The calorific value of H2(g) at STP is 12.78 KJ/L. Hence, the approximate standard enthalpy of formation of H2O is:
Correct answer: B. -286 KJ
- A. -143 KJ
- B. -286 KJ
- C. Zero
- D. +286 KJ
Explanation
The calorific value of H2(g) at STP is given as 12.78 kJ/L. At STP, 1 mole of gas occupies 22.4 L. Therefore, 1 L of H2 corresponds to 1/22.4 moles. The energy released from 1/22.4 moles of H2 is 12.78 kJ, meaning the energy released for 1 mole of H2 is 12.78 × 22.4 = 286.27 kJ. Thus, the approximate standard enthalpy of formation of H2O(l) is -286 kJ, confirming option B is correct. The other options either underestimate, suggest no change, or incorrectly indicate an endothermic process, which doesn't align with the nature of the reaction.
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About Thermochemistry and Energetics of Chemical Reactions
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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