Environmental impact - heavy metals questions

Questions on Environmental impact - heavy metals

1. Lead(II) ions can be removed from polluted water by precipitating them as lead(II) carbonate.
The Ksp value for lead(II) carbonate is 7.40 x 10−14 at 298 K.

(a) Determine the concentration of lead(II) carbonate (in g dm−3) in a saturated solution of lead(II) carbonate at 298 K.

(b) Explain why the solubility of lead(II) carbonate in water at 298 K is lower if sodium carbonate is added to a saturated solution of lead(II) carbonate.

(c) Determine the mass of lead(II) carbonate that will be precipitated from 1.00 dm3 of a saturated solution of lead(II) carbonate at 298 K if 10.6 g of sodium carbonate are then dissolved in the solution. Assume that the addition of the sodium carbonate causes no change to the total volume of the solution.

2. (a) Explain how EDTA can function as a polydentate ligand.

(b) The equilibrium constant for the formation of the copper EDTA complex, [Cu(EDTA)]2− is very high which explains why the addition of EDTA ions to a solution of hydrated copper(II) ions results in the rapid formation of the copper EDTA complex ion.
Explain why the polydentate copper(II) complex ion is much more stable than the hexahydrated copper(II) ion.

3. When a solution of ethane−1,2−diamine is added to an aqueous solution of chromium(III ions the following reaction occurs:

[Cr(H2O)6]3+(aq) + 3H2NCH2CH2NH2(aq) ⇌ [Cr(H2NCH2CH2NH2)3]3+(aq) + 6H2O(l)

(a) Explain why three molecules of ethane−1,2−diamine bond to the transition metal ion.

(b) State the coordination number of the copper(II) ion in

(i) [Cr(H2O)6]3+ and (ii) [Cr(H2NCH2CH2NH2)3]3+.

4. (a) Hydrogen peroxide can be formed in biological systems by the reaction of enzymes reacting with the superoxide free-radical ion in acid solution.

2O2−.(aq) + 2H+(aq) → O2(g) + H2O2(aq)

Explain why this can be described both as a termination reaction and as a disproportionation reaction.

(b) The hydrogen peroxide can react slowly with the superoxide free-radical ion to produce the hydroxyl radical, OH., as well as oxygen and a hydroxide ion. This reaction is known as the Haber-Weiss reaction.

O2−.(aq) + H2O2(aq) → O2(g) + OH−(aq) + OH.(aq)

The Haber-Weiss reaction is speeded up considerably in the presence of a catalyst of iron(II) ions. This two-step reaction is known as the Fenton reaction.

State the equations that illustrate the two steps of the Fenton reaction.


(c) Explain why the Fenton reaction is used industrially in waste water plants .

Download the  Environmental impact - heavy metals questions to give to your studentsTo see the worked answers go to Environmental impact - heavy metals answers.
All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.