We’re being asked to determine the standard enthalpy change of formation (ΔH˚f) for acetone. Recall that ΔH˚rxn can be calculated from the enthalpy of formation (ΔH˚f) of the reactants and products involved:
We’re given the ΔH˚rxn for the combustion of 1 mole of acetone. The chemical equation for that reaction is:
CH3COCH3(l) + O2(g) → CO2(g) + H2O(l)
This equation is not yet balanced. To balance it, we have to make sure that the number of elements on both sides is equal.
Balance C: We have 3 C on the reactant side and 1 C on the product side – add a coefficient of 3 to CO2:
CH3COCH3(l) + O2(g) → 3 CO2(g) + H2O(l)
Balance H: We have 6 H on the reactant side and 2 H on the product side – add a coefficient of 3 to H2O:
CH3COCH3(l) + O2(g) → 3 CO2(g) + 3 H2O(l)
Balance O: We have 1 + 2 = 3 O on the reactant side and 3(2) + 3(1) = 9 O on the product side – add a coefficient of 4 to O2:
CH3COCH3(l) + 4 O2(g) → 3 CO2(g) + 3 H2O(l)
Acetone, CH3COCH3, is a liquid solvent. The enthalpy change at 25 ˚C and 1 atm for the complete burning of 1 mol of acetone in oxygen to give CO2(g) and H2O(l) is -1791 kJ. Calculate the standard enthalpy of formation of acetone.
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