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Q1 (a) Calorimetry results for a typical beer (3.5 % alcohol by volume, or 2.8% alcohol by mass) show a fuel value (energy content) of 1.1 kJ g-1.
i) Use the data given to calculate enthalpy of combustion of ethanol, C2H5OH(l).
ii) Hence, calculate the fuel value of the ethanol, in kJ per gram of beer.
iii) What accounts for the remaining energy content of the beer?
Concentration in beer
1.2 % by mass
0.3 % by mass
-278 2.8% by mass
(b) The manufacturers of a new engine want to know how efficient it is. They think that the petrol used to run the engine can be approximated well by octane and have asked you to tell them how much energy is available from burning octane, according to the following equation:
C8H18(l) + 12.5O2(g) → 8CO2(g) + 9H2O(l)
(i) Use the equation and the data given to calculate values of ΔrHo298 and ΔrUo298 for this reaction.
(ii) Comment on the relative magnitudes of the values you obtain. Substance ΔfHo298 /kJ mol-1
Octane C8H18(l) −249.9
Carbon dioxide CO2(g) −393.5
Water H2O(l) −285.8
Next, the chief scientist points out that the chemical equation as written does not exactly mimic the operating conditions of the engine. The exhaust gases are hot and contain water in the vapour phase; octane is also in the vapour phase (boiling point 399K).
(iii) Estimate ΔrHo for the reaction at 1000 K. The enthalpy of vaporisation of water, ΔvapHo, is +40.7 kJ mol-1 and the enthalpy of vaporisation of octane, ΔvapHo, is +41.5 kJ mol-1.
Substance Cp, m /J K-1 mol-1
Octane C8H18(l) 187.8
Oxygen O2(g) 29.4
Carbon dioxide CO2(g) 37.1
Water H2O(l) 75.3
THere you go!!!!!!!!!!