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Date May 2011 Marks available 1 Reference code 11M.3.sl.TZ1.D2
Level SL Paper 3 Time zone TZ1
Command term Suggest Question number D2 Adapted from N/A

Question

Two substances commonly used in antacid tablets are magnesium hydroxide and aluminium hydroxide.

State an equation to represent a neutralization reaction with one of the above antacids.

[1]
a.

State and explain whether 0.1 mol of magnesium hydroxide is more effective or less effective than 0.1 mol of aluminium hydroxide.

[1]
b.

Suggest why compounds such as sodium hydroxide or potassium hydroxide cannot be used as an antacid.

[1]
c.

Markscheme

\({\text{Al(OH}}{{\text{)}}_3} + {\text{3HCl}} \to {\text{AlC}}{{\text{l}}_3} + {\text{3}}{{\text{H}}_2}{\text{O}}/{\text{Mg(OH}}{{\text{)}}_2} + {\text{2HCl}} \to {\text{MgC}}{{\text{l}}_2} + {\text{2}}{{\text{H}}_2}{\text{O}}\);

Accept ionic equations.

a.

less effective and (magnesium hydroxide) 2/0.2 mol \({\text{O}}{{\text{H}}^ - }\) ions available as compared to (aluminium hydroxide) 3/0.3 mol OHions for neutralization / neutralizes \({\text{2}}{{\text{H}}^ + }\)/0.2 mol acid as compared to \({\text{3}}{{\text{H}}^ + }\)/0.3 mol acid; [1]

Do not accept aluminium hydroxide can neutralize more acid.

b.

strong base / corrosive / harmful to the body;

c.

Examiners report

Whilst quite a few candidates could write one of the required balanced equations a surprising large number could not succeed in this simple task. Answers to the second part of this question often lacked the stoichiometric rigour required and the reasons for not using strong alkalis provoked an amazing range of responses, mostly incorrect and many exposing a worrying lack of basic chemical knowledge. Only a minority of the students could correctly identify the function of alginates and dimethicone in antacid preparations.

a.

Whilst quite a few candidates could write one of the required balanced equations a surprising large number could not succeed in this simple task. Answers to the second part of this question often lacked the stoichiometric rigour required and the reasons for not using strong alkalis provoked an amazing range of responses, mostly incorrect and many exposing a worrying lack of basic chemical knowledge. Only a minority of the students could correctly identify the function of alginates and dimethicone in antacid preparations.

b.

Whilst quite a few candidates could write one of the required balanced equations a surprising large number could not succeed in this simple task. Answers to the second part of this question often lacked the stoichiometric rigour required and the reasons for not using strong alkalis provoked an amazing range of responses, mostly incorrect and many exposing a worrying lack of basic chemical knowledge. Only a minority of the students could correctly identify the function of alginates and dimethicone in antacid preparations.

c.

Syllabus sections

Options » D: Medicinal chemistry » D.4 pH regulation of the stomach
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