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Question 23M.3.SL.TZ2.7

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Date May 2023 Marks available [Maximum mark: 8] Reference code 23M.3.SL.TZ2.7
Level SL Paper 3 Time zone TZ2
Command term Calculate, Compare, Explain Question number 7 Adapted from N/A
7.
[Maximum mark: 8]
23M.3.SL.TZ2.7

Lipids are another group of biomolecules.

(a)

Compare the hydrolytic and oxidative rancidity and contrast the site where the chemical changes occur.

 

Compare rancidity: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Contrast reaction site: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

[2]

Markscheme

Compare rancidity:
«both produce» disagreeable smell/taste/texture/appearance ✓

Contrast reaction site:
hydrolytic reaction occurs at ester link/COOC link AND oxidative reaction occurs at carbon-carbon double bond/C=C ✓

 

Do not accept “double bond” alone for oxidative reaction site.

(b)

Calculate the iodine number for ozubondo acid, C21H33COOH.

Mr = 330.56

[2]

Markscheme

5 C=C ✓

«100 g/330.56 g mol−1 × 5 × 253.8 g mol−1 =» 383.89 «g I2» ✓

 

Award [2] for correct final answer.

(c)

Explain two ways in which carbohydrates and lipids differ as sources of energy.

[2]

Markscheme

lipids are more reduced AND release/store more energy than carbohydrates «per gram» ✓

lipids are less «water» soluble AND energy is released slower/less available than in carbohydrates ✓

 

Accept converse arguments.

Award [1 max] for “carbohydrates used for short-term energy supply AND lipids used for long-term energy supply”
OR
“lipids more reduced AND less «water» soluble”.

(d)

Explain why stearic acid has a higher melting point than linoleic acid based on their structural differences. Use section 34 of the data booklet.

[2]

Markscheme

«stearic acid» straight chain/chain has no kinks/more regular structure
OR
«stearic acid» saturated/no «carbon–carbon» double bonds ✓

«stearic acid» chains pack more closely together
OR
stronger London/dispersion/instantaneous induced dipole-induced dipole forces «between molecules» ✓

 

Accept greater surface area/electron density for M1.

Accept stronger intermolecular/van der Waals'/vdW forces for M2.