For which one of the following reactions is the value of δh°rxn equal to δh°f for the product?

a) 2ca (s) + o₂ (g) → 2cao (s)
b) c₂h₂ (g) + h₂ (g) → c₂h₄ (g)
c) 2c (graphite) + o₂ (g) → 2co (g)
d) 3mg (s) + n₂ (g) → mg₃n₂ (s)
e) c (diamond) + o₂ (g) → co₂ (g)

The reaction D has the value of ΔH°rxn equal to ΔH°f for the product.

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.