IB Chemistry SL Reactivity 3.2.2 Redox Half-Equations

Practise separating oxidation and reduction, balancing atoms and charge, and combining half-equations in acidic or neutral solution. Show electrons on the correct side, scale…

Exam points

place electrons on the product side for oxidation and reactant side for reduction
balance oxygen with H₂O and hydrogen with H⁺ when working in acidic solution
multiply and add half-equations so electrons cancel and total charge is conserved

[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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3.2.2—Half-equations question 1

Question (a)

Question (b)