The standard enthalpy of formation of hydrogen bromide (HBr) is -36.40 kJ/mol.

This value is for the formation of one mole of HBr from its elements in their standard states, which are:

* Hydrogen (H₂) as a gas

* Bromine (Br₂) as a liquid

The reaction for the standard enthalpy of formation of HBr is:

```

1/2 H₂(g) + 1/2 Br₂(l) → HBr(g) ΔHf° = -36.40 kJ/mol

```