Acids, Alkalis and Titrations
6 marks · NormalA teacher claims that both sodium hydroxide and sodium carbonate solutions can be used in titrations with hydrochloric acid, but there are important reasons to choose one over the other.
Explain why sodium carbonate solution is considered an alkali, stating the ion responsible for its alkaline nature.
Describe two reasons why, in a titration of hydrochloric acid, sodium hydroxide might be preferred over sodium carbonate.
A student uses phenolphthalein as an indicator for a titration of sodium carbonate with hydrochloric acid. Predict the colour change at the endpoint and what this shows about the pH change.
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(a) Sodium carbonate is an alkali because it provides hydroxide ions (OH^-) when dissolved in water; the carbonate ions react with water to produce OH^- ions.
(b) Sodium hydroxide fully dissociates, giving a sharper endpoint in titration; sodium carbonate produces carbon dioxide gas during reaction and is only weakly alkaline, making the endpoint less precise and possibly leading to bubbling.
(c) The solution changes from pink to colourless at the endpoint, showing the pH changes from above 8 (alkaline) to below 8. The change is rapid, so phenolphthalein is a suitable indicator for this titration.
Common mistakes: incorrectly stating Na+ is responsible for alkalinity; not mentioning formation of CO2 gas affects titration endpoint; incorrect colour change for phenolphthalein
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Marking points
B1 (a) · 1 mark · States sodium carbonate forms OH^- in water
B2 (a) · 1 mark · Explains this is due to carbonate ions reacting with water as: CO3^{2-} + H2O ⇌ OH^- + HCO3^-
B3 (b) · 1 mark · Sodium hydroxide gives a sharper endpoint (fully dissociates, strong alkali)
B4 (b) · 1 mark · Reaction of sodium carbonate produces CO2 gas, which can affect the endpoint and cause bubbling; weakly alkaline nature makes endpoint less precise
B5 (c) · 1 mark · Phenolphthalein changes from pink (alkali) to colourless (acid) at endpoint
B6 (c) · 1 mark · Links rapid colour change to sudden pH shift around the endpoint
Worked solution
(a) Sodium carbonate is an alkali as it produces hydroxide ions, OH^-, in water. The carbonate ions react with water: CO_3^{2-}(aq) + H_2O(l) ⇌ OH^-(aq) + HCO_3^-(aq).
(b) Sodium hydroxide is a strong alkali, fully dissociating and giving a sharp, clear endpoint in titrations. Sodium carbonate, by contrast, is weakly alkaline and reacts with acid to produce carbon dioxide gas, which can cause bubbling and makes it harder to judge the endpoint accurately.
(c) With phenolphthalein, the colour at the endpoint changes from pink (alkaline) to colourless as enough acid is added. This indicates the pH drops suddenly at the endpoint, making phenolphthalein suitable.