The pH of gastric juice is 1.5. Identify the compound responsible for its acidity and state whether it is a strong or weak acid.

Compound:

Strong or weak acid:

Antacid tablets are often taken for an upset stomach. Identify the reaction involved in this treatment and state the general ionic equation for this reaction type.

Type of reaction:

Ionic equation:

One active ingredient in a commercial brand of antacid tablets is a complex of aluminium hydroxide and sodium carbonate, dihydroxyaluminium sodium carbonate, \({\text{Al(OH}}{{\text{)}}_{\text{2}}}{\text{NaC}}{{\text{O}}_{\text{3}}}{\text{(s)}}\).

Deduce the balanced equation, including state symbols, for the reaction of \({\text{Al(OH}}{{\text{)}}_{\text{2}}}{\text{NaC}}{{\text{O}}_{\text{3}}}{\text{(s)}}\) with the acid present in gastric juice.

Compound:

hydrochloric acid/HCl;

Strong or weak acid:

strong (acid);

Type of reaction:

neutralization;

Accept acid-base.

Ionic equation:

\({{\text{H}}^ + }{\text{(aq)}} + {\text{O}}{{\text{H}}^ - }{\text{(aq)}} \to {{\text{H}}_2}{\text{O(l)}}/{\text{2}}{{\text{H}}^ + }{\text{(aq)}} + {\text{CO}}_3^{2 - }{\text{(aq)}} \to {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\) /

\({{\text{H}}^ + }{\text{(aq)}} + {\text{HCO}}_3^ - {\text{(aq)}} \to {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\);

Accept equations such as Mg(OH)₂(s) + 2H⁺(aq) \( \to \) Mg²⁺(aq) + 2H₂O(l).

Ignore state symbols.

H₃O⁺ or H⁺ may be used in the equation.

Do not allow the inclusion of spectator ions.

\({\text{Al(OH}}{{\text{)}}_2}{\text{NaC}}{{\text{O}}_3}{\text{(s)}} + {\text{4HCl(aq)}} \to {\text{AlC}}{{\text{l}}_3}{\text{(aq)}} + {\text{NaCl(aq)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} + {\text{3}}{{\text{H}}_2}{\text{O(l)}}\) /

\({\text{Al(OH}}{{\text{)}}_2}{\text{NaC}}{{\text{O}}_3}{\text{(s)}} + {\text{4}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{A}}{{\text{l}}^{3 + }}{\text{(aq)}} + {\text{N}}{{\text{a}}^ + }{\text{(aq)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} + {\text{3}}{{\text{H}}_2}{\text{O(l)}}\);

correct reactants and products;

correct state symbols and balanced;

M2 can only be awarded if M1 is correct.

Very well answered by most candidates recognizing the acid in the stomach as hydrochloric acid and that it is a strong acid.

Surprisingly only about half of the candidates recognized the reaction of antacids in the stomach as a neutralization reaction and only few candidates gave a correct ionic equation for the reaction.

This was a discriminating question. Only the strong candidates were able to identify the correct products scoring one mark, and very few were able to add the correct state symbols and balance the equation. Common mistakes included writing solid state symbols for the salt products and writing an incorrect formula of aluminium chloride such as \({\text{AlC}}{{\text{l}}_{\text{2}}}\). A number of candidates had aluminium hydroxide as a product.