Data response Example 5

Data Response question on Vitamin C and iodine

 Teacher only box

I have made up this question to help prepare students for the data response questions which appear on Section A of Paper 3 for the examinations on the 2014 programme (i.e. for examinations after May 2016). The information given is new to the students but they should be able to use their knowledge and understanding of the chemistry content of the programme to answer the questions. Because I have tried to make the chemistry interesting it may be a little harder than an actual exam question but exposure to questions like this beforehand should make students able to face the data response question in the final examination with considerable confidence. All the answers require a knowledge and understanding of material from the core only so this particular question is suitable for both Standard and Higher Level students.

As well as including model answers I have also explained the chemistry fully and given the 'Complete course for students' links to the pages which cover the theory behind each individual question. If you set it as an assigned task using Student Access, students will not be able to see the model answers until after they have submitted their answers to you. If you simply give students access to the page (rather than assigning it as a task) they will be able to access the model answers themselves for review/revision/self-study etc.

If you do not wish to use student access, links to downloadable versions of the questions and, separately the worked answers, can be found at Printable versions of written tasks.

 

 

The Royal Society of Chemistry has introduced a global experiment for school students to determine the amount of vitamin C in various fruits.

Students first determine how many drops of a given iodine solution are required to react with a known amount of vitamin C by calibrating the iodine solution with a known amount of vitamin C using starch as an indicator.

All the Vitamin C content is then extracted from a selected fruit and the number of drops of the standardised iodine solution required to react with the vitamin C is determined. The amount of vitamin C in mg g-1 in the selected fruit is then calculated.

The relevant equation for the reaction in aqueous solution is

The molar mass of vitamin C = 176.12 g mol−1

(a) (i) Deduce the two half-equations for the oxidation of vitamin C and the reduction of iodine in aqueous solution. [2]

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   (ii) Explain why vitamin C is soluble in water. [1]

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(b) A student in a particular school obtained the following calibration curve:



She found that the vitamin C extracted from 3.04 g of a fresh red pepper required 82 drops of the iodine solution to react completely.
 

(i) Deduce the concentration of vitamin C in mg g-1 of the fresh red pepper. [2]

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(ii) It was determined that 103 drops of the iodine solution had a total volume of 1.00 cm3.
Calculate the concentration of the iodine solution in mol dm−3. [2]

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(c) Iodine can be formed by the reaction between iodide and iodate ions in acidic solution:

I−(aq) + IO3− (aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)

This redox reaction is known as a disproportionation reaction as iodine is simultaneously oxidised (from -1 to zero) and reduced (from +5 to zero).

Iodine is insoluble in water but it does dissolve in a solution of potassium iodide as it forms the complex triiodide ion, I3−.

I2(aq) + I− (aq) → I3−(aq)

Discuss whether this reaction between iodine and iodide ions can also be considered to be a disproportionation reaction. [2]

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