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Date	November 2016	Marks available	6	Reference code	16N.3.sl.TZ0.8
Level	SL	Paper	3	Time zone	TZ0
Command term	Calculate, Deduce, and Suggest	Question number	8	Adapted from	N/A

Question

Lipids are an important part of the human diet.

Fatty acids react with glycerol to form fats and oils. State the name of the chemical link formed in this reaction and the name of the other product.

[1]

The table below shows average figures for the percentage fatty acid composition of some common fats and oils.

(i) Deduce, with a reason, which fat or oil from the table above has the lowest iodine number.

(ii) Deduce, with a reason, which fat or oil from the table above is most likely to become rancid when exposed to the air.

(iii) The P/S index of a fat or oil is the ratio of polyunsaturated fat to saturated fat present. It is sometimes used to compare the relative health benefits of different lipids in the diet. Calculate the P/S index of beef fat and soybean oil.

(iv) Suggest why a P/S index of greater than 1 is considered beneficial to health.

(v) Cotton seed oil and corn oil have similar iodine numbers but the melting point of cotton seed oil is higher than that of corn oil. Suggest an explanation in terms of the structure and bonding in these two oils.

[6]

Markscheme

Name of the chemical link: ester/ethoxycarbonyl
AND
Name of the other product: water

Do not accept formulas.
Do not accept “esterification”

i
coconut oil AND lowest «percentage of» unsaturated fatty acids
OR
coconut oil AND smallest number of C=C bonds
OR
coconut oil AND highest «percentage of» saturated fatty acids
Accept “fats” for “fatty acids”.

ii
soybean oil AND highest «percentage of» polyunsaturated fatty acids
OR
soybean oil AND greatest number of C=C bonds
OR
soybean oil AND lowest «percentage of» saturated fatty acids
Accept “fats” for “fatty acids”.

iii
Beef fat: «P/S = $\frac{3}{{59}}$ = » 0.05
AND
Soybean oil: «P/S = $\frac{{50 + 8}}{{14}}$ =» 4.1

iv
«higher proportion of» polyunsaturated fatty acids decrease risk of atherosclerosis/heart disease/cardiovascular disease/CVD
OR
«higher proportion of» polyunsaturated fatty acids which are less likely to be deposited on the walls of arteries «than saturated fatty acids»

Accept converse arguments.

Accept correct arguments in terms of HDL and LDL but not in terms of “good” and “bad” cholesterol.

Accept “fats” for “fatty acids”.

Any two of:
cotton seed oil has «a higher proportion of» longer chain/greater molar mass fatty acids

molecules of cotton seed oil have greater surface area/have higher electron density

Accept “molecules of cotton seed oil are packed more closely/have more regular structure” for M2.

stronger London/dispersion/instantaneous induced dipole-induced dipole forces between chains in cotton seed oil

Accept converse arguments.

Accept “fats” for “fatty acids”.

Examiners report

[N/A]

Syllabus sections

Options » B: Biochemistry » B.3 Lipids

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