Answer:
The reaction D has the value of ΔH°rxn equal to ΔH°f for the product.
Step-by-step explanation:
The ΔH°f for product is equal to ΔH°rxn when the reagents are in their elemental state (ΔH°f = 0) and form one mole of product.
We have to find the reagents that are in their elemental state and that only form one mole of product:
A) 2Ca (s) + O₂ (g) → 2CaO (s)
The reagents are in their elemental state but the reaction forms two mole of product.
B) C₂H₂ (g) + H₂ (g) → C₂H₄ (g)
C₂H₂ (g) is not in its elemental state.
C) 2C (graphite) + O₂ (g) → 2CO (g)
Graphite and Oxygen are in their elemental state but the reaction forms two mole of product.
D) 3Mg (s) + N₂ (g) → Mg₃N₂ (s)
Magnesium and Nytrogen are in their elemental state and the reaction forms one mole of product.
E) C (diamond) + O₂ (g) → CO₂ (g)
Diamond is not in its elemental state.