alleryStart learning →Using formation values of -278 for C2H5OH(l), -286 for H2O(l) and -394 kJ/mol for CO2(g), what is the heat of combustion of 1 mol of ethanol?
A-680 kJ/mol
B-958 kJ/mol
C-1368 kJ/mol
D-1924 kJ/mol
Answer & Solution
Correct answer: C. -1368 kJ/mol
1. The combustion equation is C2H5OH + 3O2 giving 2CO2 + 3H2O.
2. Oxygen is an element in its most stable form, so its formation value is zero.
3. Products give 2 times minus 394 plus 3 times minus 286.
4. That is minus 788 plus minus 858, which is minus 1646 kJ.
5. Reactants give 1 times minus 278 plus zero, that is minus 278 kJ.
6. Subtract: minus 1646 minus minus 278 equals minus 1646 plus 278.
7. The heat of combustion is therefore minus 1368 kJ per mole of ethanol.
8. Forgetting the coefficients of 2 and 3 would give minus 958 kJ, the common slip here.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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