Unit 2 – Redox
Unit 2 - Jan 2009
Q1.
This question is about boron and nitrogen compounds.
(a)  Draw and name the shape of a boron trifluoride, BF3, molecule.  Suggest the FBF bond angle.
(3)

Name of shape
 ............................................................................................................................................. 
FBF bond angle
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(b)  Ammonia has the formula NH3.  Its HNH bond angle is less than the FBF bond angle in boron trifluoride.
(i)   Estimate the HNH bond angle in NH3.
(1)
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(ii)   Explain why the HNH bond angle is less than that for FBF.
(1)
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 (iii)   Name the strongest intermolecular force between BF3 molecules.
(1)
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(iv)   Name the strongest intermolecular force between NH3 molecules.
(1)
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(c)  Ammonia will react with oxygen in the presence of a platinum catalyst at 750°C forming water and nitrogen(II) oxide, NO.
(i)   What is the oxidation number of nitrogen in ammonia, NH3?
(1)
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(ii)   The diagram below shows the distribution of molecular energies in the reaction at 750°C.  On the same diagram, draw a curve to show the distribution at 500°C and explain what effect this change in temperature would have on the rate of the reaction.
(3)

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*(d)  Explain how a catalyst speeds up the rate of a reaction.
(3)
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 (Total for question = 14 marks)

Q2.
Which one of these reactions is not a disproportionation reaction?
   A     2H2O2(aq) → O2(g) + 2H2O(l)
   B     S2O32−(aq) + 2H+(aq) → SO2(g) + S(s) + H2O(l)
   C     Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq)
   D     2Cu+(aq)→ Cu(s) + Cu2+(aq)
(Total for question = 1 mark)

Q3.
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)

Q4.
10.0 cm3 of a solution containing Cu2+(aq) ions was added to excess potassium iodide solution and the following reaction occurred.

(a)  What happens to the Cu2+(aq) during this reaction?  Justify your answer.
(2)
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 (b)  All of the mixture containing iodine was titrated using sodium thiosulfate solution of concentration 0.200 mol dm−3.  The volume of sodium thiosulfate solution added at the end-point was 12.75 cm3.

       The equation for the reaction is

(i)   The end-point is shown most effectively using an indicator.  State a suitable indicator and the colour change you would expect to see at the end-point.
(2)
Indicator
 ............................................................................................................................................. 
Colour change at end-point
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(ii)   Calculate the number of moles of iodine in the solution.
(2)

*(iii)   Use your answer from (ii), and the equation for the reaction between Cu2+(aq) and I−(aq), to calculate the concentration of the Cu2+(aq) in the original sample of solution.

           Give your answer to three significant figures and justify why this is an
           appropriate level of accuracy.
(3)

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(iv)   The whole of the solution containing iodine was used in one titration.  Explain how this affects the reliability of your answer to (iii).
(1)
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 (Total for question = 10 marks)

Q5.
What is the oxidation number of oxygen in dioxygen difluoride, O2F2?
   A     −1
   B     −2
   C     +1
   D     +2
(Total for question = 1 mark)

Unit 2 - Jun 2009
Q6.
What is the oxidation number of chlorine in the ClO3−ion?
   A     −1
   B     +4
   C     +5
   D     +6
(Total for question = 1 mark)

Q7.
Iodine can react with sodium hydroxide solution to form NaIO3(aq), according to the equation below.

Which of the statements about the reaction is false?
   A     The oxidation number of some iodine atoms goes up.
   B     At high temperatures NaIO(aq) also forms.
   C     Sodium ions are spectator ions.
   D     The oxidation number of some iodine atoms goes down.
(Total for question = 1 mark)

Q8.
Which of these reactions is not a redox reaction?
   A     Mg(NO3)2(s) → MgO(s) + 2NO2(g) + ½O2(g)
   B     HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
   C     Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
   D     Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq)
(Total for question = 1 mark)

Unit 2 - Jan 2010
Q9.
This question is about some reactions of halogens and halide ions.
(a)  (i)  When chlorine is added to a solution containing bromide or iodide ions, a colour change occurs. What solvent would you add to the mixture to confirm the identity of the halogen produced?
(1)
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(ii)  Give the result for the test with this solvent in a reaction in which bromine is produced.
(1)
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(b)  (i)  Solid potassium bromide and potassium iodide can be distinguished by their reactions with concentrated sulfuric acid.
Potassium bromide reacts with concentrated sulfuric acid initially to produce hydrogen bromide. This reacts further, as shown below, to produce a sharp smelling gas and a brown fuming liquid.

Show, by use of oxidation numbers for sulfur, that the sulfuric acid has been
 reduced.
(2)
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 (ii)  State TWO observations, which would differ from those with potassium bromide, when potassium iodide reacts with concentrated sulfuric acid.
(2)
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(iii)  One product of the reaction with potassium iodide is hydrogen sulfide, H2S. How does this show that iodide ions are more powerful reducing agents than bromide ions?
(1)
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 (c)  In areas where the natural concentration of fluoride ions in rocks is low, some water authorities add fluoride to the water supply to improve the dental health of children. An alternative would be to supply free fluoride tablets.
       Give ONE reason why it could be considered more ethical to supply free fluoride tablets rather than to add fluoride compounds to the water supply.
(1)
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(Total for question = 8 marks)
Q10.
Chemical reactions may involve
A     oxidation
B     reduction
C     no change in oxidation number
D     disproportionation

Which of the terms above best describes what happens to the chlorine in the following reactions?

(a) Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq)
(1)
   A
   B
   C
   D

(b) Cl2(g) + 2Na(s) → 2NaCl(s)
(1)
   A
   B
   C
   D

(c) NaCl(s) + H2SO4(l) → HCl(g) + NaHSO4(s)
(1)
   A
   B
   C
   D
(Total for question = 3 marks)
Unit 2 - Jun 2010
Q11.
(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
 ............................................................................................................................................. 
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)

Unit 2 - Jan 2011
Q12.

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
Q13.
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
Q14.
Chlorine disinfectants are essentially solutions containing chlorine molecules and chlorate(I) ions in an equilibrium summarised by the equation

The chlorine content of a disinfectant was determined using the following procedure.
1. 10.0 cm3 of the disinfectant was transferred to a 250 cm3 volumetric flask.
2. Approximately 20 cm3 of nitric acid and 20 cm3 potassium iodide solution (both in excess) were added to the volumetric flask.
3. The solution in the volumetric flask was made up to the mark with distilled water and then mixed thoroughly.
4. 10.0 cm3 portions of the solution in the volumetric flask were titrated against a solution of sodium thiosulfate, concentration 0.109 mol dm-3. Starch solution was added near the end-point of the titration and the mean (average) titre was 27.35 cm3.
The equations for the reactions involved in this procedure are

(a)  (i) Calculate the number of moles of sodium thiosulfate used in the titration.
(2)

(ii) Calculate the number of moles of iodine, I2, that reacted in the titration (step 4).
(1)

(iii) Hence state the number of moles of chlorine, Cl2, in 10.0 cm3 of the solution in the volumetric flask.
(1)

(iv) Calculate the concentration of chlorine, in mol dm-3, in the original disinfectant.
(2)

(b)  Equation 1 is an example of a disproportionation reaction. Define the term 'disproportionation' and explain, by considering the relevant oxidation numbers, why this reaction is a disproportionation.
(3)
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 (c)  State the colours of the titration solution just before the starch solution is added, after the starch solution is added and the colour change at the end-point of the reaction.
(2)
Colour just before adding the starch
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Colour after adding the starch
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Colour at the end-point
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(Total for question = 11 marks)

Q15.
What is the oxidation number of oxygen in OF2?
   A   -2
   B   -1
   C   +1
   D   +2
(Total for question = 1 mark)

Q16.
In which of the following reactions is sulfuric(IV) acid, H2SO3, acting as an oxidizing agent?
   A   2NaOH + H2SO3 → Na2SO3 + 2H2O
   B   2FeCl3 + H2SO3 + H2O → 2FeCl2 + H2SO4 + 2HCl
   C   2H2S + H2SO3 → 3H2O + 3S
   D   H2SO3 → H2O + SO2
(Total for question = 1 mark)

Unit 2 - Jun 2012
Q17.
      Which of the following is not a disproportionation reaction?
   A      Cl2 + 2OH− → Cl− + ClO− + H2O
   B      Cu2O + H2SO4 → CuSO4 + Cu + H2O
   C       3IO− → 2I− + IO3+
   D      Cu + 4HNO3 → Cu(NO3)2 + 2H2O + 2NO2
(Total for question = 1 mark)

Q18.
      The oxidation number of sulfur in sodium hydrogensulfide, NaHS, is
   A      −2
   B      −1
   C      +1
   D      +2
(Total for question = 1 mark)

Q19.
(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
Q20.
(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.  .................................................................................................................................................... 
    ....................................................................................................................................................
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)

Q21.
CO(g) + 2H2(g)  CH3OH(g)       ΔH = −91 kJ mol−1

The conditions which would produce the greatest yield of methanol are

   A    high pressure and high temperature.
   B    high pressure and low temperature.
   C    low pressure and low temperature.
   D    low pressure and high temperature.
(Total for Question = 1 mark)
Q22.
(a)  (i)  An alkaline solution is produced when barium reacts with cold water. Write the equation for this reaction, including all state symbols.
(2)

(ii)  The reaction in (a)(i) is a redox reaction. State the initial and final oxidation number of any element that changes its oxidation number.
(2)
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 (b)  Dilute hydrochloric acid is added to the solution produced in (a)(i). Write the equation for the reaction which occurs. State symbols are not required.
(1)

(c)  Dilute sulfuric acid is added to another sample of the solution produced in (a)(i). How would the appearance of the resulting mixture differ from the mixture produced in (b)? Explain this difference.
(2)
Appearance
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Explanation
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(d)  (i)  Two white powders are known to be barium carbonate and magnesium carbonate.
How could you distinguish between the two powders by heating them? [No practical details are required.]
Include the equation for the action of heat on one of these carbonates. State symbols are not required.
(2)
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Equation:

(ii)  Suggest another test, other than heating or the use of an acid, which could be used to distinguish between magnesium carbonate and barium carbonate. State the results for both compounds.
(2)
Test
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Result with magnesium carbonate
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Result with barium carbonate
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(Total for Question = 11 marks)

Q23.
The thermite reaction, shown below, is a useful industrial process.
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s)
The iron in this reaction undergoes
   A    disproportionation.
   B    oxidation.
   C    redox.
   D    reduction.
(Total for Question = 1 mark)

Unit 2 - Jun 2014
Q24.
This is a question about Group 2 elements and their compounds.
*(a) Explain why the first ionization energy of calcium (590 kJ mol−1) is greater than that of strontium (550 kJ mol−1). 
(2)
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(b)  (i)  Describe how you would carry out a flame test on a sample of a Group 2 metal salt.
(2)
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(ii)  What result of the flame test would confirm the presence of a barium salt?
(1)
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*(iii)  Explain, in terms of electronic transitions, how the result of the flame test arises.
(3)
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(c)  Barium reacts with water to form a clear, colourless solution.
(i)  Give the name or formula of the barium compound formed.
(1)
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(ii)  State another observation that would be made when barium reacts with water.
(1)
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(d)  Barium reacts with chlorine gas to form barium chloride as shown in the equation below.
Ba(s) + Cl2(g) → BaCl2(s)
(i)  Use the changes in oxidation numbers to show that this is a redox reaction.
(2)
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(ii)  Write the ionic equation for the reaction between barium chloride solution and dilute sulfuric acid. Include state symbols in the equation.
(2)

(iii)  The reaction in (d)(ii) is used to test for sulfate ions.
Why is dilute hydrochloric acid added with the barium chloride solution in this test?
(1)
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(e)  Magnesium carbonate, MgCO3, readily reacts with hydrochloric acid.
(i)  Write the equation for this reaction. State symbols are not required.
(1)

*(ii)  The rate of the reaction between powdered magnesium carbonate and dilute hydrochloric acid was monitored using the experimental apparatus shown below.

State two factors that would decrease the rate of this reaction, other than by changing the reaction temperature.
Explain how these two factors decrease the reaction rate.
(4)
Factor 1
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Explanation 1
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Factor 2
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Explanation 2
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(f)  Suggest why pressure has little or no effect on the rate of the reaction of magnesium oxide and hydrochloric acid, the equation for which is given below.
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
(1)
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(Total for question = 21 marks)

Q25.
Potassium dichromate(VI), K2Cr2O7, can be used to accurately determine the concentration of other chemicals, such as sodium thiosulfate, Na2S2O3.
(a) The dichromate(VI) ion has two chromium atoms sharing one oxygen to give two tetrahedral units. Each chromium atom uses six electrons in bonding and expands its outer shell to accommodate a total of 12 electrons.
Complete the dot and cross diagram for this ion below. Only show outer shell electrons.
Use x for chromium electrons and • for oxygen electrons. Use the symbol * to represent the extra electrons which give the ion its charge.
(3)

(b)  Four chemistry students were given a solution of sodium thiosulfate with a concentration of approximately 0.1 mol dm−3 and asked to determine its exact concentration using potassium dichromate(VI) solution. They were each given separate tasks to carry out as described below.
(i)  The first student was given the task of making up a potassium dichromate(VI) solution. A mass of 14.71 g of K2Cr2O7 was weighed out, dissolved in deionized water, the volume made up to 250 cm3 in a volumetric flask and the mixture shaken.
Calculate the concentration of this potassium dichromate(VI) solution, in mol dm−3.
Use the Periodic Table as a source of data.
(2)

(ii)  The second student was asked to calculate the mass of potassium iodide that would be required to add to 0.00250 mol of potassium dichromate(VI) to ensure complete reaction. The equation for the reaction is
Cr2O72−(aq) + 14H+(aq) + 6I−(aq) → 2Cr3+(aq) + 3I2(aq) + 7H2O(l)
Calculate the minimum mass of potassium iodide, KI, required and hence suggest a suitable mass to use if the potassium iodide is to be in excess.
You must show your working and your mass should be reasonable.
(2)

Minimum mass required ........................................................... g
Suitable mass to use ........................................................... g

(iii)  The third student was given the following equation.
I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)
This student was asked to estimate the titration reading at the end-point if a solution that contained 0.00260 mol of iodine was in the conical flask and the concentration of sodium thiosulfate was about 0.16 mol dm−3.
Calculate the volume of sodium thiosulfate solution, in cm3, that would have been added at the end-point of the titration.
(2)

*(iv)  The fourth student was given the following experimental information.
0.1 g of potassium dichromate(VI) was dissolved in a total volume of 25.00 cm3. An excess of potassium iodide and acid was added and then used in a titration with the sodium thiosulfate solution of concentration approximately 0.1 mol dm−3. The titre was 25.15 cm3.
The student suggested that the greatest uncertainty in the result arose from the mass that was measured and that the procedure was unreliable.
Explain why these views were justified. No calculation is required.
(2)
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(c)  (i)  Acidified sodium dichromate(VI) solution is commonly used for the oxidation of alcohols. It is important not to use hydrochloric acid in this reagent mixture because the chloride ions are oxidized to chlorine.
Write the ionic half-equation for the oxidation of chloride ions. State symbols are not required.
(1)

(ii)  Fumes of hydrogen chloride gas can be identified by bringing the fumes into contact with another gas, X. Identify gas X and state the observation you would make.
(2)
Gas X
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Observation
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(d)  Potassium bromide can be distinguished from potassium chloride by its reaction with silver nitrate solution, followed by the addition of aqueous ammonia solution.
State what you would see on the addition of silver nitrate solution to potassium bromide solution.
How could both dilute and concentrated ammonia be used to confirm that silver bromide is formed, rather than silver chloride?
(3)
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(Total for question = 17 marks)

Q26.
What is the oxidation number of phosphorus in P4O6?
   A    +3
   B    +4
   C    +5
   D    +6
(Total for question = 1 mark)