SRJC_H2_CHEM_P3_ANS

1 Turn Over] SRJC 2014 Prelim H2 Chemistry Paper 3 Answers 1 Singapore has hosted the Formula One Grand Prix since 2008. This car racing event sees race car drivers from around the world compete in a circuit to win the top honour of a race car champion. These race cars are fuelled by petrol, which is a mixture of hydrocarbons. One such hydrocarbon is pure liquid octane, C8H18. (a) (i) Define the term standard enthalpy change of combustion. Standard enthalpy change of combustion (Hc ) of a substance is the energy evolved when one mole of the substance is completely burnt in oxygen under standard conditions. OR C8H18 (l) + 2 25 O2 (g)  8CO2 (g) + 9H2O(l) Hc  < 0, standard conditions (ii) Use the data in the table below to calculate the standard enthalpy change of combustion of octane. Hence calculate the heat evolved per gram of octane. species ΔHf / kJ mol-1 C8H18(l) -269 CO2(g) -394 H2O(l) -286 C8H18(l) + 2 25 O2(g)  8CO2(g) + 9H2O(l) 8C(s) + 9H2(g) + 2 25 O2(g) By Hess’ Law, ΔHc = 269 + 8(-394) + 9(-286) = - 5460 kJ mol-1 OR ΔHc = ΔHf (products) – ΔHf (reactants) = 269 + 8(-394) + 9(-286) = - 5460 kJ mol-1 Heat evolved per gram of octane = 114 5460 = 47.9 kJ g-1 -269 8(-394) + 9(-286)

2 Turn Over] (iii) Methanol, CH 3OH, is also commonly used as a fuel for high -performance engines in race cars. Due to the high temperature in car engines, methanol is converted to the vapour state. Use the bond energies given in the Data Booklet to calculate the standard enthalpy change of combustion of methanol. Hence calculate the heat evolved per gram of methanol. CH3OH(l) + 2 3 O2(g)  CO2(g) + 2H2O(g) Bonds broken in the reactants : 3C-H, C-O, O-H, 2 3 O=O Bonds formed in the products: 2C=O, 4 O-H ΔHc = ∑BE(reactants) – ∑BE(products) = 3(410) + 360 + 460 + 2 3 (496) – 2(740) – 4(460) = -526 kJ mol-1 Heat evolved per gram of methanol = 32 526 = 16.4 kJ g-1 (iv) Using your answers in (a)(ii) and (a)(iii), suggest an advantage of using liquid octane over methanol as the fuel in race cars. It gives out more heat energy per gram of fuel. [6] (b) Methylbenzene is a component of lead -free petrol. It is synthesised by cyclising and dehydrogenating heptane. Methylbenzene can be made by catalytic reforming of heptane. This process causes straight chain hydrocarbons between 6 to 8 carbon atoms to rearrange into compounds containing benzene rings. This process requires a platinum catalyst, high temperature of 500 oC and pressure of about 20 atm. C7H16(g) C7H8(g) + 4H2(g) ∆H > 0 (i) State Le Chatelier’s Principle. Le Chatelier’s Principle states that when a system in equilibrium is disturbed , the system will react to counteract the effect of the change until a new equilibriu