Which of the following substances have an enthalpy of formation equal to zero? a. \(C l_{2}(g)\) b. \(\mathrm{H}_{2}(g)\) c. \(\mathrm{N}_{2}(l)\) d. \(\mathrm{Cl}(g)\)

The standard state of an element is defined as its most stable form at 1 atm of pressure and 298 K (25°C). For each substance, we will determine if it is in its standard state: a. \(Cl_{2}(g)\): Chlorine is a diatomic molecule and exists as a gas at standard conditions. Therefore, this substance is in its standard state. b. \(H_{2}(g)\): Hydrogen is also a diatomic molecule and exists as a gas at standard conditions. Therefore, this substance is in its standard state. c. \(N_{2}(l)\): Nitrogen is a diatomic molecule, but it is not in liquid form at standard conditions. It exists as a gas. Therefore, this substance is not in its standard state. d. \(Cl(g)\): Chlorine is a diatomic molecule, and it does not exist as a single atom in the gaseous phase at standard conditions. Therefore, this substance is not in its standard state.

Now that we have determined the standard states of each substance, we can apply the definition of enthalpy of formation. The enthalpy of formation of an element in its standard state is equal to zero. Based on this definition, we can conclude the following for each substance: a. \(Cl_{2}(g)\): This substance has an enthalpy of formation equal to zero because it is in its standard state. b. \(H_{2}(g)\): This substance has an enthalpy of formation equal to zero because it is in its standard state. c. \(N_{2}(l)\): This substance does not have an enthalpy of formation equal to zero because it is not in its standard state. d. \(Cl(g)\): This substance does not have an enthalpy of formation equal to zero because it is not in its standard state.

From the analysis above, we can conclude that the substances with an enthalpy of formation equal to zero are: a. \(Cl_{2}(g)\) b. \(H_{2}(g)\)