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Date November 2019 Marks available 2 Reference code 19N.3.hl.TZ0.12
Level HL Paper 3 Time zone TZ0
Command term State and Identify Question number 12 Adapted from N/A

Question

Stearic acid (Mr = 284.47) and oleic acid (Mr = 282.46) have the same number of carbon atoms. The structures of both lipids are shown in section 34 of the data booklet.

The iodine number is the number of grams of iodine which reacts with 100 g of fat. Calculate the iodine number of oleic acid.

[1]
a.

The chemical change in stored fats causes rancidity characterized by an unpleasant smell or taste.

Compare hydrolytic and oxidative rancidity.

[2]
b.

State one similarity and one difference in composition between phospholipids and triglycerides.

Similarity:

Difference:

[2]
c.

Markscheme

«one C=C bond»
«1 mole iodine : 1 mole oleic acid»

«  100 × 253.80 282.46 =» 89.85 «g of I2» ✔

NOTE: Accept “90 «g of I2»”.

a.

NOTE: Award [1] for any two sites or conditions from any of the four listed.
Accept “high temperature” for “heat”. Accept "lipase" for "enzyme".
Do not accept just “double bond”.
Accept “air” for “oxygen” and “UV/sun” for “light”.
Ignore any reference to heat/high temperature as a condition for oxidative.

b.

Similarity:

«derived from» propane-1,2,3-triol/glycerol/glycerin/glycerine

OR
«derived from» at least two fatty acids
OR
contains ester linkages
OR
long carbon chains ✔

NOTE: Do not accept “two fatty acids as both a similarity and a difference”.
Do not accept just “hydrocarbon/carbon chains”.

 

Difference:

phospholipids contain two fatty acids «condensed onto glycerol» AND triglycerides three
OR
phospholipids contain phosphate/phosphato «group»/residue of phosphoric acid AND triglycerides do not ✔

NOTE: Accept “phospholipids contain phosphorus AND triglycerides do not".
Accept “phospholipids are amphiphilic AND triglycerides are not” OR “phospholipids have hydrophobic tails and hydrophilic heads AND triglycerides do not”.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 10: Organic chemistry » 10.1 Fundamentals of organic chemistry
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Core » Topic 10: Organic chemistry » 10.2 Functional group chemistry
Options » B: Biochemistry » B.3 Lipids
Options » B: Biochemistry
Core » Topic 10: Organic chemistry
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