Skip to main content
Pearson+ LogoPearson+ Logo
Ch.19 - Chemical Thermodynamics
All textbooksBrown 14th EditionCh.19 - Chemical ThermodynamicsProblem 115
Chapter 19, Problem 115

Hydrogen gas has the potential for use as a clean fuel inreaction with oxygen. The relevant reaction is2 H21g2 + O21g2 ¡ 2 H2O1l2Consider 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 electricity directly by using fuel cells that operate at 85 °C.(a) Use data in Appendix C to calculate ∆H° and ∆S° forthe reaction. We will assume that these values do notchange appreciably with temperature.

Verified step by step guidance
1
Identify the balanced chemical equation for the reaction: \(2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(l)\).
Use Appendix C to find the standard enthalpies of formation (\(\Delta H_f^\circ\)) for each substance involved in the reaction: \(\text{H}_2(g)\), \(\text{O}_2(g)\), and \(\text{H}_2\text{O}(l)\).
Calculate the standard enthalpy change (\(\Delta H^\circ\)) for the reaction using the formula: \(\Delta H^\circ = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants})\).
Use Appendix C to find the standard molar entropies (\(S^\circ\)) for each substance involved in the reaction: \(\text{H}_2(g)\), \(\text{O}_2(g)\), and \(\text{H}_2\text{O}(l)\).
Calculate the standard entropy change (\(\Delta S^\circ\)) for the reaction using the formula: \(\Delta S^\circ = \sum S^\circ(\text{products}) - \sum S^\circ(\text{reactants})\).

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
4m
Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
Recommended video:
Guided course
02:34
Enthalpy of Formation

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.
Recommended video:
Guided course
01:46
Entropy in Phase Changes

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.
Recommended video:
Guided course
01:51
Gibbs Free Energy of Reactions
Related Practice
Textbook Question

The reaction SO2(g) + 2 H2S(g) ⇌ 3 S(s) + 2 H2O(g) is the basis of a suggested method for removal of SO2 from power-plant stack gases. The standard free energy of each substance is given in Appendix C. (b) In principle, is this reaction a feasible method of removing SO2?

Textbook Question

The reaction SO2(g) + 2 H2S(g) ⇌ 3 S(s) + 2 H2O(g) is the basis of a suggested method for removal of SO2 from power-plant stack gases. The standard free energy of each substance is given in Appendix C. (d) Would you expect the process to be more or less effective at higher temperatures?

Textbook Question

When most elastomeric polymers (e.g., a rubber band) are stretched, the molecules become more ordered, as illustrated here:

Suppose you stretch a rubber band. (a) Do you expect the entropy of the system to increase or decrease?