Answers to questions on Environmental impact - heavy metals

1. (a) [Pb²⁺(aq)] = [CO₃²⁻(aq)] = √(7.40 x 10⁻¹⁴) = 2.72 x 10⁻⁷ mol dm⁻³
so concentration of lead(II) carbonate at 298 K = 2.72 x 10⁻⁷ mol dm⁻³
M_r(PbCO₃) = (207.2 + 12.01 + 48.00) = 267.21
[PbCO₃] = 267.21 x (2.72 x 10⁻⁷) = 7.27 x 10⁻⁵ g dm⁻³

(b) K_sp = [Pb²⁺(aq)] x [CO₃²⁻(aq)]
If sodium carbonate is added, [CO₃²⁻(aq)] increases, so Pb²⁺(aq)] must decrease by a corresponding amount to keep K_sp constant. It does this by precipitating out of solution a lead(II) carbonate.

(c) M_r(Na₂CO₃) = 106 so [CO₃²⁻(aq)] = 0.100 mol dm⁻³
[Pb²⁺(aq)] = K_sp ÷ [CO₃²⁻(aq)]
Assuming the [CO₃²⁻(aq)] from PbCO₃ is negligible
[Pb²⁺(aq)] = 7.40 x 10⁻¹⁴ ÷ 0.100 = 7.40 x 10⁻¹³
Mass of PbCO₃ dissolved after adding Na₂CO₃ = 267.21 x (7.40 x 10⁻¹³) = 1.98 x 10⁻¹⁰ g
Mass of PbCO₃ present before the addition of Na₂CO₃ = 7.27 x 10⁻⁵ g
Mass of PbCO₃ precipitated by adding Na₂CO₃ = (7.27 x 10⁻⁵) – (1.98 x 10⁻¹⁰) = 7.27 x 10⁻⁵ g

2. (a) EDTA, either as the acid or as its acid anion EDTA⁴-, has six non-bonding pairs of electrons that can be used to form coordinate bonds to metal atoms or ions. It may act as a bidentate, quadradentate or hexadentate polydentate ligand

(b) When the two species EDTA and the hydrated copper ion react there is a positive entropy increase as the four or six monodentate water molecules surrounding the copper(II) ion are released to give five or seven species so that there are more ways of distributing the effective energy (or, in simpler terms, the amount of disorder increases).
Since ΔG = ΔH – TΔS, the Gibbs free energy is more exothermic (i.e. the reaction is more spontaneous) and from the expression ΔG = −RTlnK it can be seen that this increases the value of the equilibrium constant and so more of the chelated products forms compared to the hydrated copper(II) ion.

3. (a) Ethane−1,2−diamine is a bidentate ligand as it contains two non-bonding pairs of electrons (one on each of the nitrogen atoms of the two –NH₂ groups). This means that three ligands can surround the chromium(III) ion to form six coordinate bonds

(b) The coordination number is six in both (i) [Cr(H₂O)₆]³⁺ and (ii) [Cr(H₂NCH₂CH₂NH₂)₃]³⁺.

4. (a) It is a termination reaction because, although one of the reactants is a free radical, there are no free radicals in the products.
It is a disproportionation reaction because the oxygen in the superoxide free-radical ion, which has an oxidation state of −½, is simultaneously oxidized to oxygen gas (oxidation state = zero) and reduced to hydrogen peroxide (oxidation state = −1).

(b) Fe³⁺(aq) + 2O₂^−.(aq) → O₂(g) + Fe²⁺(aq)
Fe²⁺(aq) + H₂O₂(aq) → Fe³⁺(aq) + OH⁻(aq) + OH^.(aq)

(c) The hydroxyl radical is extremely reactive. It can be used to decompose pollutants such as organic material by oxidizing them to less harmful substances in waste water.

Answers to Environmental impact - heavy metals questions