Hydrogen gas has the potential for use as a clean fuel in reaction with oxygen. The relevant reaction is 2 H21g2 + O21g2 ¡ 2 H2O1l2 Consider two possible ways of utilizing this reaction as an electrical energy source: (i) Hydrogen and oxygen gases are combusted and used to drive a generator, much as coal is currently used in the electric power industry; (ii) hydrogen and oxygen gases are used to generate electricity directly by using fuel cells that operate at 85 °C. (a) Use data in Appendix C to calculate ∆H° and ∆S° for the reaction. We will assume that these values do not change appreciably with temperature.

Key Concepts

Enthalpy Change (∆H°)

Enthalpy change (∆H°) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ∆H° < 0) or endothermic (absorbs heat, ∆H° > 0). In the context of the combustion of hydrogen, calculating ∆H° helps determine the energy released when hydrogen reacts with oxygen to form water.

Entropy Change (∆S°)

Entropy change (∆S°) quantifies the degree of disorder or randomness in a system. A positive ∆S° indicates an increase in disorder, while a negative ∆S° suggests a decrease. For the reaction of hydrogen and oxygen forming water, understanding ∆S° is crucial for assessing the spontaneity of the reaction and its feasibility as an energy source.

Gibbs Free Energy (∆G°)

Gibbs free energy (∆G°) combines enthalpy and entropy to determine the spontaneity of a reaction at constant temperature and pressure. It is calculated using the equation ∆G° = ∆H° - T∆S°, where T is the temperature in Kelvin. A negative ∆G° indicates that the reaction can occur spontaneously, which is essential for evaluating the efficiency of hydrogen as a clean fuel source.