A-LevelChemistryChemical periodicity9701/43 · May/June 2024 · Q313 Marks
When a sample of hydrated lithium ethanedioate, Li2C2O4•H2O, is gently heated, two gaseous products are formed and a white solid residue remains. The residue is added to HNO3(aq). A gas is produced that turns limewater milky. Complete the equation for the decomposition of Li2C2O4•H2O. Li2C2O4•H2O → +
- (a)When a sample of hydrated lithium ethanedioate, Li2C2O4•H2O, is gently heated, two gaseous products are formed and a white solid residue remains. The residue is added to HNO3(aq). A gas is produced that turns limewater milky. Complete the equation for the decomposition of Li2C2O4•H2O. Li2C2O4•H2O → +[1]
- (b)The trend in the decomposition temperatures of the Group 2 ethanedioates is similar to that of the Group 2 nitrates. Suggest which of CaC2O4 and BaC2O4 will decompose at the lower temperature. Explain your answer.[2]
- (c)Potassium iron(III) ethanedioate, K3[Fe(C2O4)3], dissolves in water to form a green solution. Explain why transition elements can form coloured complexes.[3]
- (d)The anhydrous iron(III) compound K3[Fe(C2O4)3] decomposes on heating to form a mixture of K2[Fe(C2O4)2], K2C2O4 and CO2. Complete the equation for the decomposition of K3[Fe(C2O4)3]. K3[Fe(C2O4)3]→ K2[Fe(C2O4)2] + ............ K2C2O4 + ............ CO2[1]
- (e)The [Fe(C2O4)3]³- complex ion shows stereoisomerism. Complete the three-dimensional diagrams in Fig. 3.1 to show the two stereoisomers of [Fe(C2O4)3]3-. The C2O42- ligand can be represented using O O. [Figure 3.1][2]
- (f)Buffer solutions are used to regulate pH. Write two equations to describe how a solution containing HC2O4¯ ions acts as a buffer solution when small amounts of acid or alkali are added.[2]
- (g)A fuel cell is an electrochemical cell that can be used to generate electrical energy by using oxygen to oxidise a fuel. Ethanedioic acid, (COOH)2, dissolved in an alkaline electrolyte is being investigated as a fuel. The relevant standard electrode potentials, Eº, for the cell are shown. O2(g) + 2H2O(l) + 4e¯ ⇌ 4OH-(aq) Eº = +0.40V 2CO2(g) + 2e¯ ⇌ C2O42-(aq) Eº = -0.59 V Use these equations to deduce the overall cell reaction. Calculate the value of Eºcell. overall cell reaction Eºcell = V[2]
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Mark scheme
Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.
- [1] Li2C2O4•H2O → Li2CO3 + H2O + CO
- [2] M1 CaC2O4 AND Ca2+ has a smaller ionic radius / Ca2+ has a higher charge density; M2 anion / C2O42- becomes more polarised / distorted
- [3] M1 d orbital(s) of different energy / d-d splitting occurs; M2 electron(s) promoted / excited; M3 light is absorbed AND colour seen is complementary
- [1] 2K3[Fe(C2O4)3] → 2K2[Fe(C2O4)2] + K2C2O4 + 2CO2
- [2] [Diagrams showing two non-superimposable mirror image octahedral complexes of [Fe(C2O4)3]3- where each oxalate ligand acts as a bidentate ligand, e.g., using the O O representation to show two chelating rings that are mirror images.]
- [2] M1 HC2O4- + H+ → H2C2O4; M2 HC2O4- + OH- → C2O42- + H2O
- [2] M1 2C2O42- + O2(g) + 2H2O(l) → 4CO2(g) + 4OH-(aq); M2 Ecell = 0.40 – (–0.59) = 0.99 V
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