R3.3 Electron sharing reactions questions for IB Chemistry

61 questions on Electron sharing 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	25	0	25
Paper 1B	9	2	11
Paper 2	22	3	25
Every paper	56	5	61

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-01481Paper 1A · 1 mark

Which species is a radical?

A
$C H_{3}^{+}$
B
$C H_{3}^{-}$
C
$∙ C H_{3}$
D
$C H_{4}$

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Question 2 · CHEM-01262Paper 1B · HL only · 10 marks

Propane was reacted with chlorine in ultraviolet light. The monochlorinated products, 1-chloropropane and 2-chloropropane, were separated from the other products, and the mixture of the two was analysed by $^{1}$ H NMR spectroscopy.

Signal	Chemical shift / ppm	Relative area
$C H_{2} C l$ of 1-chloropropane	3.5	1.10
CHCl of 2-chloropropane	4.1	0.62

Boiling points: 1-chloropropane 47 °C; 2-chloropropane 36 °C.

(a)
Classify 1-chloropropane and 2-chloropropane as primary, secondary or tertiary.
1 mark
(b)
State the type of reaction that forms the products.
1 mark
(c)
Determine the percentage of each isomer in the mixture, by amount (in mol).
2 marks
(d)(i)
Propane has six primary and two secondary hydrogen atoms. Predict the percentage of 1-chloropropane if every hydrogen atom were equally likely to be substituted.
1 mark
(d)(ii)
Deduce how many times more reactive each secondary hydrogen atom is than each primary hydrogen atom in this experiment.
1 mark
(e)
Suggest why the $C H_{2} C l$ and CHCl signals, rather than the $C H_{3}$ signals, were used for the analysis.
1 mark
(f)
Suggest how the two isomers could be separated, and explain why this is possible.
2 marks
(g)
Explain why the molecular ion peaks in the mass spectra cannot distinguish the two isomers.
1 mark

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Question 3 · CHEM-00297Paper 2 · HL only · 9 marks

Ozone, O₃, in the stratosphere absorbs ultraviolet radiation. Both oxygen, O₂, and ozone are dissociated by ultraviolet light.

(a)
Draw two resonance structures for ozone.
2 marks
(b)
State the bond order of the oxygen–oxygen bonds in ozone.
1 mark
(c)
Calculate the formal charge on each oxygen atom in one of your resonance structures.
2 marks
(d)
Explain why ozone is dissociated by ultraviolet light of a longer wavelength than that needed to dissociate oxygen.
2 marks
(e)
State the molecular geometry of ozone and the hybridization of the central oxygen atom.
2 marks

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Question 4 · CHEM-01482Paper 1A · 1 mark

Which species contains an unpaired electron?

A
$N O$
B
$C O$
C
$N_{2}$
D
$C N^{-}$

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Question 5 · CHEM-01335Paper 1B · HL only · 9 marks

A student tested three hydrocarbons with a dilute solution of bromine in an inert solvent. In each test, 5 drops of the orange bromine solution were added to 1.0 cm³ of the hydrocarbon at room temperature, and the time for the orange colour to disappear was recorded. Tests were stopped after 600 s.

Hydrocarbon	Conditions	Observation
cyclohexene, C₆H₁₀	dark	decolourized in less than 2 s
cyclohexane, C₆H₁₂	dark	no change after 600 s
cyclohexane, C₆H₁₂	ultraviolet light	decolourized after 240 s; misty fumes at the mouth of the tube
benzene, C₆H₆	dark	no change after 600 s
benzene, C₆H₆	dark, with iron(III) bromide added, warmed	decolourized after 180 s; misty fumes at the mouth of the tube

(a)
State the type of reaction between cyclohexene and bromine.
1 mark
(b)
Explain how the results provide evidence that benzene does not contain three localized C=C bonds.
2 marks
(c)
Identify the misty fumes and state what they show about the type of reaction.
1 mark
(d)
Write the equation for the reaction of benzene with bromine in the presence of iron(III) bromide.
1 mark
(e)
State the role of iron(III) bromide.
1 mark
(f)
Identify one variable, other than temperature, that was controlled in these tests.
1 mark
(g)
Judging when the colour disappears is a source of random error. Suggest how it could be reduced.
1 mark
(h)
State one safety precaution needed when using bromine and benzene.
1 mark

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Question 6 · CHEM-00800Paper 2 · 6 marks

Methane reacts with halogens in the presence of ultraviolet light.

C H_{4} (g) + C l_{2} (g) \to C H_{3} C l (g) + H C l (g)

C H_{4} (g) + B r_{2} (g) \to C H_{3} B r (g) + H B r (g)

(a)
Calculate the enthalpy change, in kJ mol⁻¹, for the reaction of methane with chlorine. Use section 12 of the data booklet.
2 marks
(b)
Calculate the enthalpy change, in kJ mol⁻¹, for the reaction of methane with bromine.
1 mark
(c)
Using section 12 of the data booklet, identify the bond that is most easily broken in the reaction mixture of methane and bromine.
1 mark
(d)
State the type of bond fission that occurs when ultraviolet light is absorbed by a chlorine molecule, and the species formed.
2 marks

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