IB Chemistry SL 3.2.2 Half Equations Questions

Use real SL exam questions to balance half-equations, combine them into overall redox equations and identify oxidation at each electrode.

Exam points

Write and balance oxidation or reduction half-equations in acidic or neutral solution, conserving atoms, charge and electrons and including requested state symbols.
Combine oxidation and reduction half-equations to produce a balanced overall redox equation, using the lowest whole-number electron ratio.
Identify which half-equation represents oxidation or reduction and assign electrode reactions from electron loss or gain, checking the electron balance.

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Study 3.2.2—Half-equations conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 2]

(a)

Deduce the half-equation for the oxidation of ethanol to ethanal.

[ 1 ]

$\mathrm{CH_3CH_2OH \rightarrow CH_3CHO + 2H^+ + 2e^-}$

Half-equation required. Do not accept:
$\mathrm{C_2H_5OH+2[O]\rightarrow CH_3CHO+H_2O}$

Accept e for $e^-$ .

(b)

Deduce the overall redox equation for the reaction of ethanol to ethanal with acidified potassium dichromate(VI) by combining your answer to part (c) (iii) with the following half-equation:

\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq})+14 \mathrm{H}^{+}(\mathrm{aq})+6 \mathrm{e}^{-} \rightarrow 2 \mathrm{Cr}^{3+}(\mathrm{aq})+7 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

[ 1 ]

$\mathrm{3CH_3CH_2OH(aq)+Cr_2O_7^{2-}(aq)+8H^+(aq) \rightarrow 2Cr^{3+}(aq)+3CH_3CHO(l)+7H_2O(l)}$

Award [1] for correct reactants and products.
Award [1] for correct balancing;
M2 can only be scored if M1 is correct.
Ignore state symbols.

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IB Chemistry SL 3.2.2 Half Equations Questions question 1

Question (a)

Question (b)