(a) Which reaction is spontaneous in the hydrogen fuel cell: hydrogen gas plus oxygen gas makes water, or water makes hydrogen gas plus oxygen gas?

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Identify the reactions involved in the hydrogen fuel cell. The two reactions to consider are: 1) Hydrogen gas (H2) reacting with oxygen gas (O2) to form water (H2O), and 2) Water (H2O) decomposing into hydrogen gas (H2) and oxygen gas (O2).

Understand the concept of spontaneity in chemical reactions. A spontaneous reaction is one that occurs naturally under certain conditions without the need for continuous external energy.

Consider the standard Gibbs free energy change (\(\Delta G^\circ\)) for each reaction. The sign of \(\Delta G^\circ\) helps determine spontaneity: if \(\Delta G^\circ < 0\), the reaction is spontaneous; if \(\Delta G^\circ > 0\), the reaction is non-spontaneous.

Recall the formation of water from hydrogen and oxygen (Reaction 1) is exothermic and releases energy, typically indicating a negative \(\Delta G^\circ\) and thus spontaneity under standard conditions.

Analyze the decomposition of water into hydrogen and oxygen (Reaction 2). This reaction requires an input of energy (endothermic), suggesting a positive \(\Delta G^\circ\) and thus it is non-spontaneous under standard conditions.

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Key Concepts

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

Spontaneity of Reactions

A reaction is considered spontaneous if it occurs naturally under given conditions without external influence. This is often determined by the change in Gibbs free energy (ΔG); if ΔG is negative, the reaction is spontaneous. In the context of the hydrogen fuel cell, understanding spontaneity helps in identifying which direction of the reaction is favored energetically.

Gibbs Free Energy

Gibbs free energy is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It combines the system's enthalpy and entropy to predict the direction of chemical reactions. A negative ΔG indicates that the products of a reaction are favored, which is crucial for determining the spontaneity of the reactions in a hydrogen fuel cell.

Electrochemical Reactions

Electrochemical reactions involve the transfer of electrons between chemical species, which is fundamental in processes like those occurring in a hydrogen fuel cell. In these cells, hydrogen gas reacts with oxygen to produce water, releasing energy in the form of electricity. Understanding the nature of these reactions helps in analyzing which reaction direction is spontaneous and how energy is harnessed.

Related Practice

Textbook Question

In some applications nickel–cadmium batteries have been replaced by nickel–zinc batteries. The overall cell reaction for this relatively new battery is: 2 H₂O(l) + 2 NiO(OH)(s) + Zn(s) → 2 Ni(OH)₂(s) + Zn(OH)₂(s) (c) A single nickel–cadmium cell has a voltage of 1.30 V. Based on the difference in the standard reduction potentials of Cd²⁺ and Zn²⁺, what voltage would you estimate a nickel–zinc battery will produce? (d) Would you expect the specific energy density of a nickel–zinc battery to be higher or lower than that of a nickel–cadmium battery?

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Textbook Question

Li-ion batteries used in automobiles typically use a LiMn2O4 cathode in place of the LiCoO2 cathode found in most Li-ion batteries. (a) Calculate the mass percent lithium in each electrode material.

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Textbook Question

(b) Can the “fuel” of a fuel cell be a solid?

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Textbook Question

Li-ion batteries used in automobiles typically use a LiMn2O4 cathode in place of the LiCoO2 cathode found in most Li-ion batteries. (b) Which material has a higher percentage of lithium? Does this help to explain why batteries made with LiMn2O4 cathodes deliver less power on discharging?

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