Question

For which substance does ∆Hf° = 0?

a. Exothermic reaction
b. Endothermic reaction
c. Substance at standard conditions
d. Substance at non-standard conditions

Answer 1

Substance at standard conditions have a ∆Hf° of 0. The provided reactions show that the first is exothermic and the second is endothermic based on the placement and sign of the energy value.

Step-by-step explanation:

The substance for which ∆Hf° = 0 is c. Substance at standard conditions. By definition, the standard enthalpy of formation (∆Hf°) of an element in its most stable form is zero under standard conditions (1 bar of pressure and 298.15 K temperature unless specified otherwise). Now to address whether each chemical reaction provided is exothermic or endothermic:

1. Exothermic reaction: 2H₂(g) + O₂(g) → 2H₂O(l) + 135 kcal. The presence of a positive number after the reaction indicates the release of energy, thus it is exothermic.
2. Endothermic reaction: N₂(g) + O₂(g) + 45 kcal → 2NO(g). The energy is written on the reactants side, which means the reaction requires an input of energy and is therefore endothermic.