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Date	November 2016	Marks available	1	Reference code	16N.3.sl.TZ0.1
Level	SL	Paper	3	Time zone	TZ0
Command term	State	Question number	1	Adapted from	N/A

Question

In order to provide safe drinking water, a water supply is often treated with disinfectants, which aim to inactivate disease-causing bacteria in the water.

To compare the effectiveness of different disinfectants, a CT value is used as a measure of the dosage of disinfectant needed to achieve a certain level of inactivation of specific bacteria.

CT value (mg min dm⁻³) = C (mg dm⁻³) concentration of disinfectant × T (min) contact time with water

The table below compares the CT values of different disinfectants necessary to achieve 99% inactivation of two types of bacteria, listed as A and B.

(i) Deduce the oxidation state of chlorine in the following disinfectants.

(ii) From the data on CT values, justify the statement that bacterium B is generally more resistant to disinfection than bacterium A.

(iii) CT values can be used to determine whether a particular treatment process is adequate. Calculate the CT value, in mg min dm⁻³, when 1.50 × 10⁻⁵ g dm⁻³ of chlorine dioxide is added to a water supply with a contact time of 9.82 minutes.

(iv) From your answer to (a) (iii) and the data in the table, comment on whether this treatment will be sufficient to inactivate 99% of bacterium A.

[4]

CT values are influenced by temperature and by pH. The table below shows the CT values for chlorine needed to achieve 99% inactivation of a specific bacterium at stated values of pH and temperature.

(i) With reference to the temperature data in the table, suggest why it may be more difficult to treat water effectively with chlorine in cold climates.

(ii) Sketch a graph on the axes below to show how the CT value (at any temperature) varies with pH.

(iii) Comment on the relative CT values at pH 6.0 and pH 9.0 at each temperature.

(iv) Chlorine reacts with water as follows:

Cl₂ (g) + H₂O (l) $\rightleftharpoons$ HOCl (aq) + HCl (aq)

HOCl (aq) $\rightleftharpoons$ OCl⁻ (aq) + H⁺ (aq)

Predict how the concentrations of each of the species HOCl (aq) and OCl⁻ (aq) will change if the pH of the disinfected water increases.

[4]

Despite widespread improvements in the provision of safe drinking water, the sale of bottled water has increased dramatically in recent years. State one problem caused by this trend.

[1]

Markscheme

HOCl: +1
AND
ClO₂: +4

Accept “I” and “IV” but not “1+/1” and “4+/4” notations.

«most» CT values are higher for «bacterium» B
OR
«generally» higher dosage needed for «bacterium» B

Accept converse arguments. Accept “concentration” for “dosage”

iii

«CT = 1.50 × 10^–5 × 10³ mg dm^–3 × 9.82 min =» 1.47 × 10^–1 «mg min dm^–3»

lower than CT value/minimum dosage/1.8 × 10^–1 «mg min dm^–3»
AND
no/insufficient

Accept “concentration” for “dosage”.

higher CT value at lower temperature
OR
higher dosage «of chlorine» needed at low temperature

Accept “effectiveness decreases at lower temperature”.
Accept “concentration” for “dosage”.
Accept converse arguments.

labeled axes ( y: CT and x: pH)
AND
curve with increasing gradient

Do not accept axes the wrong way round.
Accept a linear sketch.

iii

values at pH 9.0 approximately 3 times values at pH 6.0
OR
increase in CT values in same ratio

The exact ratio is 2.9 times
Do not accept just “increase in value”.

[HOCl] decreases AND [OCl⁻] increases

plastic disposal/pollution
OR
plastic bottles use up petroleum/non-renewable raw material
OR
chemicals in plastic bottle can contaminate water
OR
«prolonged» storage in plastic can cause contamination of water
OR
plastic water bottles sometimes reused without proper hygiene considerations

Accept other valid answers.
Accept economic considerations such as “greater production costs”, “greater transport costs” or “bottled water more expensive than tap water”

Examiners report

[N/A]

Syllabus sections

Core » Topic 9: Redox processes » 9.1 Oxidation and reduction

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