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Date	November 2020	Marks available	2	Reference code	20N.3.sl.TZ0.2
Level	SL	Paper	3	Time zone	TZ0
Command term	Calculate	Question number	2	Adapted from	N/A

Question

An investigation was carried out to determine the effect of chain length of the alcohol on the equilibrium constant, $K_{c}$ , for the reversible reaction:

$ROH + {CH}_{3} COOH \overset{H^{+} (aq)}{⇌} {CH}_{3} COOR + H_{2} O$

The reactants, products and the catalyst form a homogeneous mixture.

Fixed volumes of each alcohol, the ethanoic acid and the sulfuric acid catalyst were placed in sealed conical flasks.

At equilibrium, the flasks were placed in an ice bath, and samples of each flask titrated with $NaOH (aq)$ to determine the ethanoic acid concentration present in the equilibrium mixture.

The following processed results were obtained.

Identify the independent and dependent variables in this experiment.

[1]

The ice bath is used at equilibrium to slow down the forward and reverse reactions. Explain why adding a large amount of water to the reaction mixture would also slow down both reactions.

[2]

Suggest why the titration must be conducted quickly even though a low temperature is maintained.

[1]

An additional experiment was conducted in which only the sulfuric acid catalyst was titrated with $NaOH (aq)$ . Outline why this experiment was necessary.

[1]

Calculate the percentage uncertainty and percentage error in the experimentally determined value of $K_{c}$ for methanol.

[2]

Comment on the magnitudes of random and systematic errors in this experiment using the answers in (e).

[2]

Suggest a risk of using sulfuric acid as the catalyst.

[1]

Markscheme

Independent variable:
chain length OR number of carbon «atoms in alcohol»
AND
Dependent variable:
volume of $NaOH$ OR $K_{c}$ /equilibrium constant OR equilibrium concentration/moles of ${CH}_{3} COOH$ ✔

dilution/lower concentrations ✔

less frequent collisions «per unit volume» ✔

Accept “lowers concentration of acid catalyst” for M1. M2 must refer to increase in activation energy or different pathway.

Do not accept responses referring to equilibrium.

equilibrium shifts to left
OR
more ethanoic acid is produced «as ethanoic acid is neutralized»
OR
prevents/slows down ester hydrolysis ✔

Accept “prevents equilibrium shift” if described correctly without direction.

to determine volume/moles of $NaOH$ used up by the catalyst/sulfuric acid «in the titration»
OR
to eliminate/reduce «systematic» error caused by acid catalyst ✔

Do not accept “control” OR “standard” alone.

Percentage uncertainty:
$« \frac{0.4 \times 100}{6.5} = » 6 « % »$ ✔

Percentage error:
$« \frac{6.5 - 5.3}{5.3} = » 23 « % »$ ✔

Award [1 max] if calculations are reversed OR if incorrect alcohol is used.

Any two:

large percentage error means large systematic error «in procedure» ✔

small percentage uncertainty means small random errors ✔

random errors smaller than systematic error ✔

Award [2] for “both random and systematic errors are significant.”

corrosive/burns/irritant/strong oxidizing agent/carcinogenic
OR
disposal is an environmental issue
OR
causes other side reactions/dehydration/decomposition ✔

Do not accept just “risk of accidents” OR “health risks” OR “hazardous”.

Examiners report

Well answered. Students mostly identified (alcohol) chain length as the independent variable and K_c at the dependent. For the latter [ethanoic acid] at equilibrium was another popular choice with some students neglecting to clarify "equilibrium" which was needed for the mark. This evidences an issue already identified in the Internal assessment that very often students only identify the processed variables. The proportion of students referring to volume of NaOH was too low for expectations.

A significant number of students scored at least one mark, usually the first and many both. Weaker students lost the second mark by referring to less collisions instead of less frequent collision or other words to this effect. Very few students referred to more diluted catalyst and of those even less were able to provide an adequate explanation in terms of the increased Ea. Many students tried to answer this question in terms of equilibrium instead of kinetics. There were also several responses that replied as if the dilution would only occur for part of the reaction or individual reactants instead of the entire solution.

Not well answered and of the few students that replied correctly most referred to preventing equilibrium shift and few candidates identified the direction of the shift. It was rather common to see answers where Le Chatelier's principle was stated without any attempt in adapting it to the context. Very few students described the specific equilibrium shift that could occur during the titration, changing the results.

Some students achieved on mark. Many answers referred simple to "control" or "standard" underlining the lack of some skills as also identified in the Internal assessment. Once again very few students had the specific details necessary to explain why this separate titration was needed in their response to receive a mark.

Many students scored both points and others at least one. Weaker students inverted the calculations.

Of the many students that obtained the mark most did through the first alternative and a lesser percentage through the third. Many students were unable to relate their calculations from 2e (percentage error and percentage uncertainty) to systematic error and random error. They either compared the calculations to incorrect errors or in some cases did not discuss the errors at all. Once again this points to a general lack on experimental understanding.

Most students received a mark for this question base on specific hazards. Very few students related disposal to environmental issues which isn't surprising as this is often missed in the Internal Assessment. Weaker students provided vague answers related to health issues which did not receive a mark. Some students misunderstood the question.

Syllabus sections

Core » Topic 11: Measurement and data processing » 11.1 Uncertainties and errors in measurement and results

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