Unit 2 – Chemical Equilibria
Unit 2 - Jan 2009
Q1.
Ethanoic acid is used industrially in the manufacture of polymers and glues and also in the food industry as an acidity regulator.

 It can be synthesized in the laboratory by the reaction of ethanol with excess sodium dichromate(VI) solution, acidified with concentrated sulfuric acid.  Ethanol is placed in a suitable flask along with some anti-bumping beads.  The concentrated sulfuric acid is then added a drop at a time.  The sodium dichromate(VI) solution is then added a drop at a time causing the mixture to boil spontaneously.  When the addition of the sodium dichromate(VI) solution is complete, the mixture is heated under reflux for approximately 15 minutes.  The ethanoic acid formed can then be separated from the reaction mixture.

Ethanoic acid can be produced industrially by the CativaTM process.  Methanol, which can be obtained from wood, is reacted with carbon monoxide in the presence of an iridium catalyst.

(a)  (i)   Balance the half-equation for the reduction of dichromate(VI) ions.
(1)

(ii)   The half-equation for the oxidation of ethanol is

Use this and your answer to (a)(i) to write a full equation for the overall reaction between acidified dichromate(VI) ions and ethanol.  State symbols are not required.
(2)

(b)  (i)   Why are the concentrated sulfuric acid and sodium dichromate(VI) added a drop at a time in the laboratory process?
(1)
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(ii)   Draw a labelled diagram of the apparatus that could be used to heat the mixture under reflux.
(3)

(iii)   What colour would the mixture be after it was heated under reflux?
(1)
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(c)  A solution containing both water and ethanoic acid is produced by distillation of the final reaction mixture.
(i)   Explain why the other products and any excess reactants are left behind in the distillation flask.
(1)
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 (ii)   Suggest a method to separate pure ethanoic acid, boiling temperature 118°C, from the water.
(1)
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 (d)  (i)   In the CativaTMprocess what effect, if any, would increasing the pressure have on the yield of ethanoic acid?  Justify your answer.
(2)
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 (ii)   Suggest TWO reasons why it might be difficult, or undesirable, to produce ethanoic acid in industry by scaling up the laboratory process.
(2)
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*(e)  An alternative industrial process for the production of ethanoic acid is the oxidation of butane using a transition metal catalyst at 150°C and 55−60 atm.

Evaluate the 'greenness' and sustainability of the two industrial processes.

 Suggest TWO additional pieces of information that would help you make a more informed decision.
(6)
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(Total for question = 20 marks)

Q2.
In the reaction between Ag+(aq) ions and Fe2+(aq) ions, what would be the effect of increasing the concentration of Ag+(aq) ions?

   A     Rate of reaction increases, yield of Fe3+(aq) stays the same.
   B     Rate of reaction increases, yield of Fe3+(aq) decreases.
   C     Rate of reaction decreases, yield of Fe3+(aq) stays the same.
   D     Rate of reaction increases, yield of Fe3+(aq) increases.
(Total for question = 1 mark)

Unit 2 - Jun 2009
Q3.
In the equilibrium below, what effect would the changes described have on the system?

(a)  Increase in temperature
(1)
   A     increase rate, decrease yield
   B     increase rate, increase yield
   C     decrease rate, decrease yield
   D     decrease rate, increase yield

(b)  Decrease in pressure
(1)
   A     increase rate, decrease yield
   B     increase rate, increase yield
   C     decrease rate, decrease yield
   D     decrease rate, increase yield
(Total for question = 2 marks)

Unit 2 - Jan 2010
Q4.
Magnesium nitrate, Mg(NO3)2, decomposes when it is heated. One product is the brown gas, nitrogen dioxide.
(a)  (i)  Write an equation for this reaction. State symbols are not required.
(2)

(ii)  Calcium nitrate decomposes in a similar way to magnesium nitrate, but at a higher temperature.
Explain why the two nitrates have different stability to heat.
(2)
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 (b)  Sodium nitrate decomposes to give different products to magnesium nitrate. Write an equation for the decomposition of sodium nitrate. State symbols are not required.
(1)

(c)  A student suggested that the structure of the nitrate ion, NO3−, is

Scientists have found that the bonds between nitrogen and oxygen in the nitrate ion are all the same length. Is the student's suggestion supported by this evidence? Explain your answer.
(1)

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(d)  Nitrogen dioxide gas can dimerize to dinitrogen tetroxide, N2O4, a very pale yellow gas, as shown in the equation below.

(i)  What would you see when an equilibrium mixture of these gases is warmed gently?  Explain your answer.
(2)
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 (ii)  Explain why an equilibrium mixture of these gases eventually becomes paler in colour when the pressure on it is increased.
(2)
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 (e)  Two Maxwell-Boltzmann distributions showing the energy of particles in a gas at different temperatures, T1 and T2, are shown below. The activation energy for the reaction is labelled EA.

Use the distributions to explain why gases react faster when the temperature is increased.
(2)
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 (Total for question = 12 marks)

Unit 2 - Jun 2010
Q5.
(a)  In the catalytic converter of a car engine's exhaust system, the following reaction occurs.

The temperature in a catalytic converter is high.
(i) State the effect, if any, on the position of equilibrium if the temperature is lowered. Give a reason for your answer.
(2)
Effect
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Reason
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 (ii) The gases from the engine are not cooled before entering the converter. Explain why this is so.
(2)
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 (iii) State the effect, if any, on the position of equilibrium if the pressure on the reacting gases is increased. Give a reason for your answer.
(2)
Effect
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Reason
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(b)  Nitrogen monoxide, NO, is formed when nitrate ions, , in acidic solution are reduced by silver metal.
(i)  Calculate the oxidation number of nitrogen in NO and in .
(2)
In NO
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In 
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(ii)  Balance the half-equation for the reduction of nitrate ions, , in acidic solution.
(1)

(iii)  Write the half-equation for the oxidation of silver metal, Ag, to silver ions, Ag+.
(1)

(iii)  (iv) Hence deduce the full ionic equation for the reaction between silver metal and nitrate ions in acidic solution. State symbols are not required.
(2)

(Total for question = 12 marks)

Q6.
For the reversible reaction

which of the following could represent the change in the concentrations of X and Y with
 time, starting with a mixture of both X and Y? Equilibrium is reached at time teqm.

(Total for question = 1 mark)

Unit 2 - Jan 2011
Q7.
This question is about some reactions which can be used in the manufacture of hydrogen.

       Reaction 1 uses two naturally occurring chemicals, water and natural gas. Steam is reacted with methane to form carbon monoxide and hydrogen in an equilibrium reaction.

       Reaction 1    CH4(g) + H2O(g)  CO(g) + 3H2(g)       ΔH = +210 kJ mol−1

       In reaction 2, carbon monoxide and steam are passed over copper at high temperature. This forms carbon dioxide and hydrogen.

       Reaction 2    CO(g) + H2O(g)  CO(g) + H2(g)

       The carbon dioxide formed is removed by passing it through potassium carbonate solution in reaction 3.

       Reaction 3    K2CO3(aq) + CO2(g) + H2O(l) → 2KHCO3(aq)

       The potassium carbonate is regenerated by heating the potassium hydrogencarbonate solution in reaction 4. The carbon dioxide gas produced is released into the atmosphere.

       Reaction 4    2KHCO3(aq) → K2CO3(aq) + CO2(g) + H2O(l)
(a) For each of the first three reactions, state the initial and final oxidation numbers of any elements that change their oxidation numbers. Hence decide which are redox reactions.
(5)
Reaction 1
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Reaction 2
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Reaction 3
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*(b) (i) Discuss, with reasons, the conditions of temperature and pressure that would favour the production of hydrogen in reaction 1. You should consider the effect of the conditions on both yield and rate.
(7)
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 (ii) Excess steam is used in reaction 1. State why an excess of a reagent is used and suggest why steam, rather than methane, is chosen.
(2)
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 (c) Copper is a catalyst in reaction 2. Explain how a catalyst increases the rate of a reaction.
(2)
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 (d) (i) State one economic advantage of reaction 4.
(1)
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*(ii) Reaction 4 contributes to global warming. Identify the substance formed in this reaction which is likely to be responsible and explain the processes that lead to an increase in global temperatures.

      Suggest two effects an increase in global temperatures might have on the environment.
(4)
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(Total for question = 21 marks)

Unit 2 - Jun 2011
Q8.
This question is about nitrogen monoxide, NO, which can be described both as a friend and a foe.

(a) (i)   What is meant by the term free radical?
(1)
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 (a) (ii)  Suggest a dot and cross diagram for nitrogen monoxide, showing outer shell electrons only, remembering that it is a free radical.
(2)

(b) (i)   Part of the unbalanced equation for the preparation of nitrogen monoxide from nitric acid is shown below.

Identify the elements which are oxidized and reduced and give their oxidation numbers.
(3)
Element oxidized .........................

Oxidation number initial .........................    final .........................

Element reduced .........................

Oxidation number initial .........................    final .........................

     (ii)  Complete and balance the equation for the reaction between silver and nitric acid.
(2)

(c) The reaction between nitrogen and oxygen to form nitrogen monoxide reaches equilibrium.

     (i)   Explain why the yield of nitrogen monoxide is increased when the temperature is increased.
(1)
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  ∗(ii)  State and explain the effect, if any, on the yield of nitrogen monoxide when the pressure is increased.
(2)
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   (iii)  State and explain how the rate of the reaction is affected by an increase in pressure.
(2)
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∗ (d) (i)  Explain why a jet aeroplane in flight causes much more damage to the ozone layer than cars carrying the same number of passengers at sea level. You should assume that the nitrogen monoxide outputs for both methods of conveying the passengers are the same.
(2)
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   (ii)  The reactions of chlorine free radicals with ozone may be represented by the following equations.

Write corresponding equations for the reactions of the free radical nitrogen monoxide with ozone. Combine your two equations to show the overall reaction.
Use these equations to explain why a small quantity of nitrogen monoxide can have a continuing effect on the ozone layer.
(5)
Equations

Explanation
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 (Total for Question = 20 marks)

Unit 2 - Jan 2012
Q9.
Brown nitrogen dioxide, NO2, exists in equilibrium with colourless dinitrogen tetroxide, N2O4.

(a) The pressure is increased. When equilibrium is restored, the appearance of the mixture of gases will be
(1)
   A  colourless.
   B  unchanged. 
   C  paler brown. 
   D  darker brown.

(b) The temperature is increased. When equilibrium is restored, the appearance of the mixture of gases will be
(1)
   A  colourless.
   B  unchanged. 
   C  paler brown. 
   D  darker brown.
(Total for question = 2 marks)

Unit 2 - Jun 2012
Q10.
(a)  Ozone, O3, is formed when oxygen is exposed to ultraviolet (UV) radiation or to an electric discharge. Ozone is a blue gas whereas oxygen is colourless. When the two gases are mixed, an equilibrium is established as shown in the following equation.
3O2(g)    2O3(g)      ΔH = +143 kJ mol−1
(i) When the temperature of the pale blue equilibrium mixture is increased at constant volume, the colour darkens. Explain this observation in terms of the changes to the equilibrium.
(2)
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 (ii) State and explain what you would see if the pressure of the system at equilibrium were increased.
(2)
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 (iii) A small amount of oxygen gas containing the isotope 18O is added to the equilibrium mixture. After a few hours, ozone containing 18O is detected. Given that the equilibrium position is not affected, explain this observation.
(1)
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 (b) The concentration of ozone in the atmosphere may be determined by bubbling air through a solution of acidified potassium iodide. Iodine is formed in solution, the concentration of which may be determined by titration with a solution of sodium thiosulfate of known concentration. The equations for the reactions are
O3 + 2I− + 2H+  →  O2 + H2O + I2     Equation 1
 I2+ 2S2O32−   →  2I− + S4O62−       Equation 2
In an experiment to determine the concentration of ozone in a sample of air, 100 m3 of air was bubbled through 100 cm3 of a solution containing an excess of acidified potassium iodide. The resulting solution was titrated against a solution of sodium thiosulfate of concentration 0.0155 mol dm−3. The volume of sodium thiosulfate solution required for complete reaction was 25.50 cm3.
(i) Calculate the number of moles of sodium thiosulfate that react.
(1)

(ii) Calculate the number of moles of iodine that reacted with the sodium thiosulfate.
(2)

(iii) Use equation 1 to deduce the number of moles of ozone that reacted with the acidified potassium iodide.
(1)

(iv) Calculate the volume of ozone, measured in m3, present in the original sample of air. Assume that all gas volumes were measured at room temperature and pressure and that the molar volume of any gas under these conditions is 0.024 m3 mol−1.
(1)

(v) Calculate the concentration of ozone in the sample of air in units of parts per million (ppm) by volume.
(1)

(vi) A student suggested that the 100 cm3 of acidified potassium iodide should be divided into four portions before the titration. Explain how this change increases the reliability and decreases the accuracy of the experiment.
(3)
Increases reliability
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Decreases accuracy
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 (c) Give the oxidation numbers of oxygen in equation 1, shown below. Hence state the role of ozone in this reaction.
(3)
O3 + 2I− + 2H+ → O2 + H2O + I2
Oxidation number of O
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Role of ozone
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(d) Ozone is used as an alternative to chlorine to disinfect flood damaged buildings, to remove residual smoke odours from fires and in the treatment of drinking water. Suggest one advantage of using ozone rather than chlorine, given that chlorine and ozone are both toxic.
(1)
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 (Total for question = 18 marks)

Unit 2 - Jun 2013
Q11.
Which of the following cannot alter the position of a chemical equilibrium?

   A    Increasing the amount of catalyst
   B    Increasing the reactant concentration
   C    Increasing the temperature
   D    Increasing the total pressure
(Total for Question = 1 mark)

Q12.
(a)  Sea water is a source of chemicals. The most abundant chemical dissolved in sea water is sodium chloride. Compounds of magnesium and bromine are also present. Magnesium occurs at 1300 parts per million (ppm) and bromine at 60 ppm by mass.

The solution left after crystallizing sodium chloride from sea water is even richer in bromine, and contains around 2.2 g dm−3 of bromine.

Bromine is extracted from this solution by passing in chlorine gas. The mixture is acidified to prevent hydrolysis of bromine by the reaction
Br2(aq) + H2O(l)  2H+(aq) + Br−(aq) + BrO−(aq)

The bromine can be separated by heating the solution to collect bromine vapour which is then condensed, or by blowing air through the solution.

(i)  Show by calculation that a solution containing 2.2 g dm−3 of bromine is richer in bromine than one containing 60 ppm.
[Assume that the mass of 1 dm3 of the bromine solution is 1000 g]
(1)

(ii)  Write an ionic equation, including state symbols, for the reaction in which chlorine gas reacts with bromide ions in solution to produce bromine.
(2)

(iii)  What would be observed when the reaction in (ii) occurs?
(1)
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(iv)  Explain why the addition of an acid, such as hydrochloric acid, prevents hydrolysis of bromine.
(2)
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(v)  Assuming the hydrolysis of bromine is endothermic, explain how an increase in temperature would affect the equilibrium position for the hydrolysis of bromine.
(2)
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(vi)  Use your knowledge of activation energy to explain why an increase in temperature increases the rate of hydrolysis of bromine.
(1)
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(vii) Use the equation for the hydrolysis of bromine to show that it is a disproportionation reaction.
Br2(aq) + H2O(l)  2H+(aq) + Br−(aq) + BrO−(aq)
(2)
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(b) At the surface of the sea, there is a dynamic equilibrium between carbon dioxide gas in air and dissolved carbon dioxide in the surface sea water.
CO2(g) CO2(aq)
(i)  State two features of a system which has reached dynamic equilibrium.
(2)
1.  .................................................................................................................................................... 
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2.  .................................................................................................................................................... 
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*(ii)  Carbon dioxide dissolves more easily in seawater than in pure water because seawater contains carbonate ions, CO32−(aq), and the following reaction occurs.

CO2(aq) + H2O(l) + CO32−(aq)  2HCO3 −(aq)

Explain how an increase in concentration of carbonate ions in sea water affects the amount of carbon dioxide gas in the atmosphere.
(2)
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(iii)  Carbon dioxide and water vapour both contain polar bonds.
What effect does infrared radiation have on the bonds in these molecules?
(1)
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*(iv)  Outline the mechanism by which molecules such as carbon dioxide and water cause global warming.
(2)
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*(v) Without water vapour in the atmosphere, the earth would be many degrees colder than it is at present. Why are many climate change scientists more concerned about warming due to carbon dioxide in the atmosphere, than warming due to the presence of water vapour? Refer to the difference between anthropogenic climate change and natural climate change in your answer.
(4)
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(Total for Question = 22 marks)

Q13.
What is the oxidation number of chlorine in Cl2O7?
   A    −1
   B    +1
   C    −7
   D    +7
(Total for Question = 1 mark)

Unit 2 - Jun 2014
Q14.
A solution of iodine in aqueous potassium iodide is brown. The following equilibrium exists in this solution.

What would be the effect, if any, on the colour of the solution if five drops of dilute sodium hydroxide solution were added to 5 cm3 of the iodine solution?
   A    No visible change.
   B    The colour gets lighter.
   C    The mixture turns colourless.
   D    The mixture goes a darker colour.
(Total for question = 1 mark)

Q15.
The following system was allowed to reach equilibrium at 300 °C.

(a)  What would you see if the equilibrium mixture was cooled to 250 °C?
(1)
   A    No visible change.
   B    The colour gets lighter.
   C    The mixture turns colourless.
   D    The mixture goes a darker purple.

(b)  The equilibrium mixture at 300 °C was compressed in a gas syringe to occupy a smaller volume. What would be seen immediately?
(1)
   A    No visible change.
   B    The colour gets lighter.
   C    The mixture turns colourless.
   D    The mixture goes a darker purple.
(Total for question = 2 marks)

Q16.
The indicator methyl orange is a weak acid and may be represented by the formula HA(aq). The equation for its dissociation is shown below.

Under certain conditions, at equilibrium, a solution of HA has a yellow colour. On addition of a small volume of dilute sodium hydroxide, the colour of this solution would
   A    change from yellow to red.
   B    change from yellow to orange.
   C    change from yellow to orange and then to red.
   D    not change.
(Total for question = 1 mark)

Q17.
When sodium is added to ethanol, which of the following observations would be made?
   A    Colour change of orange to green
   B    Effervescence
   C    Yellow flame
   D    No change
(Total for question = 1 mark)