A-LevelChemistryChemical periodicity9701/42 · Feb/Mar 2019 · Q417 Marks

Complete the electronic configuration of a copper atom. 1s²2s²2p⁶

(a)(i)Complete the electronic configuration of a copper atom. 1s²2s²2p⁶[1]
(a)(ii)• Explain why most copper(II) salts are coloured. • Suggest why copper(I) salts are usually white.[4]
(b)Brass is an alloy of copper and zinc. The following reaction can be used to determine the amount of copper in a sample of brass. 2Cu²⁺(aq) + 4I⁻(aq) → 2CuI(s) + I₂(aq) The procedure was carried out using the following steps. • A solution of Cu²⁺(aq) was obtained by dissolving a 1.50 g sample of brass in concentrated sulfuric acid and diluting with water. • An excess of I⁻(aq) was added. • The iodine produced was titrated against a 0.500 mol dm⁻³ solution of thiosulfate ions, S₂O₃²⁻(aq). I₂(aq) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(aq) • The volume of S₂O₃²⁻ solution needed to reach the end-point was 28.35 cm³. Calculate the percentage by mass of copper in the sample of brass. percentage by mass of copper =[3]
(c)(i)Use standard electrode potential data from the Data Booklet to calculate E°cell for the reaction. 2Cu²⁺(aq) + 4I⁻(aq) → 2CuI(s) + I₂(aq) E°cell = V[1]
(c)(ii)Explain how the value of E°cell calculated in (i) predicts that the reaction is not likely to occur.[1]
(c)(iii)In an experiment, a solution of I⁻(aq) is added to a solution of Cu²⁺(aq). A reaction does occur and a precipitate of sparingly soluble CuI(s) is formed. The concentration of Cu²⁺(aq) remaining in the solution is 1.00 mol dm⁻³. The concentration of Cu⁺(aq) in a saturated solution of CuI is 1.3 × 10⁻⁶ mol dm⁻³. Use the Nernst equation to calculate the electrode potential, E, for the Cu²⁺/Cu⁺ half cell in this experiment. E(Cu²⁺/Cu⁺) = V[2]
(c)(iv)Copper(I) chloride is also sparingly soluble in water. Suggest why the following reaction does not occur. 2Cu²⁺(aq) + 4Cl⁻(aq) → 2CuCl(s) + Cl₂(aq)[1]
(d)(i)State the colours of Cu²⁺(aq) and [CuCl₄]²⁻(aq). Cu²⁺(aq) [CuCl₄]²⁻(aq)[1]
(d)(ii)Name the type of reaction that occurs when [CuCl₄]²⁻(aq) is formed from Cu²⁺(aq).[1]
(d)(iii)Write an expression for the stability constant, Kstab, for [CuCl₄]²⁻(aq). Include the units in your answer. Kstab = units =[2]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

[1] (1s²2s²2p⁶)3s²3p⁶3d¹⁰4s¹
[4] M1 d orbitals / sub-shell split into two levels by repulsion of approaching ligands
M2 light absorbed and complementary colour observed
M3 (d) electron(s) promoted / excited OR (d) electron(s) moves to higher (d) orbital
M4 (in Cu(I) complexes) all the orbitals in Cu are full OR Cu(I) is d¹⁰
[3] n(S₂O₃²⁻) = 28.35 × 0.5 / 1000 = 0.0142 (0.014175)
this also equals n(Cu²⁺)
mass of Cu = 0.014175 × 63.5 = 0.90 g
% of Cu = 100 × 0.90 / 1.5 = 60%
[1] E°cell = 0.15 - 0.54 = −0.39 (V)
[1] since E°cell is negative (reaction is not likely to occur) OR since E°cell < 0 (reaction is not feasible / not spontaneous)
[2] E = E° + (0.059/1) log(1.0/1.3 × 10⁻⁶)
= +0.15 + 0.059 × 5.89
= +0.50/0.497 V
[1] E°cell is very negative OR calculation (E°cell = 0.15 – 1.36 =) −1.21 V
[1] Cu²⁺(aq) is (light) blue AND [CuC₄]²⁻(aq) is yellow
[1] ligand displacement / replacement / substitution / exchange
[2] Kstab = [[CuCl₄]²⁻] / ([Cu²⁺][Cl⁻]⁴)
units: mol⁻⁴ dm¹²