| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.sl.TZ1.7 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Calculate | Question number | 7 | Adapted from | N/A |
Question
Starch is a natural polymer of glucose.
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.
Markscheme
continuation bonds AND -O- attached to just one end AND both H-atoms on end carbons must be on the same side [✔]
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.
Type of linkage:
glycosidic [✔]
Note: 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= =» 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.
Examiners report
Hardly any students were able to draw the required repeating unit, but in contrast almost all knew that the monomers were joined by a glycosidic linkage.
It was very unusual to find a candidate who could give a correct equation for starch hydrolysis.
Nearly half the students correctly calculated the enthalpy change and some of these went on to find the value in kJ g-1. The most common mistakes were to use the mass of starch rather than the mass of water and adding 273 to the temperature change.
Whilst many could quote from 7b, that starch undergoes hydrolysis, very few linked this to a biochemical mechanism. Other factors, relating to the reduction of intermolecular forces between the polymer chains were also rarely encountered.
Syllabus sections
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18M.3.sl.TZ1.8a:
State the specific type of linkage formed between α-glucose fragments in both maltose and amylose.
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16N.3.sl.TZ0.9b:
The structures of two molecules, X and Y, are shown below.
(i) Justify why both these molecules are carbohydrates.
(ii) Distinguish between these molecules in terms of their functional groups.
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17M.3.sl.TZ2.10a:
Identify the functional groups which are present in only one structure of glucose.
- 19N.3.hl.TZ0.15c: Explain why maltose, C12H22O11, is soluble in water.
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17M.3.sl.TZ1.12a.ii:
Using the partial structure given, complete the structural formula of the molecule formed from the condensation of two cyclic -glucose molecules.
- 18N.3.sl.TZ0.8a: Name the type of link between the two monosaccharide residues.
- 18N.3.sl.TZ0.8b: Outline how the two monomer structures, galactose and glucose, differ.
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17M.3.sl.TZ1.12a.i:
Deduce the straight chain structure of ribose from its ring structure drawn in section 34 of the data booklet.
- 18N.3.hl.TZ0.10b: Outline how the two monomer structures, galactose and glucose, differ.
- 18N.3.hl.TZ0.10a: Name the type of link between the two monosaccharide residues.
- 19N.3.sl.TZ0.10c: Explain why maltose, C12H22O11, is soluble in water.
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19M.3.hl.TZ1.8a:
Draw the structure of the repeating unit of starch and state the type of linkage formed between these units.
Type of linkage:
-
19M.3.hl.TZ2.12c:
Sketch the cyclic structures of the two monosaccharides which combine to form sucrose.
-
19M.3.sl.TZ1.7a:
Draw the structure of the repeating unit of starch and state the type of linkage formed between these units.
Type of linkage:
-
19M.3.sl.TZ2.8b:
Sketch the cyclic structures of the two monosaccharides which combine to form sucrose.
-
19M.3.sl.TZ2.8a:
State the name of the functional group forming part of the ring structure of each monosaccharide unit.
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19M.3.hl.TZ2.12a:
State the name of the functional group forming part of the ring structure of each monosaccharide unit.
- 17N.3.sl.TZ0.9b: Draw the structure of galactose on the skeleton provided.
-
17M.3.sl.TZ1.11a:
List the building blocks of triglycerides and carbohydrates.
-
16N.3.sl.TZ0.9c:
Amylose is an unbranched polysaccharide composed of repeating units of glucose.
(i) Draw the structure of the repeating unit of amylose. Use section 34 of the data booklet.
(ii) Amylose is a major component of starch. Corn starch can be used to make replacements for plastics derived from oil, especially for packaging. Discuss one potential advantage and one disadvantage of this use of starch.
-
17M.3.sl.TZ2.10b:
Sucrose is a disaccharide formed from -glucose and β-fructose.
Deduce the structural formula of sucrose.
-
17M.3.sl.TZ1.12b:
Constructing models that allow visualizations of the stereochemistry of carbohydrates is essential to understand their structural roles in cells.
Describe how Haworth projections help focus on the position of attached groups.
-
18M.3.sl.TZ2.6e:
Determine, to the correct number of significant figures, the energy produced by the respiration of 29.9 g of C5H10O5.
ΔHc (C5H10O5) = 205.9 kJ mol−1
-
19M.3.hl.TZ1.8c:
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.
-
18M.3.sl.TZ1.8b:
A person with diabetes suffering very low blood sugar (hypoglycaemia) may be advised to consume glucose immediately and then eat a small amount of starchy food such as a sandwich. Explain this advice in terms of the properties of glucose and starch.
-
17M.3.sl.TZ1.11b.i:
The drain pipe of a kitchen sink can become clogged by fatty acids, such as linoleic acid, C18H32O2, but not by the trisaccharide, raffinose, C18H32O16, containing the same number of carbon atoms.
Explain why raffinose is far more water soluble than linoleic acid.
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20N.3.hl.TZ0.8b(i):
The diverse functions of biological molecules depend on their structure and shape.
Deduce the straight chain structure of deoxyribose from its ring structure drawn in section 34 of the data booklet.
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20N.3.sl.TZ0.7b(ii):
The diverse functions of biological molecules depend on their structure and shape.
Sucrose is a disaccharide formed in the reaction of glucose with fructose.
Identify the reaction type and the newly formed functional group that joins the monosaccharide units in the product.
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20N.3.sl.TZ0.7b(i):
The diverse functions of biological molecules depend on their structure and shape.
Deduce the straight chain structure of deoxyribose from its ring structure drawn in section 34 of the data booklet.
