R3.2 Electron transfer reactions questions for IB Chemistry

201 questions on Electron transfer reactions, written by Augment in the style of IB exam papers. Six of them are below in full, to answer on paper.

IB Chemistry
Part of R3 What are the mechanisms of chemical change?
SL and HL

Questions by paper and level

SL and HL questions are for students at either level. HL only questions cover what only HL students study.

Paper	SL and HL	HL only	All
Paper 1A	54	25	79
Paper 1B	17	11	28
Paper 2	55	39	94
Every paper	126	75	201

Counted on 10 November 2026

Questions to try

Written by Augment in the style of IB exams, not past papers. Answer them on paper, then check each one against its markscheme with a free account.

Question 1 · CHEM-00067Paper 1A · 1 mark

Magnesium burns in chlorine to form magnesium chloride.

M g (s) + C l_{2} (g) \to M g C l_{2} (s)

Which statement about this reaction is correct?

A
Each magnesium atom gains two electrons.
B
Each chlorine atom loses one electron.
C
Magnesium is oxidized and each chlorine atom gains one electron.
D
Magnesium shares one pair of electrons with each chlorine atom.

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Question 2 · CHEM-01053Paper 1B · 9 marks

A student investigated the reactivity of the halogens. 2 cm³ of aqueous halogen was added to 2 cm³ of each aqueous potassium halide. The mixture was shaken, 1 cm³ of cyclohexane was added and the tube was shaken again. Cyclohexane does not mix with water and forms the upper layer. The colour of the upper layer was recorded.

Halogen added	KCl(aq)	KBr(aq)	KI(aq)
chlorine water	—	orange	purple
bromine water	orange	—	purple
aqueous iodine	purple	purple	—

(a)
Suggest why cyclohexane was added to each mixture.
1 mark
(b)
Explain why the upper layer is orange when bromine water is added to KCl(aq), even though no reaction occurs.
1 mark
(c)
Deduce the order of reactivity of the three halogens, justifying your answer with the results.
2 marks
(d)
Write the ionic equation for the reaction between bromine water and aqueous potassium iodide, and identify the species that is reduced.
2 marks
(e)
Suggest a control experiment that would help the student interpret the colours.
1 mark
(f)
Suggest why the reaction between chlorine water and potassium bromide might give only a very pale colour if the chlorine water had been stored for several weeks.
1 mark
(g)
State one safety precaution for this experiment.
1 mark

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Question 3 · CHEM-00301Paper 2 · HL only · 9 marks

Dinitrogen monoxide, N₂O, is used as an anaesthetic. Three possible Lewis formulas for N₂O are described, with the number of lone pairs on each atom given in brackets.

Structure	Terminal N	Bond	Central N	Bond	O
I	N (2)	=	N (0)	=	O (2)
II	N (1)	≡	N (0)	–	O (3)
III	N (3)	–	N (0)	≡	O (1)

(a)
Calculate the formal charge on each atom in structures I, II and III.
3 marks
(b)
Deduce, giving two reasons, which structure is preferred.
2 marks
(c)
Predict the molecular geometry of N₂O.
1 mark
(d)
State the number of σ and π bonds in structure II.
1 mark
(e)
State the hybridization of the central nitrogen atom.
1 mark
(f)
Formal charge and oxidation state are both ways of assigning electrons to atoms. Outline how the assumption behind formal charge differs from the one behind oxidation state.
1 mark

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Question 4 · CHEM-00183Paper 1A · 1 mark

Aluminium reacts with oxygen to form aluminium oxide, Al₂O₃.

How many electrons are transferred from aluminium to oxygen in forming one formula unit of Al₂O₃?

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Question 5 · CHEM-01054Paper 1B · 10 marks

The concentration of chlorine in a sample of chlorine water was determined. 10.00 cm³ of chlorine water was added to excess aqueous potassium iodide. The iodine formed was titrated with 0.0100 mol dm⁻³ sodium thiosulfate solution, with starch added near the end point.

C l_{2} (a q) + 2 I^{-} (a q) \to 2 C l^{-} (a q) + I_{2} (a q)

I_{2} (a q) + 2 S_{2} O_{3}^{2 -} (a q) \to 2 I^{-} (a q) + S_{4} O_{6}^{2 -} (a q)

Titration	Rough	1	2	3
Final burette reading / cm³ ± 0.05 cm³	18.60	18.35	36.40	19.15
Initial burette reading / cm³ ± 0.05 cm³	0.00	0.40	18.35	1.15

(a)
State the absolute uncertainty in each titre.
1 mark
(b)
Calculate the mean titre, using only the appropriate results.
1 mark
(c)
Calculate the percentage uncertainty in the mean titre.
1 mark
(d)
Determine the concentration of chlorine in the chlorine water, in mol dm⁻³.
3 marks
(e)
Deduce the oxidation state of sulfur in $S_{2} O_{3}^{2 -}$ and the average oxidation state of sulfur in $S_{4} O_{6}^{2 -}$ .
2 marks
(f)
Explain why the potassium iodide is added in excess.
1 mark
(g)
The chlorine water was left in an open beaker for 20 minutes before the samples were taken. Suggest the effect of this on the result and whether it is a random or systematic error.
1 mark

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Question 6 · CHEM-00434Paper 2 · 8 marks

Chlorine water is added separately to colourless aqueous solutions of potassium bromide and potassium iodide.

(a)
State the colour of each solution after the chlorine water is added.
2 marks
(b)
Write the ionic equations for the two reactions.
2 marks
(c)
Explain why chlorine is a stronger oxidizing agent than bromine.
2 marks
(d)
State why there is no reaction when iodine solution is added to aqueous potassium bromide.
1 mark
(e)
Deduce the change in the oxidation state of chlorine in the reactions in (b).
1 mark

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