Dynamic Equilibria (Edexcel GCSE Chemistry)

What essential plant nutrients do fertilisers supply?

Ammonia and sulfuric acid can react in the laboratory to make ammonium sulfate fertiliser.

What is the formula of the product?

Higher Tier Only

Ammonia is manufactured by the Haber process.

The reaction that occurs is:

N₂ (g) + 3H₂ (g)  2NH₃ (g)

A temperature of 450 ^oC is used instead of room temperature, and an iron catalyst can also be added.

Which row of the table shows how the temperature and the catalyst affect the rate of attainment of equilibrium?

Higher Tier Only 

Ammonia is manufactured by the Haber process.

The reaction that occurs is:

N₂ (g) + 3H₂ (g) → 2NH₃ (g)

The reaction is carried out at a pressure of 200 atmospheres. A higher pressure would give a greater equilibrium yield of ammonia.

Which of these statements is not a correct reason for the choice of pressure in the Haber process?

Figure 1 shows the dot and cross diagram for a molecule of ammonia.

Figure 1

i) What do the dots and crosses represent in the diagram?

(1)

ii) Give the formula for the molecule of ammonia. (1)

i) Ammonia can be manufactured by the Haber process. The word equation for the reaction is

nitrogen + hydrogen  ammonia

State the meaning of the symbol.

(1)

ii) In the Haber process, the percentage yield of ammonia at equilibrium changes with temperature.

Figure 2 shows how the percentage yield of ammonia at equilibrium changes with temperature.

Figure 2

State what happens to the percentage yield of ammonia at equilibrium as the temperature increases.

(1)

iii) Use the graph to find the percentage yield of ammonia at equilibrium at 450 °C. (1) percentage yield of ammonia at equilibrium = ..............................................

Ammonia reacts with nitric acid to form ammonium nitrate.

i) Complete the word equation for this reaction. (1)

................. + .................. → ........................

ii) An ammonium ion has the formula NH₄⁺.

A nitrate ion has the formula NO₃^-.

Which of the following is the formula for ammonium nitrate?

(1)

iii) Explain why farmers spread ammonium nitrate on their fields.

(2)

Ammonium sulfate can be made by a laboratory preparation or an industrial process.

Which of the following statements is not a correct difference between the laboratory preparation and the industrial process?

Fertilisers contain compounds that promote plant growth.

i) State the name of an element in these compounds that promotes plant growth.  

(1)

ii) Potassium nitrate is present in some fertilisers. Potassium nitrate is formed by the reaction of potassium hydroxide solution with nitric acid. Complete the balanced equation for this reaction. 

(2)

KOH + HNO₃ → .............................. + ..............................

In the Haber process, hydrogen and nitrogen react to form ammonia. 

hydrogen + nitrogen  ammonia

i) The  symbol in the word equation shows that the reaction goes forwards and backwards at the same time.

Give the name of this type of reaction.

(1)

ii) State the formula of a molecule of ammonia. 

(1)

iii) Figure 7 shows a graph of world ammonia production, in millions of tonnes, from 1945 to 2015. 

Figure 7

State the overall trend in world ammonia production from 1945 to 2015.

(1)

Hydrogen can also be used in a hydrogen-oxygen fuel cell.

Give the name of the product formed in this fuel cell.  

Ammonia solution and dilute sulfuric acid are used to prepare pure, dry ammonium sulfate crystals. In an experiment a titration is carried out to determine the volumes of ammonia solution and dilute sulfuric acid that react together.

Then an ammonium sulfate solution is prepared from which the pure, dry crystals are obtained.

Describe in detail, using suitable apparatus, how this experiment should be carried out. 

What numbers are required to balance the equations showing the reaction between ammonia and sulfuric acid?

___ NH₃ (aq) + ___ H₂SO₄ (aq)   ___ (NH₄)₂SO₄  (aq)

Ammonia is made by reacting nitrogen with hydrogen.

The nitrogen and hydrogen are obtained from raw materials.

Draw one straight line from each gas to the raw material it is obtained from.

When nitrogen and hydrogen are reacted together, the reaction can reach a dynamic equilibrium.

Use words from the box to complete the sentences about dynamic equilibrium.

In a dynamic equilibrium two reactions occur at the same time.

These are the forward reaction and the ............................... reaction.

The rates of the two reactions are ...............................

The reaction between nitrogen and hydrogen happens at a pressure of 200 atmospheres.

Another unit of pressure is Pascals, Pa (1 atmosphere = 101325 Pa).    

Calculate the value of 200 atmospheres in Pascals.

pressure = ............................................................. Pa

Figure 5 shows molecules of nitrogen, hydrogen and ammonia before the reaction and at equilibrium.

Figure 5

i) Complete the table showing

   • the number of hydrogen molecules before reaction    • the number of hydrogen molecules at equilibrium    • the change in the number of hydrogen molecules.

(1)

ii) Complete the equation for this reaction.

(2)

......................... + ............................... 2NH₃

The Haber process is used to make ammonia, and ammonia is used to make nitric acid.

Ammonia and nitric acid can be reacted to make a salt that can be used as a fertiliser.

What is the formula of the salt made from this reaction?

Higher Only

Sulfur trioxide is produced by reacting sulfur dioxide with oxygen.

2SO₂ + O₂ 2SO₃

i) This reaction takes place in industry at 1–2 atm pressure and can reach a dynamic equilibrium. Explain the effect on the rate of attainment of equilibrium, if the process is carried out at a pressure higher than 1–2 atm.

(3)

ii) What volume of oxygen, in cm³, would react completely with 500 cm³ sulfur dioxide?

(1)

Higher Only

When there are alternative methods of producing a product, the final pathway is chosen by considering atom economy, cost of energy, yield of product and rates of reactions.

State another factor that should also be considered.

Higher Only

The reaction between nitrogen and hydrogen is exothermic.

N₂ + 3H₂  2NH₃

If nitrogen and hydrogen were reacted at 150 atm pressure and 300°C, without a catalyst, some ammonia would be formed.

In the Haber process a pressure of 150 atm and a temperature of 450°C are used, in the presence of an iron catalyst.

Explain why the conditions used in the Haber process are better than the first set of conditions for the manufacture of ammonia.

Nitric acid can be titrated with a solution of ammonia.

i) State the type of reaction occurring when nitric acid reacts with ammonia.

(1)

ii) What salt is formed in this reaction?                                                                                 

(1)

Higher Only

In one stage of the production of nitric acid, nitrogen oxide, NO, is reacted with oxygen to make nitrogen dioxide, NO₂. 

2NO + O₂ → 2NO₂

 Calculate the minimum volume of air, measured at room temperature and pressure, required to react with 1000 g nitrogen oxide to form nitrogen dioxide.

 Assume that the air contains 20% oxygen by volume.

(relative atomic masses: N = 14, O = 16

1 mol of gas occupies 24 dm³ at room temperature and pressure)

volume of air = .............................. dm³

Higher Only

In another stage in the production of nitric acid, ammonia is reacted with oxygen to form nitrogen oxide and water.

4NH₃ (g) + 5O₂ (g)  4NO (g) + 6H₂O (g)

Heat energy is given out when ammonia reacts with oxygen.

The conditions chosen for the reaction are

• excess air, rather than just the right amount

• a pressure of 10 atm, rather than atmospheric pressure

• a temperature of 900 °C, rather than room temperature.

Explain the effect of the conditions chosen on the equilibrium yield of nitrogen oxide and on the rate of attainment of equilibrium.

Higher Tier Only 

Ammonia is manufactured by the Haber process.

The equation for the reaction is

N₂ (g) + 3H₂ (g) 2NH₃ (g)

The reaction is reversible and can reach equilibrium.

i) An iron catalyst can be used in the reaction.

Which row of the table shows how adding the iron catalyst affects the rate of attainment of equilibrium and the equilibrium yield of ammonia?

(1)

ii) Which of the following statements is correct when the reaction reaches equilibrium?

(1)

iii) The reaction is carried out at a pressure of 200 atmospheres.

Explain what effect a pressure higher than 200 atmospheres would have on the rate of attainment of equilibrium and on the equilibrium yield of ammonia.

(4)

Ammonium sulfate and ammonium nitrate are used as fertilisers as they both contain nitrogen, which will increase the yield of crops.

i) Suggest one other reason for using solid ammonium sulfate and solid ammonium nitrate as nitrogenous fertilisers.

(1)

ii) Ammonium nitrate can be made by the reaction of ammonia with nitric acid.

Write the balanced equation for this reaction.

(2)

iii) Describe one similarity and one difference between the industrial production of ammonium sulfate and the laboratory preparation of ammonium sulfate.

(2)

similarity...................................................

difference.................................................

When hydrogen and nitrogen react to form ammonia, the reaction can reach a dynamic equilibrium.

N₂  +   3H₂      2NH₃

Explain what is meant by a dynamic equilibrium.

Higher Only

The pressure used in the Haber process is approximately 200 atmospheres.

Explain how the use of a higher pressure would affect the equilibrium yield of ammonia.

Higher Only

The reaction to form ammonia is exothermic. The temperature used is approximately 450 °C.

Explain how the use of a lower temperature would affect the equilibrium yield of ammonia.

Higher Only

Undertaking the Haber process at 450 ^oC still means the reaction is very slow. 

Explain how this problem can be overcome in industry.

Ammonia can be used to produce salts used as fertilisers. 

Two of these salts include ammonium sulfate and ammonium nitrate. 

Write a balanced symbol equation for the production of ammonium sulfate from ammonia and sulfuric acid. 

Compare the industrial production of ammonium sulfate with laboratory production. 

Describe a method that could be used to prepare ammonium sulfate in the laboratory. 

Higher Only

Ammonia is reacted with excess nitric acid, HNO₃, to make ammonium nitrate, NH₄NO₃.

NH₃ + HNO₃ → NH₄NO₃

Calculate the mass of ammonium nitrate produced by the complete reaction of 46 kg of ammonia.

Give your answer in grams to three significant figures.

(Relative atomic masses H = 1.0, N = 14, O = 16)