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IB DP Chemistry B.3 Lipids HL Paper 3

Question

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:

(b) Calculate the iodine number for ozubondo acid, \(C_{21}H_{33}COOH\).

\(M_r\) = 330.56

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

Answer/Explanation

Answer:

(a) 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

(b) 5 C=C
«100 g/330.56 g \(mol^{-1}\) x 5 x 253.8 g \(mol^{-1}\) =» 383.89 «g \(I_2\)»

(c) 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

Question

Lipids and carbohydrates play an essential role in the body.

(a) (i) State the general formula of a carbohydrate.
(ii) Fructose is a carbohydrate. Determine the energy, in kJ, released by the respiration of 10.5g of fructose, \(C_6H_{12}O_6\).
Enthalpy of (\(ΔH^{\theta }_c\)) of fructose = -2810kJ \(mol^{-1}\)

(b) Glucose, an isomer of fructose, exists as two isomeric ring forms. Annotate the diagram below to complete the structure of β-glucose. Use section 34 of the data booklet.

(c) Cellulose is a polymer of β-glucose. Describe the importance of cellulose in the human diet.

(d) Lipids and carbohydrates both release energy in the body.
(i) Write balanced equations for the complete oxidation of fructose, \(C_6H_{12}O_6\), and the triglyceride tristearin, \(C_{57}H_{110}O_6\).

\(C_6H_{12}O_6\):
\(C_{57}H_{110}O_6\):

(ii) Predict, giving one reason, whether 10.5g of the triglyceride tristearin would release more or less energy than 10.5g of fructose when completely oxidized.

(e) Describe the chemical composition of phospholipids and their function in the body, other than as energy storage.

Chemical composition:
Function:

Answer/Explanation

Answer:

(a) (i) \(C_x(H_2O)_y\)
(ii) «n=10.5g/180.18g \(mol^{-1}\)=» 0.0583 «mol» 
«0.0583 mol x 2810 kJ \(mol^{-1}\) =» 164 «kJ»

(b)

(c) «provides dietary» fibre/roughage

(d) (i)

(ii) more AND contains less oxygen/contains more carbon/more reduced

(e) Chemical composition:
glycerol AND «two» fatty acids AND «one» phosphate
Function:
cell/plasma membrane

Question 

The main fatty acid composition of cocoa butter and coconut oil is detailed below.

a. The melting points of cocoa butter and coconut oil are $34^{\circ} \mathrm{C}$ and $25^{\circ} \mathrm{C}$ respectively.
Explain this in terms of their saturated fatty acid composition.
b. Fats contain triglycerides that are esters of glycerol and fatty acids. Deduce an equation for the acid hydrolysis of the following triglyceride.

c. The addition of partially hydrogenated cocoa butter to chocolate increases its melting point and the content of trans-fatty acids (trans-fats).
Outline one effect of trans-fatty acids on health.

▶️Answer/Explanation

Markscheme
a. coconut oil has higher content of lauric/short-chain «saturated» fatty acids
OR
cocoa butter has higher content of stearic/palmitic/longer chain «saturated» fatty acids $[\boldsymbol{V}]$
longer chain fatty acids have greater surface area/larger electron cloud [ $[\boldsymbol{V}]$
stronger London/dispersion/instantaneous dipole-induced dipole forces «between triglycerides of longer chain saturated fatty acids» [ $\boldsymbol{C}$ ]
Note: Do not accept arguments that relate to melting points of saturated and unsaturated fats.

b.

correct products [ $\boldsymbol{U}]$
correctly balanced [ $[$ ]
c. Any one of:
«increased risk of» coronary/heart disease [ [ ]
«increased risk of» stroke $[\boldsymbol{V}]$
«increased risk of» atherosclerosis $[\boldsymbol{V}]$
«increased risk of type-2» diabetes
$[\boldsymbol{v}]$
increase in LDL cholesterol
decrease in HDL cholesterol
[ $\checkmark]$
«increased risk of» obesity

 

 

 
 

 

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