The variation in energy of the lowest energy state of molecules during adsorption was evaluated for hydrogen chemisorption on iron films by treating the adsorption data with statistical mechanics as a theoretical tool. The validity of the evaluated value of this variation was examined. The application of this evaluated value to the heat of adsorption led to the microscopic understanding of the heat of adsorption on a quantitative level, and the interpretation of the observed temperature independence of the heat of adsorption.