Reversible Reactions & Equilibrium (Cambridge (CIE) IGCSE Chemistry)

At present the most important method of manufacturing hydrogen is steam reforming of methane.

In the first stage of the process, methane reacts with steam at 800°C.

 CH₄(g) + H₂O(g) 3H₂(g) + CO(g)

In the second stage of the process, carbon monoxide reacts with steam at 200 °C.

CO(g) + H₂O(g)    CO₂(g) + H₂(g)

i) Explain why the position of equilibrium in the first reaction is affected by pressure but the position of equilibrium in the second reaction is not.

[2]

ii) Suggest why a high temperature is needed in the first reaction to get a high yield of products but in the second reaction a high yield is obtained at a low temperature.

[2]

Two other ways of producing hydrogen are cracking and electrolysis.

There are three products of the electrolysis of concentrated aqueous sodium chloride. Hydrogen is one of them.

i) Write an equation for the electrode reaction which forms hydrogen.

 [2]

ii) Name the other two products of the electrolysis of concentrated aqueous sodium chloride and give a use of each one.

product ............................................
use ............................................

product ............................................
use ............................................ 

[4]

Hydrogen can be manufactured from methane by steam reforming.

CH₄ (g) + H₂O (g) CO (g) + 3H₂ (g)

The reaction is carried out using a nickel catalyst at temperatures between 700 °C and 1100 °C and using a pressure of one atmosphere.

The forward reaction is endothermic.

What is meant by the term catalyst?

Suggest two reasons why a temperature lower than 700 °C is not used.

Suggest one advantage of using a pressure greater than one atmosphere.

Suggest one disadvantage of using a pressure greater than one atmosphere.

Hydrogen can also be manufactured by electrolysis. The electrolyte is concentrated aqueous sodium chloride. The electrodes are inert.
The products of electrolysis are hydrogen, chlorine and sodium hydroxide.

 i) Define the term electrolysis.

[2]

ii) Name a substance that can be used as the inert electrodes.

[1]

iii) Write an ionic half-equation for the reaction in which hydrogen is produced.

[1]

iv) Where is hydrogen produced in the electrolytic cell?

[1]

v) Describe a test for chlorine.

test ...............................................................................................................................
result ............................................................................................................................

[2]

The electrolysis of concentrated aqueous sodium chloride can be represented by the following word equation.

  sodium chloride + water → sodium hydroxide + hydrogen + chlorine

Construct a chemical equation to represent this reaction. Do not include state symbols.

Carbonyl chloride is made by the reversible reaction given below.

CO(g) + Cl₂(g)  COCl₂(g)

The forward reaction is exothermic. The reaction is catalysed by carbon within a temperature range of 50 to 150 °C.

i) Predict the effect on the yield of carbonyl chloride of increasing the pressure. Explain your answer.

 [2]

ii) If the temperature is allowed to increase to above 200 °C, very little carbonyl chloride is formed. Explain why.

[2]

iii) Explain why a catalyst is used. 

[1]

The structural formula of carbonyl chloride is given below.

Draw a diagram showing the arrangement of the outer (valency) electrons in one molecule of this covalent compound.

Use o to represent an electron from a carbon atom.

Use x to represent an electron from a chlorine atom.

Use ● to represent an electron from an oxygen atom.

Nitric acid is now made by the oxidation of ammonia. It used to be made from air and water. This process used very large amounts of electricity.

Air was blown through an electric arc and heated to 3000 °C.  

 The equilibrium mixture leaving the arc contained 5% of nitric oxide. This mixture was cooled rapidly. At lower temperatures, nitric oxide will react with oxygen to form nitrogen dioxide. 

 Nitrogen dioxide reacts with oxygen and water to form nitric acid.

 i) Suggest a reason why the yield of nitric oxide in equilibrium 1 increases with temperature.

 [1]

 ii) What effect, if any, would increasing the pressure have on the percentage of nitric oxide in equilibrium 1? Explain your answer.

  [2]

 iii) Deduce why equilibrium 2 is only carried out at lower temperatures.

 [2]

 iv) Complete the equation for the reaction between nitrogen dioxide, water and oxygen to form nitric acid.

 ........NO₂ + O₂ + ............ → ............HNO₃

 [2]

 v) Ammonia is more expensive than water and air. Suggest a reason why the ammonia-based process is preferred to the electric arc process.

[1]

Nitric acid is used to make the fertiliser ammonium nitrate, NH₄NO₃.

 i) What advantage has this fertiliser over another common fertiliser, ammonium sulfate, (NH₄)₂SO₄?

 [1]

 ii) Plants need nitrogen to make chlorophyll. Explain why chlorophyll is essential for plant growth.

 [4]

Ammonia is made by the Haber process.

N₂ (g) + 3H₂ (g) 2NH₃ (g) 

State one major use of ammonia.

Describe how hydrogen is obtained for the Haber process.

This reaction is carried out at a high pressure, 200 atmospheres.

State, with an explanation for each, two advantages of using a high pressure.

i) What is the difference between an endothermic and an exothermic reaction?

 [1]

 ii) Bond breaking is an endothermic process. Bond energy is the amount of energy needed to break or form one mole of the bond.

 Complete the table and explain why the forward reaction is exothermic. 

 [3]

Carbonyl chloride, COCl₂, is a colourless gas. It is made by the following reaction.

cool

CO (g) + Cl₂ (g) COCl₂ (g)

heat

When the pressure on the equilibrium mixture is decreased, the position of equilibrium moves to left.

 i) How does the concentration of each of the three chemicals change?

 [2]

 ii) Explain why the position of equilibrium moves to the left.

 [2] 

Using the information given with the equation, is the forward reaction exothermic or endothermic? Give a reason for your choice.

Carbonyl chloride reacts with water to form two acidic compounds.

Suggest which acidic compounds are formed.

1. .......................................................................................

2. .......................................................................................

The structural formula of carbonyl chloride is given below. 

Draw a diagram that shows the arrangement of the valency electrons in one molecule of this covalent compound.

Use x for an electron from a chlorine atom.

Use o for an electron from a carbon atom.

Use ● for an electron from an oxygen atom. 

Sulfuric acid is made by the Contact process in the following sequence of reactions.

sulfur → sulfur dioxide → sulfur trioxide → sulfuric acid

How is sulfur dioxide made from sulfur?

The equation for a stage of the Contact process is

 2SO₂ + O₂ 2SO₃

 The percentage of sulfur trioxide in the equilibrium mixture varies with temperature.

i) How does the percentage of sulfur trioxide in the equilibrium mixture vary as the temperature increases? Circle the correct answer.

 increases      stays the same      decreases

 [1]

 ii) Is the forward reaction in the equilibrium 2SO₂ + O₂ 2SO₃ exothermic or endothermic? Give a reason for your choice. 

[2]

 iii) Explain, mentioning both rate and percentage yield, why the temperature used in the Contact process is 450 °C. 

[2]

Ammonia is manufactured by the Haber process. Nitrogen and hydrogen are passed over a catalyst at a temperature of 450 °C and a pressure of 200 atmospheres. The equation for the reaction is as follows.

N₂ + 3H₂  2NH₃

The forward reaction is exothermic.

State one use of ammonia.

What is the meaning of the symbol ?

What are the sources of nitrogen and hydrogen used in the Haber process?

nitrogen ......................
hydrogen ....................

Name the catalyst in the Haber process.

i)If a temperature higher than 450 °C was used in the Haber process, what would happen to the rate of the reaction? Give a reason for your answer.

[2]

ii) If a temperature higher than 450 °C was used in the Haber process, what would happen to the yield of ammonia? Give a reason for your answer.

 [2]

i) If a pressure higher than 200 atmospheres was used in the Haber process, what would happen to the yield of ammonia? Give a reason for your answer.

[2]

ii) Explain why the rate of reaction would be faster if the pressure was greater than 200 atmospheres.

 [1]

iii) Suggest one reason why a pressure higher than 200 atmospheres is not used in the Haber process.

 [1]

Draw a dot-and-cross diagram to show the arrangement of the outer (valency) electrons in one molecule of ammonia.

Ammonia acts as a base when it reacts with sulfuric acid.

i) What is a base?

[1]

ii) Write a balanced equation for the reaction between ammonia and sulfuric acid.

[2]

The main use of sulfur dioxide is the manufacture of sulfuric acid.

The Contact process changes sulfur dioxide into sulfur trioxide.

2SO₂ (g) + O₂ (g) 2SO₃ (g)

The forward reaction is exothermic Temperature 400 to 450 °C Low pressure 1 to 10 atmospheres Catalyst vanadium(V) oxide

i) What is the formula of vanadium(V) oxide?

[1]

ii) Vanadium(V) oxide is an efficient catalyst at any temperature in the range 400 to 450 °C. Scientists are looking for an alternative catalyst which is efficient at 300°C. What would be the advantage of using a lower temperature?

[2]

iii) The process does not use a high pressure because of the extra expense. Suggest two advantages of using a high pressure? Explain your suggestions.

[4]

Sulfur is used to make sulphuric acid. In the UK, the annual production of the acid is about 2.5 million tonnes.

The reactions in the manufacture of sulfuric acid by the Contact Process are shown below.

Sulfur                                    Sulfur dioxide

S                                         reaction 1                            SO₂

                     Sulfur dioxide + oxygen                             Sulfur trioxide

2SO₂ + O₂                                          reaction 2                                 2SO₃

Sulfur trioxide                                                Oleum

2SO₃                                reaction 3                               H₂S₂O₇

Oleum + water                                              Sulfuric acid

H₂S₂O₇                           reaction 4                                         H₂SO₄

i) Name the catalyst used in reaction 2.

[1]

ii) Reaction 2 is exothermic. Why is a catalyst, rather than a higher temperature, used to increase the rate of this reversible reaction?

[2]

iii) Write a symbol equation for reaction 4.

[1]

About one third of this production of acid is used to make nitrogen and phosphorus-containing fertilisers.

i) Name the third element that is essential for plant growth and is present in most fertilisers.

[1]

ii) Name a nitrogen-containing fertiliser that is manufactured from sulphuric acid.

[1]

iii) Rock phosphate (calcium phosphate) is obtained by mining. It reacts with concentrated sulphuric acid to form the fertiliser, superphosphate. Predict the formula of each of these phosphates.

[2]

iv) The ionic equation for the reaction between the phosphate ion and sulphuric acid is shown below.

PO₄^3– + 2H₂SO₄ → H₂PO₄^– + 2HSO₄^–

Explain why the phosphate ion is described as acting as a base in this reaction.

[2]

Ammonia is made by the Haber process.

N₂ (g) + 3H₂ (g)    2NH₃ (g)

The forward reaction is exothermic. Typical reaction conditions are:

• finely divided iron catalyst,

• temperature 450 °C,

• pressure 200 atmospheres. 

Explain why the catalyst is used as a very fine powder and larger pieces of iron are not used.

Using the above conditions, the equilibrium mixture contains about 15% ammonia.

State two changes to the reaction conditions which would increase the percentage of ammonia at equilibrium.

Suggest why the changes you have described in (b) are not used in practice.

In 1909, Haber discovered that nitrogen and hydrogen would react to form ammonia. The yield of ammonia was 8%.

N₂ (g) + 3H₂ (g) 2NH₃ (g)

The forward reaction is exothermic

catalyst: platinum
temperature: 600 ^oC
pressure: 200 atm

 Describe how hydrogen is obtained for the modern process.

i) What is the catalyst in the modern process?

[1]

ii) Explain why the modern process, which uses a lower temperature, has a higher yield of 15%.

[2]

i) Complete the following table that describes the bond breaking and forming in the reaction between nitrogen and hydrogen to form ammonia.

[4]

ii) Explain, using the above data, why the forward reaction is exothermic.

[2]

Ammonia contains the elements nitrogen and hydrogen. It is manufactured from these elements in the Haber process.

    N₂(g) + 3H₂(g)      2NH₃

The forward reaction is exothermic.

i) Nitrogen is obtained from liquid air by fractional distillation. Why does this technique separate liquid oxygen and nitrogen?

[1]

ii) Name two raw materials from which hydrogen is manufactured.

[2]

The table shows how the percentage of ammonia in the equilibrium mixture varies with pressure at 600 °C.

i) Explain why the percentage of ammonia increases as the pressure increases.

[2]

ii) How would the percentage of ammonia change if the measurements had been made at a lower temperature? Explain your answer.

[2]

iii) State two of the reaction conditions used in the Haber Process.

[2]

Ammonia is a base. 

i) Name a particle that an ammonia molecule can accept from an acid.

[2]

ii) Write an equation for ammonia acting as a base.

[1]

Another compound that contains nitrogen and hydrogen is hydrazine, N₂H₄.

i) Draw the structural formula of hydrazine. Hydrogen can form only one bond per atom but nitrogen can form three.

[1]

ii) Draw a diagram that shows the arrangement of the valency electrons in one molecule of hydrazine. Hydrazine is a covalent compound. Use x to represent an electron from a nitrogen atom. Use o to represent an electron from a hydrogen atom.

[2]

The simplest alcohol is methanol.

It is manufactured by the following reversible reaction.

           300 ^oC            
30 atm
CO (g) + 2H₂ (g)  CH₃OH (g)                 

i) Reversible reactions can come to equilibrium. Explain the term equilibrium.

[1]

ii) At 400 ^oC, the percentage of methanol in the equilibrium mixture is lower than at 300 ^oC. Suggest an explanation.

[2]

iii) Suggest two advantages of using high pressure for this reaction. Give a reason for each advantage.

[5]

i) Complete the equation for the combustion of methanol in an excess of oxygen.

.......... CH₃OH + .......... O₂ → .............................. + ..............................

[2]

ii) Complete the word equation.

methanol + ethanoic acid → .......................  ..................... + ....................

[2]

iii) Methanol can be oxidised to an acid. Name this acid.

[1]

Carbonyl chloride, COCl₂, is widely used in industry to make polymers, dyes and pharmaceuticals.

Carbonyl chloride was first made in 1812 by exposing a mixture of carbon monoxide and chlorine to bright sunlight.

This is a photochemical reaction.

 CO (g) + Cl₂ (g) → COCl₂ (g)

 i) Explain the phrase photochemical reaction.

 [2]

 ii) Give another example of a photochemical reaction and explain why it is important either to the environment or in industry.

 [3]

Carbonyl chloride is now made by the reversible reaction given below.

CO (g) + Cl₂ (g)  COCl₂ (g)

The forward reaction is exothermic.

The reaction is catalysed by carbon within a temperature range of 50 to 150 °C.

 i) Predict the effect on the yield of carbonyl chloride of increasing the pressure.

Explain your answer.

 [2]

 ii) If the temperature is allowed to increase to above 200°C, very little carbonyl chloride is formed. Explain why.

 [2]

 iii) Explain why a catalyst is used.

 [1]

The structural formula of carbonyl chloride is given below. 

Draw a diagram showing the arrangement of the outer (valency) electrons in one molecule of this covalent compound.

Use o to represent an electron from a carbon atom.

Use x to represent an electron from a chlorine atom.

Use ● to represent an electron from an oxygen atom.

Reversible reactions can come to equilibrium. The following are three examples of types of gaseous equilibria.

A₂ (g) + B₂ (g)2AB (g)    reaction 1

A₂ (g) + 3B₂ (g) 2AB₃ (g)    reaction 2

2AB₂ (g) 2AB(g) + B₂ (g)    reaction 3

Explain the term equilibrium.

The following graphs show how the percentage of products of a reversible reaction at equilibrium could vary with pressure.
For each graph, decide whether the percentage of products decreases, increases or stays the same when the pressure is increased, then match each graph to one of the above reactions and give a reason for your choice.

i).

effect on percentage of products .....................................................................
reaction ......................................................................................................................
reason ........................................................................................................................

[3]

ii)

effect on percentage of products .....................................................................
reaction ......................................................................................................................
reason ........................................................................................................................

[3]

iii).

effect on percentage of products .....................................................................
reaction ......................................................................................................................
reason ........................................................................................................................

[3]

Reversible reactions can come to equilibrium. They have both a forward and a backward reaction.

When water is added to an acidic solution of bismuth(III) chloride, a white precipitate forms and the mixture slowly goes cloudy.

BiCl₃ (aq) + H₂O (l)       BiOCl (s) + 2HCl (aq)

             colourless             white

i) Explain why the rate of the forward reaction decreases with time.

[2]

ii) Why does the rate of the backward reaction increase with time?

[1]

iii) After some time why does the appearance of the mixture remain unchanged?

[2]

iv) When a few drops of concentrated hydrochloric acid are added to the cloudy mixture, it changes to a colourless solution. Suggest an explanation.

[2]

Both of the following reactions are reversible.

reaction 1    N₂ (g) + O₂ (g)  2NO (g)
reaction 2    2NO (g) + O₂ (g) 2NO₂ (g)

i) Suggest a reason why an increase in pressure does not affect the position of equilibrium for reaction 1.

[1]

ii) What effect would an increase in pressure have on the position of equilibrium for reaction 2? Give a reason for your answer.

[2]

Separate: Chemistry and Extended Only

Sulfur dioxide, SO₂, is used in the manufacture of sulfuric acid.

In the first stage of the process, sulfur dioxide is obtained from sulfur-containing ores.

The next stage of the process is a reaction which can reach equilibrium. The equation for this stage is shown.

Describe two features of an equilibrium.

Separate: Chemistry and Extended Only

Name the catalyst used in this stage.

Separate: Chemistry and Extended Only

In this stage, only a moderate temperature of 450 °C is used.

What does this suggest about the forward reaction?

Separate: Chemistry and Extended Only

Sulfuric acid is made from sulfur in a four-stage process.

stage 1  Sulfur is converted into sulfur dioxide.
stage 2  Sulfur dioxide is converted into sulfur trioxide.
stage 3  Sulfur trioxide is converted into oleum.
stage 4  Oleum is converted into sulfuric acid.

Describe how sulfur dioxide is converted into sulfur trioxide in stage 2.

Your answer should include:

• an equation for the reaction

• the temperature used

• the name of the catalyst used.

Separate: Chemistry and Extended Only

The reaction in stage 2 can reach equilibrium.

What is meant by the term equilibrium?

Ammonia is made by the Haber process.

N₂ (g) + 3H₂ (g)  2NH₃ (g)

The forward reaction is exothermic.

The conditions in the reaction chamber are:

• a pressure of 200 atmospheres,

• a catalyst of finely divided iron,

• a temperature of 400 to 450 °C.

What are the two advantages of using a high pressure? Give a reason for both.

advantage 1 ...............................................................................................................................

reason ........................................................................................................................................

advantage 2 ...............................................................................................................................

reason ........................................................................................................................................

A higher temperature would give a faster reaction rate.

 Why is a higher temperature not used?

i) Why is the iron catalyst used as a fine powder?

 [1]

 ii) Give two reasons why a catalyst is used.

 [2]

The equilibrium mixture leaving the reaction chamber contains 15% ammonia. Suggest how the ammonia could be separated from the mixture. 

Ammonia is used to make nitrogen trifluoride, NF₃.

 Nitrogen trifluoride is essential to the electronics industry. It is made by the following reaction.

 Determine if the above reaction is exothermic or endothermic using the following bond energies and by completing the following table. The first line has been done as an example. Bond energy is the amount of energy, in kJ / mole, needed to break or make one mole of the bond. 

Almost all samples of nitrogen dioxide are an equilibrium mixture of nitrogen dioxide, NO₂, and dinitrogen tetroxide, N₂O₄.  

 In the forward reaction, a bond forms between the two nitrogen dioxide molecules.

 NO₂ + NO₂ → O₂N–NO₂ 

i) Explain the term equilibrium mixture.

 [1]

 ii) The syringe contains a sample of the equilibrium mixture. The plunger was pulled back reducing the pressure.

How would the colour of the gas inside the syringe change? Give an explanation for your answer. 

 [3]

iii) A sealed tube containing an equilibrium mixture of nitrogen dioxide and dinitrogen tetroxide was placed in a beaker of ice cold water. The colour of the mixture changed from brown to pale yellow.

 Is the forward reaction exothermic or endothermic? Give an explanation for your choice.

 [2]

 iv) What other piece of information given in the equation supports your answer to (iii)?

 NO₂ + NO₂ → O₂N–NO₂

 [1]

This question is about compounds of nitrogen.

i) Describe the Haber Process giving reaction conditions and a chemical equation.

Reference to rate and yield is not required.

[5]

ii) Give one use of ammonia. 

[1]

The diagram shows the structure of a hydrazine molecule.

Draw the electron arrangement of a hydrazine molecule. Show the outer shell electrons only.

Hydrazine is a base.

Define the term base.

Nitrogen dioxide is an atmospheric pollutant.

i) State one environmental problem caused by nitrogen dioxide.

[1]

ii) Explain how oxides of nitrogen, such as nitrogen dioxide, are formed in car engines.

[2]

Ammonia is manufactured by the Haber process.

 N₂(g) + 3H₂(g)    2NH₃

The forward reaction is exothermic.

Describe how the reactants are obtained.

i) Nitrogen ....................................................................................................................................

[2]

ii) Hydrogen ..................................................................................................................................

[3]

The percentage of ammonia in the equilibrium mixture varies with temperature and pressure.

i) Which pair of graphs, A, B or C, shows correctly how the percentage of ammonia at equilibrium varies with temperature and pressure?

[1]

ii) Give a full explanation of why the pair of graphs you have chosen in (i) is correct.

[6]

iii) Catalysts do not alter the position of equilibrium. Explain why a catalyst is used in this process.

[2]

Nitrogen dioxide is a brown gas. It can be made by heating certain metal nitrates.

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

At most temperatures, samples of nitrogen dioxide are equilibrium mixtures.

2NO₂ (g)     N₂O₄ (g)
dark brown            pale yellow

i) At 25 °C, the mixture contains 20 % of nitrogen dioxide. At 100 °C this has risen to 90%. Is the forward reaction exothermic or endothermic? Give a reason for your choice.

[2]

ii) Explain why the colour of the equilibrium mixture becomes lighter when the pressure on the mixture is increased.

[2]

A 5.00 g sample of impure lead(II) nitrate was heated. The volume of oxygen formed was 0.16 dm³ measured at r.t.p. The impurities did not decompose.

Calculate the percentage of lead(II) nitrate in the sample.

2Pb(NO₃)₂    →   2PbO   +   4NO₂  +   O₂

Number of moles of O₂ formed = .......................................

Number of moles of Pb(NO₃)₂ in the sample = .......................................

Mass of one mole of Pb(NO₃)₂ = 331 g

Mass of lead(II) nitrate in the sample = ....................................... g

Percentage of lead(II) nitrate in sample = .....................................

Carbonyl chloride is made from carbon monoxide and chlorine.

CO (g) + Cl₂ (g)   COCl₂ (g)

Two methods of preparing carbon monoxide are from methane and oxygen, and from methane and steam.

The reaction between methane and oxygen can also form carbon dioxide.

i) How can carbon monoxide be made instead of carbon dioxide?

[1]

ii) The following reaction is used to make carbon monoxide and hydrogen.

The reaction is carried out at 1100 °C and normal pressure.

CH₄ (g) + H₂O (g)    CO (g) + 3H₂ (g)

The reaction is reversible and comes to equilibrium. Suggest why a high temperature is used.

[2]

iii) What is the disadvantage of using a high pressure for the reaction given in (a)(ii)?

[2]

Chlorine is made by the electrolysis of concentrated aqueous sodium chloride.

Describe this electrolysis. Write ionic equations for the reactions at the electrodes and name the sodium compound formed.

The structural formula of carbonyl chloride is given below. 

Draw a diagram showing the arrangement of the valency electrons around the atoms in one molecule of this covalent compound.

Use ○ to represent an electron from a carbon atom. Use × to represent an electron from a chlorine atom. Use ● to represent an electron from an oxygen atom.

The halogens are a group of non-metals in Group VII of the Periodic Table.

The reactivity of the halogens decreases down the group.

Describe an experiment which shows that chlorine is more reactive than iodine. Include an equation in your answer.

The halogens form interhalogen compounds. These are compounds which contain two different halogens.

Deduce the formula of the compound which has the composition 0.013 moles of iodine atoms and 0.065 moles of fluorine atoms.

Iodine reacts with chlorine to form a dark brown liquid, iodine monochloride.

I₂ (s) + Cl₂ (g) → 2ICl (l)

                               dark brown

When more chlorine is added and the tube is sealed, a reversible reaction occurs and the reaction comes to equilibrium.

ICl (l) + Cl₂ (g) ICl₃ (s)

                                          dark brown                 yellow

i) Give another example of a reversible reaction.

[1]

ii) Explain the term equilibrium.

[2]

Chlorine is removed from the tube and a new equilibrium is formed.

Explain why there is less of the yellow solid and more dark brown liquid in the new equilibrium mixture.

A sealed tube containing the equilibrium mixture is placed in ice-cold water. There is an increase in the amount of yellow solid in the equilibrium mixture.

What can you deduce about the forward reaction in this equilibrium?

ICl (l) + Cl₂ (g) ICl₃ (s)

Explain your deduction.

Ammonia is a compound which only contains the elements nitrogen and hydrogen. It is a weak base.

i) Define the term base.

[1]

ii) Given aqueous solutions of ammonia and sodium hydroxide, both having a concentration of 0.1mol / dm³, how could you show that ammonia is the weaker base?

[2]

Ammonia is manufactured by the Haber Process. The economics of this process require that as much ammonia as possible is made as quickly as possible. 

Explain how this can be done using the following information.

The conditions for the following reversible reaction are:

• 450°C

• 200 atmospheres pressure

• iron catalyst

N₂(g) + 3H₂(g)  2NH₃(g)                       the reaction is exothermic

Another compound which contains only nitrogen and hydrogen is hydrazine, N₂H₄.

Complete the equation for the preparation of hydrazine from ammonia. 

.....NH₃ + NaClO → N₂H₄ + ............ + H₂O

The structural formula of hydrazine is given below.

Draw a diagram showing the arrangement of the valency electrons in one molecule of the covalent compound hydrazine.

Use x to represent an electron from a nitrogen atom.

Use o to represent an electron from a hydrogen atom.

Hydrazine is a weak base and it removes dissolved oxygen from water. It is added to water in steel boilers to prevent rusting.

i) One way it reduces the rate of rusting is by changing the pH of water. What effect would hydrazine have on the pH of water?

[1]

ii) Give a reason, other than pH, why hydrazine reduces the rate of rusting.

[1]

Sulfuric acid is produced by the Contact process. The steps of the Contact process are shown.

Describe step 2, giving reaction conditions and a chemical equation. Reference to reaction rate and yield is not required.

Dilute sulfuric acid is a typical acid. A student adds excess dilute sulfuric acid to a sample of solid copper(II) carbonate in a test-tube.

i) Give three observations the student would make.

[2]

ii) Give the names of all products formed.

[1]

Sulfuric acid is made by the Contact process.

The following equation represents the equilibrium in the Contact process.

2SO₂ (g) + O₂ (g) 2SO₃ (g)

Oxygen is supplied from the air. The composition of the reaction mixture is 1 volume of sulfur dioxide to 1 volume of oxygen.

What volume of air contains 1 dm³ of oxygen?

Sulfur dioxide is more expensive than air.

What is the advantage of using an excess of air?

The forward reaction is exothermic. The reaction is usually carried out at a temperature between 400 and 450 °C.

i) What is the effect on the position of equilibrium of using a temperature above 450 °C?
Explain your answer.

[2]

ii) What is the effect on the rate of using a temperature below 400 °C?
Explain your answer.

[3]

A low pressure, 2 atmospheres, is used. At equilibrium, about 98% SO₃ is present.

i) What is the effect on the position of equilibrium of using a higher pressure?

[1]

ii) Explain why a higher pressure is not used.

[1]

Name the catalyst used in the Contact process.

Sulfur is needed for the production of sulfuric acid. Two of the major sources of sulfur are:

• Underground deposits of the element sulfur,

• Sulfur compounds from natural gas and petroleum.

Explain why sulfur and its compounds are removed from these fuels before they are burned.

Describe how sulfur dioxide is changed into sulfur trioxide. Give the reaction conditions and an equation.

Sulfuric acid is made by the Contact Process.

2SO₂ (g) + O₂ (g) → 2SO₃ (g)   forward reaction is exothermic

i) What are the reaction conditions for the Contact Process?

[3]

ii) Would the yield of sulfur trioxide increase, decrease or stay the same when the temperature is increased? Explain your answer.

[2]

There are three ways of making salts from sulphuric acid.

Titration using a burette and indicator Precipitation by mixing the solutions and filtering Neutralisation of sulphuric acid using an excess of an insoluble base Complete the following table of salt preparations.

The results of an investigation into the action of heat on copper(II) sulphate-5-water, a blue crystalline solid, are given below. The formula is CuSO₄.5H₂O and the mass of one mole is 250 g A 5.0 g sample of the blue crystals is heated to form 3.2 g of a white powder. With further heating this decomposes into a black powder and sulphur trioxide.

i) Name the white powder.

[1]

ii) What is observed when water is added to the white powder?

[1]

iii) Name the black powder.

[1]

iv) Calculate the mass of the black powder. Show your working.

[3]

The Carlsbad caverns in New Mexico are very large underground caves. Although the walls of these caves are coated with gypsum (hydrated calcium sulfate), the caves have been formed in limestone.

It is believed that the caves were formed by sulfuric acid reacting with the limestone.

i) Complete the word equation. 

 [1]

 ii) Describe how you could test the water entering the cave to show that it contained sulfate ions.

 test ......................................

 result ......................................

 [2]

 iii) How could you show that the water entering the cave has a high concentration of hydrogen ions?

 [1]

Hydrogen sulfide gas which was escaping from nearby petroleum deposits was being oxidised to sulfuric acid.

 i) Complete the equation for this reaction forming sulfuric acid.

 H₂S + .................... O₂ → ....................

 [2]

ii) Explain why all the hydrogen sulfide should be removed from the petroleum before it is used as a fuel.

 [1]

 iii) Draw a diagram to show the arrangement of the valency electrons in one molecule of the covalent compound hydrogen sulfide.

Use o to represent an electron from a sulfur atom.
Use x to represent an electron from a hydrogen atom.

 [2]

Sulfuric acid is manufactured by the Contact Process. Sulfur dioxide is oxidised to sulfur trioxide by oxygen.

 2SO₂ + O₂ 2SO₃

 i) Name the catalyst used in this reaction.

 [1]

ii) What temperature is used for this reaction?

[1]

Gypsum is hydrated calcium sulphate, CaSO₄.xH₂O. It contains 20.9% water by mass. 

Calculate x.

M_r: CaSO₄, 136; H₂O, 18.

79.1 g of CaSO₄ = .............................. moles

20.9 g of H₂O = .............................. moles

x = ..............................