Which substances have ah = 0 kj/mol

Oxygen, bromine, iron, helium

Explanation:

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 .Although nitrogen is a gas at standard conditions, it is diatomic, so .Water is a liquid at standard conditions, however, it consists of two types of atoms, hydrogen and oxygen, so .Bromine is a liquid at standard conditions, so .Iron is a solid at standard conditions, it's a metal, so .Helium is a gas at standard conditions, it belongs to noble gases, so .Sulfur is a solid at room conditions, however, the conformation it has is and not , so .

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