IB Chemistry HL 3.1.14 Hl Acid Base Indicators Questions

Use real HL exam questions to explain indicator equilibria, predict colours from pH and pKa, and connect conjugated structure to absorbed light.

Exam points

Explain acid-base indicator colour change with a weak-acid equilibrium, identifying protonated and deprotonated coloured forms and the equilibrium shift.
Predict the predominant indicator form or observed colour at a stated pH by comparing pH with the indicator pKa or transition range.
Relate indicator structure and conjugation to the wavelength or colour of absorbed light, and identify the colour-change property of an indicator.

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Study 3.1.14 (HL)—Acid-base indicators conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

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Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})

Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.

$\mathrm{HIn} \rightleftharpoons \mathrm{H}^{+}+\mathrm{In}^{-}$ ;
(weak acid in which the) acid/HIn and conjugate base/In- have different colours / OWTTE;
excess alkali shifts the equilibrium to the RHS/towards the conjugate base;

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IB Chemistry HL 3.1.14 Hl Acid Base Indicators Questions question 1