Ch.18 - Thermodynamics: Entropy, Free Energy & Equilibrium

Problem 141a

Chapter 18, Problem 141a

Consider the unbalanced equation: (a) Balance the equation for this reaction in basic solution.

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Identify the elements that are not balanced in the given chemical equation.

Assign oxidation numbers to each element in the reaction to identify the elements undergoing oxidation and reduction.

Write the half-reactions for the oxidation and reduction processes.

Balance each half-reaction for mass and charge. Start by balancing all elements except hydrogen and oxygen, then balance oxygen atoms by adding H2O, and hydrogen atoms by adding H+.

Since the reaction occurs in a basic solution, add OH- to both sides of the equation to neutralize the H+ ions, forming water. Combine and simplify the half-reactions to get the balanced equation.

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Key Concepts

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

Balancing Chemical Equations

Balancing chemical equations involves ensuring that the number of atoms for each element is the same on both sides of the equation. This is crucial for obeying the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. Balancing can be achieved by adjusting coefficients in front of the chemical formulas.

Basic Solution

A basic solution is one where the pH is greater than 7, indicating a higher concentration of hydroxide ions (OH-) than hydrogen ions (H+). In balancing reactions in basic solutions, hydroxide ions and water are often added to neutralize any hydrogen ions produced during the reaction, which helps in achieving a balanced equation.

Half-Reaction Method

The half-reaction method is a systematic approach to balancing redox reactions by separating the oxidation and reduction processes. Each half-reaction is balanced for mass and charge, and then combined to form the overall balanced equation. This method is particularly useful in basic solutions, where additional steps may be required to account for hydroxide ions.

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Related Practice

Textbook Question

A mixture of 14.0 g of N2 and 3.024 g of H2 in a 5.00 L container is heated to 400 °C. Use the data in Appendix B to calculate the molar concentrations of N2, H2, and NH3 at equilibrium. Assume that ∆H° and ∆S° are independent of temperature, and remember that the standard state of a gas is defined in terms of pressure.

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Textbook Question

The lead storage battery uses the reaction: (b) Calculate ∆G for this reaction on a cold winter's day (10 °F) in a battery that has run down to the point where the sulfuric acid concentration is only 0.100 M.

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Textbook Question

Chloroform has ΔHvaporization = 29.2 kJ>mol and boils at 61.2 °C. What is the value of ΔSvaporization for chloroform?

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Textbook Question

Consider the unbalanced equation: (b) Use the data in Appendix B and ΔG°f for IO₃^-(aq)= -128.0 kJ/mol to calculate ΔG° for the reaction at 25 °C.

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Textbook Question

Consider the unbalanced equation: I₂(s) → I^-(aq) + IO₃^-(aq) (d) What pH is required for the reaction to be at equilibrium at 25°C when [I^-] = 0.10M and [IO₃^-] = 0.50 M?

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Textbook Question

Methanol (CH₃OH) is made industrially in two steps from CO and H₂. It is so cheap to make that it is being considered for use as a precursor to hydrocarbon fuels, such as methane (CH₄):

Step 1. CO(g) + 2 H₂(g) S CH₃OH(l) ΔS° = - 332 J/K

Step 2. CH₃OH₁l₂ → CH₄(g) + 1/2 O₂(g) ΔS° = 162 J/K

(k) Calculate an overall ΔG°, ΔH°, and ΔS° for the formation of CH₄ from CO and H₂.

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