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If C (diamond) → C(graphite) + X kJ mol−1 C(diamond) + O2(g) → CO2(g) + Y kJ mol−1 C(graphite) + O2(g) → CO2(g) + Z kJ mol−1 At constant temperature. Then
- X = Y + Z
- −X = Y + Z
- X = −Y + Z
- X = Y − Z
Correct answer: X = Y + Z
Solution
The reaction involving the conversion of diamond to graphite (X) plus the combustion of both forms of carbon (Y for diamond and Z for graphite) reflects the principle of Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps. Therefore, the enthalpy change for converting diamond to graphite (X) must equal the sum of the enthalpy changes for the combustion of diamond (Y) and graphite (Z).
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