Question

Which substances have AH = 0 kJ/mol

Answer 1

Answer:

Oxygen, bromine, iron, helium

Explanation:

$\Delta H^o_f$ is defined as the standard enthalpy of formation. By definition, the standard enthalpy of formation is equal to 0 kJ/mol for the substances in their standard states, that is, at room temperature and 1 atm pressure.

Simply speaking, looking at the substances given, we need to understand whether their states agree with what we expect to see at standard conditions (e. g., sodium is a metal, fluorine is a gas, bromine is a liquid at standard conditions). Those are substances consisting of just one type of atoms.

• Firstly, oxygen is a gas at standard conditions and it is diatomic, so its $\Delta H^o_f=0 kJ/mol$.

• Although nitrogen is a gas at standard conditions, it is diatomic, so $\Delta H^o_f\neq 0 kJ/mol$.

• Water is a liquid at standard conditions, however, it consists of two types of atoms, hydrogen and oxygen, so $\Delta H^o_f \neq 0 kJ/mol$.
• Bromine is a liquid at standard conditions, so $\Delta H^o_f=0 kJ/mol$.
• Iron is a solid at standard conditions, it's a metal, so $\Delta H^o_f=0 kJ/mol$.
• Helium is a gas at standard conditions, it belongs to noble gases, so $\Delta H^o_f=0 kJ/mol$.
• Sulfur is a solid at room conditions, however, the conformation it has is $S_8$ and not $S_6$, so $\Delta H^o_f\neq 0 kJ/mol$.