Choosing a Fuel for a Small Pump Engine
A science club compares ethanol and petrol as fuels for a small four-stroke engine used to drive a water pump. Burning 1.00 g of ethanol raises the temperature of 100 g of water by 28.0 °C. Burning 1.00 g of petrol raises the temperature of 100 g of water by 45.0 °C under the same conditions. The club also finds that ethanol costs more per litre, is renewable, and produces only carbon dioxide and water when burned completely. Petrol is a mixture of hydrocarbons with an average formula C₈H₁₈. The teacher says the engine starts easily on petrol but runs badly on ethanol and that ethanol is described as 'carbon neutral'. The club must recommend a fuel for the pump, taking into account energy released, carbon dioxide emissions, renewability, cost and engine suitability. The specific heat capacity of water is 4.2 J/g°C; M(ethanol) = 46; M(C₈H₁₈) = 114.
1. Calculate the energy released per gram and per mole for each fuel from the water-heating data. State one reason these values underestimate the true enthalpy of combustion.
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Use q = mcΔT, with m = 100 g water and c = 4.2 J/g°C. Ethanol: q = 100 × 4.2 × 28.0 = 11,760 J = 11.76 kJ for 1.00 g. Per mole: 11.76 × 46 = 541 kJ/mol. Petrol: q = 100 × 4.2 × 45.0 = 18,900 J = 18.9 kJ for 1.00 g. Per mole: 18.9 × 114 = 2.15 × 10^3 kJ/mol. These are lower than data-book values because heat is lost to the apparatus and surroundings, combustion may be incomplete, and not all heat reaches the water. The comparison is still fair because the same apparatus and method were used.
2. Explain, in terms of bonds and oxygen content, why ethanol releases less energy per gram than petrol.
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Petrol hydrocarbons such as C8H18 contain only carbon and hydrogen and have a high proportion of C-C and C-H bonds per gram. Burning them forms many strong C=O and O-H bonds, releasing a lot of energy. Ethanol, C2H5OH, is already partly oxidised: one carbon is bonded to oxygen, so it has fewer C-H bonds and is less reduced. A gram of ethanol therefore needs less oxygen and releases less energy on complete combustion. Per mole the comparison is less fair because the molecules are different sizes, but per gram the energy density is lower.
3. The teacher calls ethanol 'carbon neutral'. Evaluate that claim and recommend a fuel for the pump, giving conditions.
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'Carbon neutral' means the CO2 released when ethanol burns is balanced by CO2 absorbed by the crops used to make it. That is only approximately true: fossil fuels are used for fertiliser, harvesting, distillation and transport, and land use may change. Ethanol also releases less energy per gram, so more fuel is needed for the same pump work; it can corrode parts and needs carburettor or jet adjustment. Petrol has higher energy density and suits the existing engine but is fossil-derived. Recommendation: use petrol in the existing pump unless the engine is modified; consider an ethanol blend or ethanol only if it is locally produced from waste, the engine is adjusted, and a life-cycle assessment shows a real CO2 saving. Cost and food-versus-fuel concerns also matter.