Equilibrium and reversible reactions

Question 1

What happens in a reversible reaction?

Answer 1

Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up

In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again

Question 2

What happens in reversible reactions? (both ways)

Answer 2

It is said that the reaction can occur in both directions:

The forward reaction forming the products

The reverse reaction forming the reactants

Question 3

Are the reactions exothermic or endothermic? In reversible reactions

Answer 3

If the forward reaction is exothermic, then the reverse reaction will be endothermic
The same amount of heat is transferred in both directions

Question 4

What happens when writing equations for reversible reactions?

Answer 4

When writing chemical equations for reversible reactions, a symbol with arrows pointing on both sides is used

Question 5

Explain an example of a reversible reaction.

Answer 5

An example is, the reaction for the Haber process which produces ammonia from nitrogen and hydrogen

N2 + 3H2 ⇌ 2NH3

The forward reaction, producing ammonia, is exothermic

So, the reverse reaction is endothermic

Question 6

What are hydrated salts?

Answer 6

Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour

Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process

Question 7

Explain the reaction of the hydrated salt:
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO4*5H2O ⇌ CuSO4 + 5H2O

What colour are the crystals?
Which is the hydrated salt and which one is anhydrous

Answer 7

The hydrated salt is copper(II) sulfate pentahydrate, CuSO4*5H2O

These are usually seen as blue crystals

The hydrated salt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO4
This reaction is endothermic as energy is taken in to remove the water

The anhydrous salt is copper(II) sulfate
This is usually seen as white crystals / powder
Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO4*5H2O

This reaction is highly exothermic

Question 8

Explain this reaction:
Another example is cobalt(II) chloride:

hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

CoCl2*6H2O ⇌ CoCl2 + 6H2O

Answer 8

The hydrated salt is cobalt(II) chloride hexahydrate, CoCl2*6H2O
These are usually seen as pink crystals

The hydrated salt can be heated / dehydrated to form anhydrous cobalt(II) chloride, CoCl2

This reaction is endothermic as energy is taken in to remove the water

The anhydrous salt is cobalt(II) chloride, CoCl2

This is usually seen as blue crystals

Adding water to the anhydrous salt forms the hydrated cobalt(II) chloride hexahydrate, CoCl2*6H2O

This reaction is highly exothermic

Question 9

The hydration of CoCl2 and CuSO4 are chemical tests used to detect the presence of water. What are the colour changes?

Answer 9

You should remember the equations and colour changes:

CoCl2 + 6H2O ⇌ CoCl2.6H2O Blue to pink

CuSO4 + 5H2O ⇌ CuSO4.5H2O White to blue

Question 10

Where can a reversible reaction reach equilibrium, what does it mean?

Answer 10

Reversible reactions occur in both the forward and backward directions

A reversible reaction can reach equilibrium in a closed system
This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter

Question 11

What happens to the rate of the reaction at equilibrium?

Answer 11

The rate of the forward reaction is equal to the rate of the reverse reaction

Question 12

What happens to the concentration of reactants at equilibrium? and why?

Answer 12

The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)

Question 13

What does it mean when we say equilibrium is dynamic?

Answer 13

This means that the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate

Question 14

An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process. Explain

Answer 14

At the start of the reaction, only nitrogen and hydrogen are present

This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest

As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease

So, the rate of the forward reaction will decrease

However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase

Ammonia will decompose to reform hydrogen and nitrogen
In a closed system, the two reactions are interlinked and none of the gases can escape
So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:

Question 15

What does Le chateliers principle state?

Answer 15

The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. It states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change

Question 16

What does the chateliers principle allow us to predict and do?

Answer 16

The principle is used to predict changes to the position of equilibrium when there are changes in:
Temperature
Pressure
Concentration

Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions

Question 17

What happens when the pressure is changed? According to CP

Answer 17

if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules

Question 18

When is the position of equilibrium said to shift to the right?

Answer 18

The position of equilibrium is said to shift to the right when the forward reaction is favoured

This means that there is an increase in the amount of products formed

Question 19

When is the position of equilibrium said to shift to the left?

Answer 19

The position of equilibrium is said to shift to the left when the reverse reaction is favoured

So, there is an increase in the amount of reactants formed

Question 20

How can we predict the effects of changes in temperature on systems of equilibrium?

Answer 20

We can predict the effect of changes in temperature on systems in equilibrium

To make this prediction it is necessary to know whether the reaction is exothermic or endothermic

Question 21

What happens when the temperature is raised?

Answer 21

If the temperature is raised:
The yield from the endothermic reaction increases
The yield from the exothermic reaction decreases

Equilibrium moves in the endothermic direction to reverse the change

Question 22

What happens when the temperature is lowered?

Answer 22

If the temperature is lowered:
The yield from the endothermic reaction decreases
The yield from the exothermic reaction increases

Equilibrium moves in the exothermic direction to reverse the change

Question 23

What happens when a change in temperature is made to a system?

Answer 23

When a change in temperature is made to a system, the system will oppose the change

E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature

It will do this by favouring the endothermic reaction

Question 24

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride.

The forward reaction is exothermic:

ICl + Cl2 ⇌ ICl3
dark brown yellow

What colour will the mixture turn when heated? Explain your answer.

Answer 24

The system will oppose the increase in temperature

Increasing the temperature of an equilibrium reaction favours the endothermic reaction

If the forward reaction is exothermic, then the backward reaction must be endothermic

Therefore, the equilibrium will move to the left and produce more of the reactants

This means that the colour of the mixture will become increasingly brown as the temperature increases

Question 25

In which reactions does pressure have an effect?

Answer 25

Changes in pressure only affect reactions where the reactants or products are gases

Question 26

How to predict the effect of changes in pressure on system in equilibrium

Answer 26

We can predict the effect of changes in pressure on systems in equilibrium

To make this prediction, the balanced symbol equation must be known

Question 27

Effect of increase in pressure on system of equilibrium

Answer 27

Equilibrium shifts in the direction that produces the least number of molecules.This reduces the effects of the change because the pressure decreases as the number of molecules decreases.

Question 28

Effect of decrease in pressure on system of equilibrium

Answer 28

Equilibrium shifts in the direction that produces the greatest number of molecules. This reduces the effects of the change because the pressure increases as the number of molecules increases.

Question 29

Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:

2NO2 (g) ⇌ N2O4 (g)
brown gas colourless gas

What will the colour change be if the pressure is increased? Explain your answer.

Answer 29

The number of gas molecules produced by the forward reaction = 1

The number of gas molecules produced by the reverse reaction = 2

An increase in pressure will cause equilibrium to shift in the direction that produces the least number of molecules of gas
This is the forward reaction

So, the equilibrium shifts to the right

This means that:
The mixture will become increasingly colourless

The concentration of N2O4 will increase

Question 30

Effect of changing concentration on equilibrium

Answer 30

The effect of changing concentration can be thought of as a balance, with the reactants on the left and the products on the right

If the concentration of a reactant increases, then the equilbrium shifts to the right to balance this balance

Question 31

Effect of Increase in concentration of a reactant

Answer 31

Equilibrium shifts to the right

Question 32

Effect of decrease in concentration of a reactant

Answer 32

Equilibrium shifts to the left

Question 33

Effect of Increase in concentration of a product

Answer 33

Equilibrium shifts to the left

Question 34

Effect of Decrease in concentration of a product

Answer 34

Equilibrium shifts to the right

Question 35

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride

ICl + Cl2 ⇌ ICl3
dark brown yellow

Explain what happens when:
- The concentration of ICl3 increases
- Some Cl2 is removed

Answer 35

• The concentration of ICl3 increases

There are more molecules of ICl3 on the products side
So, the position of the equilibrium moves to the left
This produces more ICl and Cl2
So, the reaction mixture gets darker / turns dark brown

• Some Cl2 is removed

There are less molecules of Cl2 on the reactants side
So, the position of the equilibrium moves to the left
This produces more Cl2 (and ICl)
So, the reaction mixture gets darker / turns dark brown

Question 36

How does the presence of a catalyst affect equilibrium?

Answer 36

The presence of a catalyst:
Does not affect the position of equilibrium
Increases the rate at which equilibrium is reached

Question 37

Why does a catalyst increase the rate at which equilibrium is reached?

Answer 37

This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway requiring lower activation energy

As a result, the equilibrium concentration of reactants and products is the same as it would be without the catalyst

Question 38

What happens when the conditions of equilibrium are changed, how does the system respond?

Answer 38

When the conditions at equilibrium are changed, the system always responds by doing the opposite.

For example if the concentration is increased the system tries to reduce it by changing the direction of the reaction or if the temperature is increased the system will try to reduce the temperature by absorbing the extra heat.