What is "which of the substances have a standard enthalpy of formation?

A standard enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a substance is formed from its constituent elements in their standard states (usually at 25°C and 1 atm pressure). Some common substances that have a standard enthalpy of formation include:

1. Carbon dioxide (CO2): ΔHf° = -393.5 kJ/mol
2. Water (H2O): ΔHf° = -285.8 kJ/mol
3. Methane (CH4): ΔHf° = -74.8 kJ/mol
4. Oxygen gas (O2): ΔHf° = 0 kJ/mol
5. Sodium chloride (NaCl): ΔHf° = -411 kJ/mol
6. Nitrogen gas (N2): ΔHf° = 0 kJ/mol
7. Sulfuric acid (H2SO4): ΔHf° = -814 kJ/mol

The standard enthalpy of formation is used to calculate the enthalpy change in a chemical reaction by subtracting the sum of the standard enthalpies of formation of the reactants from the sum of the standard enthalpies of formation of the products. It is an important thermodynamic property that helps to predict the feasibility and energetics of chemical reactions.