Chapter 10 - Reaction rates and equilibrium

Question 1

Describe the collision theory.

Answer 1

For a reaction to occur, particles must collide.

• The colliding particles must have enough energy to break the existing bonds (activation energy).
• The particles must be in the correct orientation so that the reactive parts of the particles collide.

Question 2

What is activation energy?

Answer 2

The minimum energy required for a reaction to take place.

Question 3

What effect does an increased concentration have on the rate of reaction of a solution?

Answer 3

There are more particles in the same volume, the reactant particles are closer together, successful collisions are more frequent, the rate of the reaction increases.

Question 4

What is the initial rate of reaction?

Answer 4

The rate at t = 0 on a concentration-time graph.

Question 5

Explain the role of a catalyst.

Answer 5

A catalyst increases the rate of a reaction without being used up over the whole reaction.
A catalyst allows a reaction to proceed via a different route with a lower activation energy.

Question 6

What is a homogeneous catalyst?

Answer 6

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

Question 7

What is a heterogeneous catalyst?

Answer 7

A catalyst that is in a different state from the reactants.

Question 8

What are the economic benefits of using catalysts?

Answer 8

Lower temperatures and pressures can be used which will be less expensive to generate.

Question 9

What are the environmental benefits of using catalysts?

Answer 9

Lower temperatures reduce the energy demand, less fossil fuels are burnt and less carbon dioxide is produced.

Question 10

What is a closed system?

Answer 10

A system which is isolated from the surroundings -nothing can enter and nothing can leave.

Question 11

What does the ⇌ symbol indicate?

Answer 11

That the reversible reaction is in equilibrium.

Question 12

A dynamic equilibrium exists in…

Answer 12

…a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change.

Question 13

What is Le Chatelier’s principle?

Answer 13

When a system in dynamic equilibrium is subjected to a change in conditions, the equilibrium position shifts to minimise the change.

Question 14

What is the effect of changing concentration on the equilibrium position?

Answer 14

When the concentration of one of the reactants is increased, the equilibrium position shifts to the right to decrease the concentration.

When the concentration of one of the reactants is decreased, the equilibrium position shifts to the left to increase the concentration.

Concentration changes only affect equilibria involving solutions or gases.

Question 15

What is the effect of changing temperature on the equilibrium position?

Answer 15

• If the forward reaction is exothermic (ΔH -ve) the reverse reaction is endothermic (ΔH +ve).
• When the temperature is increased, the position of equilibrium shifts in the endothermic direction to decrease the temperature.
• When the temperature is decreased, the position of equilibrium shifts in the exothermic direction to increase the temperature.

Question 16

What is the effect of changing pressure on the equilibrium position?

Answer 16

• An increase in pressure shifts the equilibrium position to the side with fewer gas molecules to decrease the pressure
• A decrease in pressure shifts the equilibrium position to the side with more gas molecules to increase the pressure.

Changes in pressure only affect equilibria involving gases.

Question 17

What is the effect of catalysts on the equilibrium position?

Answer 17

A catalyst increases the rate of both forward and reverse reactions in an equilibrium by the same amount resulting in an unchanged position of equilibrium.

Question 18

If the equilibrium constant, Kc
Kc = 1
then…

Answer 18

The equilibrium position is halfway between reactants and products.

Question 19

If the equilibrium constant, Kc
Kc > 1
then…

Answer 19

The equilibrium position favours the right-hand side (products side).

Question 20

If the equilibrium constant, Kc
Kc < 1
then…

Answer 20

The equilibrium position favours the left-hand side (reactants side).

Question 21

What effect does increased pressure have on the rate of reaction?

Answer 21

There is the same number of gas molecules in a smaller volume, increasing the concentration and rate of reaction.

Question 22

What effect does the presence of a catalyst have on the rate of reaction?

Answer 22

More particles have sufficient energy to react at the lower activation energy, increasing the rate of reaction.

Question 23

Catalysts work by:

Answer 23

• Reacting to form an intermediate.

- Being regenerated later in the reaction.