Scottish Highers Chemistry Oxidising and Reducing Agents: 12. (a) The concentration of sod

Question

12. (a) The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration.

Step 1
A 100.0 cm³ sample from the swimming pool is first reacted with an excess of acidified potassium iodide solution forming iodine.

NaOCl(aq) + 2 I⁻(aq) + 2 H⁺(aq) → I₂(aq) + NaCl(aq) + H₂O(ℓ)

Step 2
The iodine formed in step 1 is titrated using a standard solution of sodium thiosulfate, concentration 0.00100 mol l⁻¹. A small volume of starch solution is added towards the endpoint.

(i) Write the ion-electron equation for the oxidation reaction occurring in step 1.

(ii) Calculate the concentration, in mol l⁻¹, of sodium hypochlorite in the swimming pool water, if an average volume of 12.4 cm³ of sodium thiosulfate was required.

(b) The level of hypochlorite in swimming pools needs to be maintained between 1 and 3 parts per million (1 - 3 ppm).

400 cm³ of a commercial hypochlorite solution will raise the hypochlorite level of 45 000 litres of water by 1 ppm.

Calculate the volume of hypochlorite solution that will be added to an Olympic-sized swimming pool, capacity 2 500 000 litres, to raise the hypochlorite level from 1 ppm to 3 ppm.

(c) The familiar chlorine smell of a swimming pool is not due to chlorine but compounds called chloramines. Chloramines are produced when the hypochlorite ion reacts with compounds such as ammonia, produced by the human body.

OC⁻(aq) + NH₃(aq) → NH₂Cl(aq) + OH⁻(aq) monochloramine.

OC⁻(aq) + NH₂Cl(aq) → NCl₂(aq) + OH⁻(aq) dichloramine.

OC⁻(aq) + NCl₂(aq) → NCl₃(aq) + HCl(aq) trichloramine.

Chloramines are less soluble in water than ammonia due to the polarities of the molecules, and so readily escape into the atmosphere, causing irritating fumes.

(i) Explain the difference in polarities of ammonia and trichloramine molecules.

(ii) Chloramines can be removed from water using ultraviolet light treatment.

One step in the process is the formation of free radicals.

NH₂Cl + UV → NH₂• + •Cl

State what is meant by the term free radical.

(iii) Another step in the process is shown below.

NH₂Cl + •Cl → NHCl + HCl

State the name for this type of step in a free radical reaction.

12. (a) The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2015

Worked Solution & Example Answer:12. (a) The concentration of sodium hypochlorite in swimming pool water can be determined by redox titration - Scottish Highers Chemistry - Question 12 - 2015

Step 1: Write the ion-electron equation for the oxidation reaction occurring in step 1.

Step 2: Calculate the concentration, in mol l⁻¹, of sodium hypochlorite in the swimming pool water, if an average volume of 12.4 cm³ of sodium thiosulfate was required.

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