what two conditions must be met by a thermochemical equation so that its standard enthalpy change can be given the symbol delta Hf?
See my link in your later post.
In order for a thermochemical equation to have its standard enthalpy change denoted by the symbol ΔHf (standard enthalpy of formation), two conditions must be met:
1. The equation must represent the formation of one mole of the compound from its elements in their standard states. This means that the reactants should be the pure elements in their most stable form at a given temperature and pressure.
2. All reactants and products must be in their standard states. The standard state of a substance refers to its pure form at a pressure of 1 bar (or 1 atmosphere) and a specified temperature, which is typically 298 K (25°C).
By meeting these two conditions, the thermochemical equation can be considered a formation reaction, and the standard enthalpy change (ΔHf) represents the enthalpy change associated with the formation of one mole of the compound from its constituent elements in their standard states.