Chapter 10 - Equilibrium and reaction rates

Question 1

What is the collision theory?

Answer 1

Collision theory is a model that explains why changes in the concentration or energy of reactants may alter the rate of reaction.

Question 2

What does the collision theory state a reaction needs to occur?

Answer 2

The reactant particles must collide.
The reactant particles must have a minimum amount of energy (the activation energy).
The reactant particles must collide in the correct orientation.

Question 3

What can to happen with reactant particles increase the rate of reaction?

Answer 3

The frequency of collisions between the reactant particles can be increased.
The energy of the reactant particles can be increased.
The reactant particles can be held in the correct orientation.

Question 4

How can you increase the frequency of collisions in a solution?

Answer 4

For a reactant in solution, increasing the concentration of the solution increases the number of dissolved particles and so increases the frequency of collisions.

Question 5

How can the frequency of collisions be increased for a gas reactant?

Answer 5

For a gas reactant, increasing its pressure increases the number of gas particles and so increases the frequency of collisions.

Question 6

How can the frequency of collisions for a solid reactant?

Answer 6

For a solid reactant, increasing its surface area increases the number of solid particles available to react and so increases the frequency of collisions.

Question 7

How could you increase the rate of reaction without a catalyst for this reaction:
Cu + S —> CuS

Answer 7

Powder the solids

Question 8

How could you increase the rate of reaction for this reaction:
Zn + CuSO4 —> ZnSO4 + Cu

Answer 8

Increase the concentration of the copper sulphate solution.

Question 9

How would you increase the rate of reaction (without using a catalyst) for this reaction:
H2 + Cl2 —> 2HCl

Answer 9

Increase the pressure of the gases

Question 10

How would you increase the rate of reaction without using a catalyst for this reaction:
CaCO3 + 2HCl —> CaCl2 + H2O + CO2

Answer 10

Increase the temperature of the acid

Question 11

Magnesium ribbon reacts with dilute hydrochloric acid. State and explain three ways to increase the rate of reaction. (6 marks)

Answer 11

Increase concentration of acid, more frequent collisions of particles in solution with magnesium.
Increase temperature of the acid, greater proportion of the collisions between particles will have the activation energy and be successful.
Use magnesium powder, larger surface area means more frequent collisions between particles in solution and surface.

Question 12

What is the rate of reaction?

Answer 12

The rate of reaction is the change in the quantity of a reactant or a product divided by the time taken.

Question 13

How can the physical quantity of any reactant or product be measured?

Answer 13

This could be the change in mass, the concentration of a solution or the volume of a gas.

Question 14

What happens to the quantity of reactants as the reaction proceeds?

Answer 14

The quantity of reactants fall

Question 15

What does the stoichiometric ratio in the equation show?

Answer 15

The relative amounts by which the quantities change.

Question 16

How do you calculate the initial rate of reaction on a graph showing quantity and time.

Answer 16

The initial rate is the gradient of a tangent (change in y/ change in x)

Question 17

What does the Boltzmann distribution show?

Answer 17

It shows the distribution of kinetic energy among particles (in a gas).

Question 18

Which particles are able to react when they collide?

Answer 18

Only the particles with enough energy (Ea) - the activation energy, will be able to react when they collide, this will only be a small proportion of the particles.

Question 19

What are on the x and y axis of the Boltzmann distribution?

Answer 19

Kinetic energy is on the x axis and Number of particles is on the y axis

Question 20

What is a catalyst and what do they do?

Answer 20

A catalyst is a substance which increases the rate of reaction without being used up by the overall reaction. It allows the reaction to proceed via a different route with lower activation energy. Catalysts can be homogeneous or heterogeneous.

Question 21

What does a catalyst do to reactant particles when they approach the catalyst surface?

Answer 21

Reactants are adsorbed onto the catalyst surface, weakening their bonds.

Question 22

What is a heterogeneous catalyst and what state are they and their reactants in?

Answer 22

A heterogeneous catalyst is one in a different phase from the reactants. Heterogeneous catalysts are usually solid with reactants that are gases or liquid.

Question 23

What are homogeneous catalysts?

Answer 23

They are catalysts that are in the same phase as the reactants and completely mixed with them. The reaction is split into two steps.
They’re usually in solution.

Question 24

What are catalyst properties?

Answer 24

Catalysts aren’t used up, so do not appear in the overall equation.
Catalysts are often transition metals or their compounds.

Question 25

Why are catalysts used in many industrial reactions?

Answer 25

They lower the temperature required for a suitable rate of reaction.
At a lower temperature, energy demand is reduced.
Less fossil fuel needs to be burnt to release energy so less carbon dioxide is released to the atmosphere.
Manufacturers spend less on energy.

Question 26

What is dynamic equilibrium?

Answer 26

When the rate of the forward reaction equals the rate of the backward reaction in a closed system.

Question 27

What happens to the concentration of the reactants and products when equilibrium is reached?

Answer 27

When equilibrium is established the forward reaction and reverse reactions continue but the concentrations of reactants and products no longer change.

Question 28

Are the concentrations of reactant and product equal at equilibrium?

Answer 28

Not necessarily

Question 29

What does Le Chatelier’s principle state?

Answer 29

Le Chatelier’s principle states that when any change is made to the conditions of an equilibrium, the position of the equilibrium moves in the direction that minimises the change.

Question 30

What happens to the position of equilibrium when the temperature is increased.

Answer 30

The position of equilibrium moves in direction that absorbs heat. (endothermic reaction)

Question 31

What happens to the position of equilibrium when the temperature is decreased.

Answer 31

The position of equilibrium moves in direction that releases heat. (exothermic reaction)

Question 32

What happens to the position of equilibrium when the pressure is increased.

Answer 32

The position of equilibrium moves in direction that reduces pressure. The direction giving fewer gas molecules.

Question 33

What happens to the position of equilibrium when the pressure is decreased.

Answer 33

The position of equilibrium moves in direction that increases pressure. The direction giving more gas molecules.

Question 34

What happens to the position of equilibrium when the concentration increases.

Answer 34

The position of equilibrium moves in direction that reduces concentration. The direction away from added substance.

Question 35

What happens to the position of equilibrium when the concentration decreases.

Answer 35

The position of equilibrium moves in direction that increases concentration. The direction towards removed substance.

Question 36

What happens to the position of equilibrium when a catalyst is added or removed?

Answer 36

The position of equilibrium doesn’t change because it increases the rate of the forward and backward reactions equally.

Question 37

What is meant by the equilibrium constant, Kc?

Answer 37

The equilibrium constant Kc is the ratio of product concentration to reactant concentrations, raised to the appropriate power.
It only varies for a reaction when the temperature is changed.
It indicates whether the position of equilibrium is to the left or right.

Question 38

What is the expression for equilibrium constant for the reaction aA + bB ⇌ cC + dD

Answer 38

Kc = (C)^c (D)^d / (A)^a (B)^b
C is the concentration of a product.
A is the concentration of a reactant.
The power is the number of moles in the equation.

Question 39

In industry why does a compromise between yield and rate have to be made?

Answer 39

For an exothermic reaction, a lower temperature gives a higher yield.
A lower temperature would save energy costs.
However, a lower temperature would slow down the forward and backward reaction so it takes longer to reach equilibrium.
A compromise temperature will have to be used.

Question 40

When does Kc apply to a reaction.

Answer 40

When the reaction has reached equilibrium.

Question 41

What does a high value of Kc indicate?

Answer 41

If Kc is greater than 1 it indicates a position of equilibrium to the right.

Question 42

What does a low value of Kc indicate?

Answer 42

If Kc is less than 1 it indicates a position of equilibrium to the left.