Final answer:
The elements must be in their most stable form under standard conditions (1 atm pressure and 25°C or 298.15 K) when calculating the standard free energy of a reaction, with the stable form having a standard free energy of formation (ΔG°f) of zero.
Step-by-step explanation:
When calculating the standard free energy (ΔG°) of a reaction, it's important to refer to elements in their most stable form under standard conditions. For elements, the standard state is considered at 1 atm pressure and 25°C (298.15 K), which is why the standard free energy of formation (ΔG°f) for any element in its stable form is defined as zero at this temperature.
This applies to elements like oxygen, which is most stable as O2(g), and carbon, which is most stable as graphite. Therefore, these states have ΔG°f values of zero. The lattice energy is significant in predicting the stability of ionic compounds, and the most stable configurations for atoms are filled energy levels like those found in noble gases.