Draw the structure of the repeating unit of starch and state the type of linkage formed between these units.

Type of linkage:

Formulate the equation for the complete hydrolysis of a starch molecule, (C₆H₁₀O₅)_n.

Calculate the energy released, in kJ g⁻¹, when 3.49 g of starch are completely combusted in a calorimeter, increasing the temperature of 975 g of water from 21.0 °C to 36.0 °C. Use section 1 of the data booklet.

Explain how the inclusion of starch in plastics makes them biodegradable.

continuation bonds AND −O attached to just one end AND both H atoms on end carbons must be on the same side    [✔]

Type of linkage:
glycosidic    [✔]

Note: Square brackets not required.

Ignore “n” if given.

Mark may be awarded if a polymer is shown but with the repeating unit clearly identified.

Accept “ether”.

(C₆H₁₀O₅)_n (s) + nH₂O (l) → nC₆H₁₂O₆ (aq)     [✔]

Note: Accept “(n-1)H₂O”.

Do not award mark if “n” not included.

q = «mcΔT = 975 g × 4.18 J g^–1 K^–1 × 15.0 K =» 61 100 «J» / 61.1 «kJ»    [✔]

«heat per gram = $\frac{61.1\text{ kJ}}{3.49\text{ g}}$ =» 17.5 «kJ g^–1»    [✔]

Note: Award [2] for correct final answer.

Any two of:
carbohydrate grains swell/break plastic into smaller pieces    [✔]

inclusion of carbohydrate makes the plastic more hydrophilic/water soluble     [✔]

carbohydrates are broken down/hydrolysed/digested by bacteria/micro-organisms    [✔]

plastic becomes more accessible to bacteria as holes/channels are created in it     [✔]

«presence of» carbohydrate weakens intermolecular/London/dispersion forces between polymer chains in the plastic    [✔]

Note: Accept “starch” for “carbohydrate” throughout.

Do not accept “carbohydrates are broken down/hydrolyzed”.

Candidates were required to draw the structure of the repeating unit of starch given the ring structure as a starting point. This proved extremely difficult with very few candidates scoring a mark. Commonly, the structure of $a$-glucose was given, or an attempt was made to draw a polymer. Naming the type of linkage formed was answered well.

Also proved challenging, with many candidates unable to write an equation for the hydrolysis of a starch molecule (C₆H₁₀O₅)_n. The n was often omitted from otherwise correct equations or the product was incorrectly given as (C₆H₁₂O₆)_n.

The incorrect mass was frequently used when calculating energy released from combustion of starch in a calorimeter. Those who used the mass of water correctly frequently stopped when energy in kJ or J was calculated, and did not seem to notice that the question asked for the energy to be calculated in kJg⁻¹ so a further calculation was required.

Responses to explain how including starch in plastics makes them biodegradable were sometimes lacking in detail. Some candidates simply said that “starch is soluble in water”. Some said that “starch can be broken down/hydrolyzed” but omitted the key words “by bacteria or microorganisms”.