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Date May 2019 Marks available 1 Reference code 19M.3.hl.TZ2.8
Level HL Paper 3 Time zone TZ2
Command term Outline Question number 8 Adapted from N/A

Question

Heavy metals are toxic even in very low concentrations.

Outline why heavy metals are toxic.

[1]
a.

Determine the maximum concentration of lead(II) ions at 298 K in a solution in which the concentration of carbonate ions is maintained at 1.10 × 10−4 mol dm−3. Use section 32 of the data booklet.

[2]
b.

State a method, other than precipitation, of removing heavy metal ions from solution.

[1]
c.

Markscheme

Any one of:
disrupt endocrine system
OR
compete for active sites of enzymes/cellular receptors
OR
form complexes with/inhibit enzymes
OR
denature proteins
OR
change shape of active site  [✔]

participate in redox reactions
OR
disturb normal redox balance «in cells»  [✔]

initiate «free» radical reactions «in electron transfer»  [✔]

a.

«Ksp = 7.40 × 10–14»
Ksp = [Pb2+][CO32–[✔]

[Pb2+] «= 7.40 × 10 14 1.10 × 10 4 » = 6.73 × 10–10 «mol dm–3»  [✔]

 

Note: Award [2] for correct final answer.

b.

Any one of:
chelation «by EDTA/polydentate ligand anchored»  [✔]

ion exchange systems [✔]

adsorption by «water» plants  [✔]

 

Note: Accept “use of zeolites”.

c.

Examiners report

The candidates seemed to have difficulty in outlining why heavy metals are toxic.

a.

Majority of the candidates managed to get two marks in determining the maximum concentration of lead(II) ions using solubility product constant.

b.

This was not well answered by most of the candidates.

c.

Syllabus sections

Options » A: Materials » A.10 Environmental impact—heavy metals (HL only)
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Options » A: Materials
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