Question

which of the substances have a standard heat of formation (δhf°) of zero?
o2(g) at 25.0°c and 1 atm
c2h6(g) at standard conditions
cl2(g) at 2 atm
fe at 1200°c

Explanation:

The standard heat of formation ($\Delta H_{f}^{\circ}$) is zero for elements in their standard states. The standard state of a substance is its most stable form at 25.0°C (298 K) and 1 atm pressure.

• For $O_{2}(g)$: Oxygen gas ($O_{2}$) is the standard state of oxygen at 25.0°C and 1 atm, so $\Delta H_{f}^{\circ}(O_{2},g) = 0$.
• For $C_{2}H_{6}(g)$: Ethane ($C_{2}H_{6}$) is a compound, not an element. Compounds have non - zero $\Delta H_{f}^{\circ}$ values (except in some special cases not relevant here).
• For $Cl_{2}(g)$ at 2 atm: The standard state of chlorine is $Cl_{2}(g)$ at 1 atm. At 2 atm, it is not in the standard state, so $\Delta H_{f}^{\circ}

eq0$.

• For Fe at 1200°C: The standard state of iron is at 25.0°C (298 K) and 1 atm. At 1200°C, it is not in the standard state, so $\Delta H_{f}^{\circ}

eq0$.

Answer:

$O_{2}(g)$ at $25.0^{\circ}C$ and 1 atm