| Date | May 2011 | Marks available | 1 | Reference code | 11M.2.sl.TZ1.4 |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate | Question number | 4 | Adapted from | N/A |
Question
Ethanol is used as a component in fuel for some vehicles. One fuel mixture contains 10% by mass of ethanol in unleaded petrol (gasoline). This mixture is often referred to as Gasohol E10.
Assume that the other 90% by mass of Gasohol E10 is octane. 1.00 kg of this fuel mixture was burned.
\[\begin{array}{*{20}{l}} {{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH(l)}} + {\text{3}}{{\text{O}}_2}{\text{(g)}} \to {\text{2C}}{{\text{O}}_2}{\text{(g)}} + {\text{3}}{{\text{H}}_2}{\text{O(l)}}}&{\Delta {H^\Theta } = - 1367{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \\ {{{\text{C}}_8}{{\text{H}}_{18}}{\text{(l)}} + {\text{12}}\frac{1}{2}{{\text{O}}_2}{\text{(g)}} \to {\text{8C}}{{\text{O}}_2}{\text{(g)}} + {\text{9}}{{\text{H}}_2}{\text{O(l)}}}&{\Delta {H^\Theta } = - 5470{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \end{array}\]
Calculate the mass, in g, of ethanol and octane in 1.00 kg of the fuel mixture.
Calculate the amount, in mol, of ethanol and octane in 1.00 kg of the fuel mixture.
Calculate the total amount of energy, in kJ, released when 1.00 kg of the fuel mixture is completely burned.
If the fuel blend was vaporized before combustion, predict whether the amount of energy released would be greater, less or the same. Explain your answer.
Markscheme
(10% 1000 g =) 100 g ethanol and (90% 1000 g =) 900 g octane;
\(n{\text{(ethanol)}} = 2.17{\text{ mol}}\) and \(n{\text{(octane)}} = 7.88{\text{ mol}}\);
\({{\text{E}}_{{\text{released from ethanol}}}} = (2.17 \times 1367) = 2966{\text{ (kJ)}}\);
\({{\text{E}}_{{\text{released from octane}}}} = (7.88 \times 5470) = 43104{\text{ (kJ)}}\);
total energy released \( = (2966 + 43104) = 4.61 \times {10^4}{\text{ (kJ)}}\);
Award [3] for correct final answer.
Accept answers using whole numbers for molar masses and rounding.
greater;
fewer intermolecular bonds/forces to break / vaporization is endothermic / gaseous fuel has greater enthalpy than liquid fuel / OWTTE;
M2 cannot be scored if M1 is incorrect.
Examiners report
Candidates were able to calculate the mass of ethanol and octane in the fuel mixture. The most common error here involved not expressing the answer in the requested units of grams. A number of candidates expressed answers in kg.
Many candidates were able to calculate the number of mole of ethanol and octane in (a) (ii) but errors in the calculation of molar mass were seen regularly. Candidates should also use the relative atomic masses, expressed to two decimal places as in the Periodic Table provided in the Data Table.
In part (a) (iii) some candidates multiplied incorrect numbers together or did not consider the number of moles of each part of the fuel mixture. Some candidates just added the enthalpies of combustion provided in the questions.
Part (b) was found to be very challenging by candidates. Very few candidates had the depth of understanding to answer this question adequately.
