Question

Which of the following substances have an enthalpy of formation equal to zero? a. ${\mathrm{Cl}}_2(g)$ b. ${\mathrm{H}}_2(g)$ c. ${\mathrm{N}}_2(l)$ d. $\mathrm{Cl}(g)$

Short answer

The substances with an enthalpy of formation equal to zero are a. ${\mathrm{Cl}}_2(g)$ and b. ${\mathrm{H}}_2(g)$.

Step-by-step solution

Definition of enthalpy of formation

Enthalpy of formation is the change in enthalpy during the formation of 1 mole of a compound from its constituent elements, with all substances in their standard states. The standard enthalpy of formation of an element in its most stable form is considered to be zero.

Analyzing each substance

a. ${\mathrm{Cl}}_2(g)$: This substance is in its elemental form and it's a gas, which is the most stable form of chlorine. Therefore, its enthalpy of formation is zero. b. ${\mathrm{H}}_2(g)$: This substance is in its elemental form and it's a gas, which is the most stable form of hydrogen. Therefore, its enthalpy of formation is zero. c. ${\mathrm{N}}_2(l)$: Nitrogen in its elemental form is ${\mathrm{N}}_2(g)$, not a liquid. Hence, its enthalpy of formation is not zero as it's not in its most stable form. d. $\mathrm{Cl}(g)$: The most stable form of chlorine is ${\mathrm{Cl}}_2(g)$. As a result, the enthalpy of formation for this substance is not zero.

Conclusion

The substances with an enthalpy of formation equal to zero are a. ${\mathrm{Cl}}_2(g)$ and b. ${\mathrm{H}}_2(g)$.