The standard enthalpy of combustion of graphite at 25oC is -393.51 kJ mol-1 and that of a diamond is -395.41 kJ mol-1 The enthalpy change for conversion of graphite to diamond is:
Correct answer: D. +1.9
- A. -1.91
- B. +2.1
- C. -2.1
- D. +1.9
Explanation
Here, we can use the equation: ΔH (graphite → diamond) = ΔH (graphite → CO₂) - ΔH (diamond → CO₂) ΔH = -393.51 kJ mol⁻¹ - (-395.41 kJ mol⁻¹). ΔH = +1.9 kJ mol. This indicates that the conversion from graphite to diamond requires an input of energy, hence the positive sign is correct.
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Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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