answer 6

Relative rate = 1 / time

Question 7

What are the units for relative rate?

Answer 7

s⁻¹ (per second)

Question 8

How can we follow the rate of a reaction?

Answer 8

• Change in concentration.
• Change in mass.
• Change in pH.
• Change in colour.
• Measure the volume of gas produced (either using a gas syringe or using an upturned measuring cylinder with displacement of water).

Question 9

Why must chemists be able to control the rate of reaction?

Answer 9

• If the rate is too slow, there will not be enough profit.
• If the rate is too high, risk of explosion!