6.2 Reversible Reactions & Equilibrium

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

	forward reaction
2NO2 (g)		N2O4 (g)
dark brown	backward reaction	colourless

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

This question is about compounds of nitrogen.

[5]

[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.

Ammonia is manufactured by the Haber process.

 N₂(g) + 3H₂(g)    2NH₃

The forward reaction is exothermic.

Describe how the reactants are obtained.

[3]

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

[1]

[6]

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

[2]

[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.

CH₄ (g) + H₂O (g)    CO (g) + 3H₂ (g)

[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)

[1]

[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.

[1]

[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.

[1]

[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.

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.

[2]

[3]

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

[1]

[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

[3]

[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.

[1]

[1]

[1]

[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.

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

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

2SO₂ + O₂ 2SO₃

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 = ..............................

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.

[2]

[1]

[1]

[1]

[2]

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

 
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.

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. 

N2 (g) + O2 (g)	2NO (g)       equilibrium 1
	nitric oxide

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.

2NO (g) + O2 (g)

2NO2 (g)       equilibrium 2

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

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

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.

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.

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.

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

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.

[2]

 [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.

[1]

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

2SO₂ (g) + O₂ (g) 2SO₃ (g)

[1]

[2]

[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

[1]

[2]

[1]

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

[1]

[1]

[2]

PO₄^3– + 2H₂SO₄ → H₂PO₄^– + 2HSO₄^–

[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.

Bonds	Energy Change	Exothermic or Endothermic?
1 mole of NN broken	+945	.......................................
3 moles of ............ broken	+1308	.......................................
6 moles of N – H formed	-2328	.......................................

[4]

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.

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

[2]

[2]

[2]

Ammonia is a base.

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

The simplest alcohol is methanol.

It is manufactured by the following reversible reaction.

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

[1]

[2]

[5]

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

[2]

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

[2]

[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.

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.

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.

[3]

[3]

[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

[2]

[1]

[2]

[2]

Both of the following reactions are reversible.

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

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

[1]

[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?