Question

Which substances have AH = 0 kJ/mol

Answer 1

Oxygen, bromine, iron, helium

Step-by-step 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\\eq 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 \\eq 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\\eq 0 kJ/mol`.