Concept:Thermochemical equations must reflect the correct sign convention indicating whether heat is released or absorbed. The formation of water from hydrogen and oxygen gases is an exothermic process.
Formula:$$ 2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(l)} $$
Solution:- The question text provides a magnitude of \( 205.5 \text{ kJ/mol} \) without a sign.
- Because the combustion of hydrogen to form water releases energy, the enthalpy change (\( \Delta H \)) must mathematically carry a negative sign to denote an exothermic process.
- Therefore, the correct representation of the enthalpy change for the reaction as written is \( -205.5 \text{ kJ mol}^{-1} \). (Note: While the universally accepted true value for \( \Delta H \) of this reaction is \( -571.6 \text{ kJ} \) or \( -285.8 \text{ kJ/mol} \) for formation, we must adapt to the magnitude given in the past paper).
Why other options are incorrect:A positive value (205.5) would falsely imply the formation of water is endothermic. Zero and 1 are mathematically arbitrary distractors.
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