May/June 2026 Paper 21

2026 · 5 questions · 38 parts · 60 marks

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Context for question 3

Separate samples of NaCl, NaBr and NaI each react with concentrated H

_{2}

_{4}

. Table 3.1 shows the observations for each reaction.

Table 3.1

reaction	compound	observations
1	NaCl	• effervescence • steamy fumes that turn blue litmus paper red
2	NaBr	• effervescence • steamy fumes initially that turn blue litmus paper red • brown vapour • pungent smell of SO $_{2}$
3	NaI	• effervescence • steamy fumes initially that turn blue litmus paper red • purple vapour • pungent smell of SO $_{2}$ and then a rotten egg smell of H $_{2}$ S

3(a)(i)Group 7 HalogensEasy1 mark

Write an equation for reaction 1.

Answer

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3(a)(ii)Acid Base Properties ReactionsEasy1 mark

Identify the reactant that behaves as a Brønsted–Lowry base in reaction 1.

3(b)Group 7 HalogensEasy1 mark

State the formulae of the species responsible for the brown vapour in reaction 2 and for the purple vapour in reaction 3.

Formula of the brown vapour, then the formula of the purple vapour

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3(c)Group 7 HalogensEasy1 mark

Explain why reaction 3 produces purple vapour.

Answer

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3(d)Oxidation Reduction Redox EquationsMedium2 marks

Complete Table 3.2 by stating the oxidation number of sulfur in each species.

species	oxidation number of sulfur
H $_{2}$ SO $_{4}$	Table answer r1_oxidation number of sulfur
SO $_{2}$	Table answer r2_oxidation number of sulfur
H $_{2}$ S	Table answer r3_oxidation number of sulfur

3(e)Group 7 HalogensEasy1 mark

Explain why reaction 3 produces H

_{2}

S but reaction 2 does not produce H

_{2}

Answer

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3(f)(i)Group 7 HalogensEasy2 marks

Describe what is observed when Ag

^{+}

(aq), followed by NH

_{3}

(aq), are added to a sample of NaI(aq).

Observation after adding Ag

^{+}

(aq)

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Observation after adding NH

_{3}

(aq)

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3(f)(ii)Group 7 HalogensEasy1 mark

Write an ionic equation for the reaction of Ag

^{+}

(aq) with NaI(aq).

Answer

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