Reversible Reactions + Equilibrium + Haber Process

Question 1

What does → mean in the reaction

magnesium + oxygen → magnesium oxide

Answer 1

Magnesium is reacting with oxygen to make the compound magnesium oxide
The arrow points in one direction only
This tells us that this reaction only goes forward, MgO will not turn back to form Mg and O2

Question 2

What are reversible reactions

Answer 2

Reactions where products of the reaction can react to produce the original reactants.

Question 3

Give an example of a reversible reaction

Answer 3

ammonium chloride
(heat) ⇌ (cool)
ammonia + hydrogen chloride

Question 4

ammonium chloride
(heat) ⇌ (cool)
ammonia + hydrogen chloride

what does this reaction show
write out the balanced symbol equation

Answer 4

It shows that when we heat the compound ammonium chloride, it reacts to form ammonia and hydrogen chloride.

Also, if we take the products and cool them down, they now react together to reform the ammonium chloride

NH4Cl(s) ⇌ NH3(g) + HCl(g)

Question 5

How can the direction of a reversible reaction be changed

Answer 5

The direction of reversible reactions can be changed by changing
the conditions.

Question 6

Give an example of a reversible reaction which involves hydrated copper sulfate

Answer 6

hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water

exothermic heat
⇌
endothermic cool

CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(l)

Question 7

hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water

describe what is happening in this reaction (also describe what is happening in terms of energy changes)

Answer 7

Hydrated copper sulfate is heated to form anhydrous copper sulfate, which is white. The reaction also produces water.

Because we are heating this, we are putting energy in. This tells us that the forward reaction is endothermic

If we take the anhydrous copper sulfate and add the water back, then the reaction reverses and hydrated copper sulfate is formed. In the reverse reaction, energy is released. The reaction gets hot.

This tells us that the reverse reaction is exothermic

Question 8

Complete the sentence

If a reversible reaction is exothermic in one direction, ___________________________

Answer 8

If a reversible reaction is exothermic in one direction, it is
endothermic in the opposite direction

Question 9

How much energy is transferred in the forward reaction compared to the reverse reaction

Answer 9

The same amount of energy is transferred in each case

Question 10

Define equilibrium

Answer 10

When the forward and reverse reactions occur at exactly the same rate in closed system

Question 11

When can equilibrium occur

Answer 11

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products (a closed system) , equilibrium is reached when the forward and reverse reactions occur at exactly the same rate.

Question 12

What is Le Chatelier’s Principle

Answer 12

If a system is at equilibrium and a change is made to the conditions, then the system responds to counteract the change

Question 13

2NO2 ⇌ N2O4
Explain what will happen if we increase the concentration of NO2

Answer 13

The position of
equilibrium moves to the right

So that the increase in the concentration of 2NO2 is reduced by producing more products (N2O4)

Question 14

2NO2 ⇌ N2O4
Explain what will happen if we decrease the concentration of N2O4

Answer 14

The position of equilibrium moves to the right

So that more NO2 will react to form N2O4 until equilibrium is reached again

Question 15

2NO2 ⇌ N2O4
Explain what will happen if we increase the concentration of N2O4

Answer 15

The position of equilibrium moves to the left

So that the increase in the concentration of N2O4 is reduced by producing more reactants (2NO2)

As a result, the yield of NO2 increases.

Question 16

2NO2 ⇌ N2O4
The forward reaction is exothermic
What does this mean in terms of temperature

Answer 16

This means that when the forward reaction takes place, energy is released and the temperature of the system increases

This also means that when the reverse reaction takes place, energy is taken in and the temperature of the system decreases

Question 17

2NO2 ⇌ N2O4
The forward reaction is exothermic

Explain what will happen if we increase the temperature of the system

Answer 17

The position of equilibrium moves to the left (in the direction of the endothermic reaction)

so that the increase in the temperature is reduced

(this is because reverse reaction is endothermic, so energy is taken in, causing the temperature to fall)

Amount of NO2 would increase and amount of N2O4 would decrease

Question 18

2NO2 ⇌ N2O4
The forward reaction is exothermic

Explain what will happen if we decrease the temperature of the system

Answer 18

The position of equilibrium moves to the right (in the direction of the exothermic reaction) to increase the
temperature

(this is because forward reaction is exothermic , so energy is released in, causing the temperature to increase)

Amount of N2O4 would increase and amount of 2NO2 would decrease

Question 19

Pressure affects reactions involving _________

Answer 19

Pressure affects reactions involving gases

Question 20

What does the pressure of a gas depend on

Answer 20

The pressure of a gas depends on the number of molecules

Question 21

2N2(g) + 3H2(g) ⇌ 2NH3(g)
Explain what will happen if we increase the pressure on this reaction

Answer 21

Equilibrium position moves to
the right (product side) to lower the pressure
because there are fewer gas molecules on product side

Question 22

2N2(g) + 3H2(g) ⇌ 2NH3(g)
Explain what will happen if we reduce the pressure on this reaction

Answer 22

Position of equilibrium moves to the left (reactant side) to increase pressure because there are more gas molecules on reactant side

Question 23

I2(g) + H2(g) ⇌ 2HI(g)
Iodine Hydrogen Hydrogen iodide

Explain what will happen if we increase the pressure on this reaction

Answer 23

There will be no change in equilibrium position because both products and reactant sides have the same number of gas molecules

Question 24

What is the haber process used for

Answer 24

The Haber process is used to manufacture ammonia, which can be used to produce nitrogen-based fertilisers

Question 25

What are the raw materials for the Haber process

Answer 25

The raw materials for the Haber process are nitrogen and
hydrogen.

Question 26

Equation for the Haber process

Answer 26

Nitrogen + Hydrogen ⇌ ammonia
N2(g) + 3H2(g) ⇌ 2NH3(g)

Question 27

State a source for the nitrogen used in the Haber Process

Answer 27

Nitrogen can be extracted from the air

Question 28

State a source for the hydrogen used in the Haber Process

Answer 28

Hydrogen can be produced by reacting methane with steam

Question 29

Describe the Haber Process for the manufacture of ammonia

Answer 29

The purified gases are passed over a catalyst of iron at a high
temperature (about 450°C) and a high pressure (about 200
atmospheres).

This causes some of the nitrogen and hydrogen molecules to react to form ammonia

This is a reversible reaction so some of the ammonia produced breaks back down into nitrogen and hydrogen

To increase the yield, we now cool the ammonia to turn it into a liquid which is removed

We can then recycle the unreacted nitrogen and hydrogen back over the catalyst

Question 30

For the Haber process which reaction is exothermic

Answer 30

The forward reaction of the Haber Process is exothermic

Question 31

Explain how the conditions for the Haber process are selected to maximise the yield of ammonia

Answer 31

The temperature and the pressure can be adjusted to shift the position of the equilibrium towards the right hand side (to produce more ammonia)

In the case of the Haber process the forward reaction is exothermic.
This means that a relatively cool temperature will shift the equilibrium to the right hand side.

The problem is that a cool temperature makes the reaction slow. So we have a trade-off between the rate of reaction and the position of the equilibrium.
This means that 450 degrees is a compromise temperature. We gets a relatively fast rate and a relatively high yield of ammonia.

Pressure also affects the equilibrium. In the case of the Haber process, a high pressure will shift the equilibrium position to the right hand side (to produce more ammonia), as there are four molecules of gas on the left hand side for every two molecules on the right hand side.

However it is extremely expensive and dangerous working with very high pressures. So we settle on a compromise pressure of 200 atmospheres

The iron catalyst also increases the rate of reaction

The catalyst has no effect at all on the position of the equilibrium