Br2 does not have a standard enthalpy of formation of 0 because

Br2 does not have a standard enthalpy of formation of 0 because

• A. It is liquid at room temperature
• B. It is solid at room temperature
• C. Standard enthalpy of formation is defined only for gases, not liquids
• D. Bromine forms a solid at room temperature

Answer: (A)

The standard enthalpy of formation of an element in its standard state is defined to be zero. For bromine at room temperature and 1 atm, the standard state is liquid bromine (Br2(l)). That means forming Br2(l) from its elements in their standard states requires no enthalpy change, so ΔHf° for Br2(l) is zero. The statement would only be true if bromine were not in its standard state; for example, Br2(g) would have a nonzero ΔHf° equal to the energy required to vaporize Br2(l) and form the gas.

The standard enthalpy of formation of an element in its standard state is defined to be zero. For bromine at room temperature and 1 atm, the standard state is liquid bromine (Br2(l)). That means forming Br2(l) from its elements in their standard states requires no enthalpy change, so ΔHf° for Br2(l) is zero. The statement would only be true if bromine were not in its standard state; for example, Br2(g) would have a nonzero ΔHf° equal to the energy required to vaporize Br2(l) and form the gas.