If ∆H is the enthalpy change and ∆U is the change in internal energy accompanying a gaseous reaction, then:
Correct answer: D. ∆H is less than ∆U if the number of moles of gaseous products is less than the number of moles of gaseous reactants.
- A. ∆H is always less than ∆U
- B. ∆H is always greater than ∆U
- C. ∆H is less than ∆U if the number of moles of gaseous products is greater than the number of moles of gaseous reactants.
- D. ∆H is less than ∆U if the number of moles of gaseous products is less than the number of moles of gaseous reactants.
Explanation
The relationship is ΔH = ΔU + ΔngRT. If the number of moles of gas decreases during the reaction, Δng is negative. This makes the ΔngRT term negative, resulting in ΔH being less than ΔU.
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About Enthalpy
Enthalpy is the heat content of a system measured at constant pressure, and its change indicates whether a reaction or physical process is endothermic or exothermic. The topic covers state functions, enthalpy diagrams, standard enthalpy of formation, combustion and neutralisation, and the relationship between ΔH and internal energy change.
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