Hydrogen gas has the potential for use as a clean fuel inreaction with oxygen. The relevant reaction is2 H2(g) + O2(g) ---->2 H2O(l)Consider two possible ways of utilizing this reaction asan electrical energy source: (i) Hydrogen and oxygengases are combusted and used to drive a generator, muchas coal is currently used in the electric power industry;(ii) hydrogen and oxygen gases are used to generate electricitydirectly by using fuel cells that operate at 85 °C.(a) Calculate ΔH° and ΔS° forthe reaction. We will assume that these values do notchange appreciably with temperature. (b) Based on thevalues from part (a), what trend would you expect forthe magnitude of ΔG for the reaction as the temperatureincreases? (c) What is the significance of the change inthe magnitude of ΔG with temperature with respect to the utility of hydrogen as a fuel? (d) Based on the analysishere, would it be more efficient to use the combustionmethod or the fuel-cell method to generate electricalenergy from hydrogen?
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Hydrogen gas has the potential for use as a clean fuel in
reaction with oxygen. The relevant reaction is
2 H2(g) + O2(g) ---->2 H2O(l)
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) Calculate ΔH° and ΔS° for
the reaction. We will assume that these values do not
change appreciably with temperature. (b) Based on the
values from part (a), what trend would you expect for
the magnitude of ΔG for the reaction as the temperature
increases? (c) What is the significance of the change in
the magnitude of ΔG with temperature with respect to the utility of hydrogen as a fuel? (d) Based on the analysis
here, would it be more efficient to use the combustion
method or the fuel-cell method to generate electrical
energy from hydrogen?
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