| Date | May 2016 | Marks available | 2 | Reference code | 16M.3.sl.TZ0.11 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Identify and Suggest | Question number | 11 | Adapted from | N/A |
Question
Hexane, C6H14, is not a suitable fuel for internal combustion engines as it has a tendency to auto-ignite, a cause of “knocking”.
(i) Hexane can be converted to different organic products in a reforming process. Identify one of these products.
(ii) Suggest why the product in (a)(i) has a lesser tendency to auto-ignite than hexane.
(i) Octane, C8H18, can undergo complete combustion under suitable conditions. Calculate the specific energy of octane, in kJg−1, using sections 1, 6 and 13 of the data booklet.
(ii) The specific energy of ethanol is 29.7kJg−1. Evaluate the addition of ethanol to octane (or its isomers) for use as a fuel in motor vehicles, giving one advantage and one disadvantage.
Advantage:
Disadvantage:
Coal can be heated with steam to produce synthetic natural gas. Formulate an equation to show the formation of methane, CH4(g), from coal, C(s), and steam, H2O(g).
Markscheme
2,2-dimethylbutane
OR
2,3-dimethylbutane
OR
3-methylpentane
OR
2-methylpentane
OR
cyclohexane
OR
methylcyclopentane
OR
benzeneAccept name or structural formula.
Accept any mono or poly-substituted cycloalkane with a total of six carbon atoms.
(ii)
increased branching (for acyclic hydrocarbons)/aromatic/aromaticity (for benzene)/cyclic hydrocarbon
OR
tertiary radicals are more stable
OR
higher octane ratingResponse in M1 must be consistent with molecule chosen in a (i)
(i)
«\(\frac{{5470}}{{114.26}}\)= » 47.9 «kJ g-1»
(ii)
Advantage:
ethanol does not produce particulates/has less incomplete combustion/CO/HCs/VOCs/is less polluting
OR
ethanol has high octane rating
OR
ethanol is renewable
OR
less environmental risks associated with spills for ethanol OR less carbon dioxide/CO2 produced if renewable energy source used
OR
economic advantages for countries that cannot produce crude oil
Accept any valid advantage and disadvantage.
Ignore any mention of cost.
Ignore any mention of NOx.
Disadvantage:
reduces efficiency/lowers specific energy/lowers energy density
OR
ethanol is more volatile/evaporates easily «than octane or its isomers»
OR
land that could be used for food production used to produce crops for ethanol
OR
biodiversity can be affected/loss of habitats «due to energy crop plantations
OR
phosphorus/nitrogen used in production has negative environmental effects
OR
modification of current engines «may be required» if ethanol used
Accept “if the fuel blend consists of nearly pure ethanol, engine is difficult to start in cold weather”.
Accept for disadvantage any engine-related problem affected by ethanol use (eg. effect on fuel pumps, incorrect fuel quantity indicators, older cars may not be suitable for ethanol use, etc.).
2C(s) + 2H2O(g) → CH4 (g) + CO2 (g)
OR
3C(s) + 2H2O(g) → CH4 (g) + 2CO(g)
Accept a two-step process.
