Chemistry · Thermochemistry
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The calorific value of H2(g) at STP is 12.78 KJ/L. Hence, the approximate standard enthalpy of formation of H2O is:
- A
-143 KJ
- B
-286 KJ
- C
Zero
- D
+286 KJ
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.
-143 KJ: This value is too low for the standard enthalpy of formation of water. The formation of H2O from hydrogen and oxygen is highly exothermic, typically releasing more energy than this option suggests.
-286 KJ: This is the correct value for the standard enthalpy of formation of H2O(l). The calculation based on the given calorific value at STP leads to this result, reflecting the exothermic nature of the reaction.
Zero: The standard enthalpy of formation cannot be zero for H2O because forming water from hydrogen and oxygen involves energy release, confirming an exothermic process.
+286 KJ: A positive value is incorrect for the standard enthalpy of formation of H2O. The reaction is exothermic, indicating a negative enthalpy change.
Tagged under Chemistry · Thermochemistry · 2021