Answers:
LEVEL: AS
CHEMISTRY – Equilibria
8. Acid-base reactions, pH titration curves and redox reactions (15 minutes)
(a) Explain the terms strong and weak when applied to aqueous acids.
(2 marks)
When dissolved in water, strong acids like hydrochloric acid are completely ionized but weak acids like
ethanoic acid are only partially ionized.
(b) When aqueous sodium hydroxide of concentration 0.10 mol dm−3 is added to 20.0 cm3
aqueous hydrochloric acid of the same concentration, the following curve is obtained.
(i) Write a balanced equation for the reaction between hydrochloric acid and aqueous sodium
hydroxide.
(1 mark)
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
(ii) Name an indicator suitable for titrating hydrochloric acid with aqueous sodium hydroxide.
Give a reason for your answer.
(2 marks)
Methyl red, because its mid-point colour occurs at pH = 7 which corresponds to the equivalence point
and to where the curve is steepest.
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Answers:
LEVEL: AS
CHEMISTRY – Equilibria
(iii) Sketch, on the axes above, the curve that would be obtained if aqueous ethanoic acid of
concentration 0.10 mol dm–3 and pH = 2.8. were used instead of hydrochloric acid.
(2 marks)
(c) Explain why the reactions of hydrochloric and nitric acids with aqueous sodium and
potassium hydroxides should give the same value for the standard molar enthalpy change of
neutralization.
(2 marks)
Both acids and both alkalis are strong and completely ionized in water so the four possible reactions are
the neutralization represented by the equation
H+(aq) + OH−(aq) → H2O(l)
(d) Describe what is observed when dilute hydrochloric acid is added to the following.
(i) Calcium carbonate.
(1 mark)
White solid dissolves and a colourless gas is evolved.
(ii) Aqueous silver nitrate.
(1 mark)
White granular precipitate forms and darkens in sunlight.
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Answers:
LEVEL: AS
CHEMISTRY – Equilibria
(e) When concentrated hydrochloric acid is heated with manganese(IV) oxide, chlorine gas is
formed.
MnO2(s) + 4HCl(aq)
→
MnCl2(aq) + Cl2(g) + 2H2O(l)
(i) Write down the oxidation states of manganese in this reaction.
Initial oxidation state:
(1 mark)
+4
Final oxidation state:
(1 mark)
+2
(ii) Write an ionic equation for this reaction.
(1 mark)
MnO2(s) + 4H+(aq) + 2Cl−(aq)
→
Mn2+(aq) + Cl2(g) + 2H2O(l)
(iii) State why this reaction should be carried out in a fume cupboard.
(1 mark)
Chlorine gas is poisonous
(Total marks 15)
ANSWERS
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