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Ch.20 - Electrochemistry
All textbooksBrown 14th EditionCh.20 - ElectrochemistryProblem 115
Chapter 20, Problem 115

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. 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?

Verified step by step guidance
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Step 1: Understand the chemical reaction involved. The reaction 2 H2(g) + O2(g) → 2 H2O(l) is an exothermic reaction, meaning it releases energy. This energy can be harnessed in different ways to generate electricity.
Step 2: Consider the efficiency of energy conversion in combustion. In the combustion method, hydrogen and oxygen gases are burned to produce heat, which is then used to drive a generator. This process involves multiple energy conversion steps: chemical energy to thermal energy, thermal energy to mechanical energy, and finally mechanical energy to electrical energy. Each step has inherent energy losses, typically making this method less efficient.
Step 3: Analyze the fuel cell method. Fuel cells convert chemical energy directly into electrical energy through electrochemical reactions. This direct conversion typically results in higher efficiency because it bypasses the intermediate steps of converting energy into heat and then into mechanical energy.
Step 4: Consider the operating conditions. Fuel cells operating at 85 °C are designed to optimize the electrochemical reaction efficiency. The lower operating temperature compared to combustion processes reduces energy losses associated with heat transfer and increases overall efficiency.
Step 5: Compare the overall efficiencies. Generally, fuel cells are more efficient than combustion methods because they convert chemical energy directly into electrical energy with fewer energy losses. Therefore, using fuel cells to generate electricity from hydrogen is likely to be more efficient than using the combustion method.

Key Concepts

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

Combustion Reaction

A combustion reaction involves the chemical reaction of a substance with oxygen, producing heat and light. In the case of hydrogen, the reaction with oxygen forms water and releases energy. This process is exothermic, meaning it releases more energy than it consumes, making it a potential source of power. However, the efficiency of energy conversion in combustion can be limited due to heat loss and other factors.
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Fuel Cells

Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electricity through a reaction with an oxidant, typically oxygen. Unlike combustion, fuel cells operate at lower temperatures and can achieve higher efficiencies because they do not rely on heat to generate power. This direct conversion minimizes energy loss, making fuel cells a promising technology for clean energy applications.
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Energy Efficiency

Energy efficiency refers to the ratio of useful output of services from an energy source to the input of energy. In the context of hydrogen energy, it is crucial to compare the efficiency of the combustion method versus fuel cells. Fuel cells generally have higher energy efficiency because they convert chemical energy directly into electrical energy, while combustion processes often waste energy as heat, leading to lower overall efficiency in power generation.
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Related Practice
Textbook Question

Aqueous solutions of ammonia (NH3) and bleach (active ingredient NaOCl) are sold as cleaning fluids, but bottles of both of them warn: 'Never mix ammonia and bleach, as toxic gases may be produced.' One of the toxic gases that can be produced is chloroamine, NH2Cl. (a) What is the oxidation number of chlorine in bleach? (active ingredient NaOCl) are sold as cleaning fluids, but bottles of both of them warn: “Never mix ammonia and bleach, as toxic gases may be produced.” One of the toxic gases that can be produced is chloroamine, NH2Cl. (b) What is the oxidation number of chlorine in chloramine? (d) Another toxic gas that can be produced is nitrogen trichloride, NCl3. What is the oxidation number of N in nitrogen trichloride?

Textbook Question

Aqueous solutions of ammonia 1NH32 and bleach (active ingredient NaOCl) are sold as cleaning fluids, but bottles of both of them warn: 'Never mix ammonia and bleach, as toxic gases may be produced.' One of the toxic gases that can be produced is chloroamine, NH2Cl. (e) Is N oxidized, reduced, or neither, upon the conversion of ammonia to nitrogen trichloride?

Textbook Question

Cytochrome, a complicated molecule that we will represent as CyFe2+, reacts with the air we breathe to supply energy required to synthesize adenosine triphosphate (ATP). The body uses ATP as an energy source to drive other reactions (Section 19.7). At pH 7.0 the following reduction potentials pertain to this oxidation of CyFe2+: O21g2 + 4 H+1aq2 + 4 e- ¡ 2 H2O1l2 Ered ° = +0.82 V CyFe3+1aq2 + e- ¡ CyFe2+1aq2 E°red = +0.22 V (a) What is ∆G for the oxidation of CyFe2+ by air? (b) If the synthesis of 1.00 mol of ATP from adenosine diphosphate (ADP) requires a ∆G of 37.7 kJ, how many moles of ATP are synthesized per mole of O2?

Textbook Question

Cytochrome, a complicated molecule that we will represent as CyFe2+, reacts with the air we breathe to supply energy required to synthesize adenosine triphosphate (ATP). The body uses ATP as an energy source to drive other reactions (Section 19.7). At pH 7.0 the following reduction potentials pertain to this oxidation of CyFe2+: O21g2 + 4 H+1aq2 + 4 e- ¡ 2 H2O1l2 Ered ° = +0.8 (b) If the synthesis of 1.00 mol of ATP from adenosine diphosphate (ADP) requires a ∆G of 37.7 kJ, how many moles of ATP are synthesized per mole of O2?