Chapter 21: Problem 65
What are \(E_{\text {cell }}^{\circ}\) and \(\Delta G^{\circ}\) of a redox reaction at \(25^{\circ} \mathrm{C}\) for which \(n=1\) and \(K=5.0 \times 10^{-6} ?\)
Short Answer
Expert verified
The standard cell potential \( E_{\text{cell}}^{\text{0}} \) is approximately -0.0316 V and the standard Gibbs Free Energy change \( \text{ΔG}^{\text{0}} \) is 3.05 kJ/mol.
Step by step solution
01
- Understand the Given Values
Given: Number of moles of electrons transferred, \( n = 1 \). Equilibrium constant, \( K = 5.0 \times 10^{-6} \). Temperature, \( T = 25^{\text{C}} \) (which is equivalent to 298 K).
02
- Use the Nernst Equation to Find Standard Cell Potential
The standard cell potential, \( E_{\text{cell}}^{\text{0}} \), can be related to the equilibrium constant, \( K \), using the following equation: \[ E_{\text{cell}}^{\text{0}} = \frac{RT}{nF} \times \text{ln}(K) \] Where: \( R \) is the gas constant (8.314 J/(mol·K)), \( T \) is the temperature in Kelvin (298 K), \( n \) is the number of moles of electrons transferred (1), and \( F \) is Faraday's constant (96485 C/mol).
03
- Calculate \( E_{\text{cell}}^{\text{0}} \)
Substitute the given values into the equation: \[ E_{\text{cell}}^{\text{0}} = \frac{8.314 \times 298}{1 \times 96485} \times \text{ln}(5.0 \times 10^{-6}) \] Calculate \( \text{ln}(5.0 \times 10^{-6}) \): \( \text{ln}(5.0 \times 10^{-6}) \thickapprox -12.206 \). Then, \[ E_{\text{cell}}^{\text{0}} \thickapprox \frac{8.314 \times 298}{96485} \times (-12.206) \thickapprox -0.0316 \text{ V} \]
04
- Use \( \text{E}_{\text{cell}}^{\text{0}} \) to Find the Standard Gibbs Free Energy Change
The standard Gibbs Free Energy change, \( \text{ΔG}^{\text{0}} \), can be calculated using the following relationship: \[ \text{ΔG}^{\text{0}} = -nF \times \text{E}_{\text{cell}}^{\text{0}} \] Given \( n = 1 \) and \( E_{\text{cell}}^{\text{0}} = -0.0316 \text{ V} \): Substitute in the values: \[ \text{ΔG}^{\text{0}} = -1 \times 96485 \times (-0.0316) \thickapprox 3050 \text{ J/mol} \] Or, \( \text{ΔG}^{\text{0}} \thickapprox 3.05 \text{ kJ/mol} \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Nernst Equation
The Nernst equation is central to understanding how cell potentials change with varying conditions. It connects the standard cell potential to non-standard conditions by accounting for concentrations of reactants and products. For the equation: \[E_{cell} = E_{cell}^{0} - \frac{RT}{nF} \times \text{ln}(Q)\]here,
- R is the gas constant (8.314 J/(mol·K))
- T is the temperature in Kelvin
- n is the number of moles of electrons transferred
- F is Faraday's constant (96485 C/mol)
- Q is the reaction quotient
Equilibrium Constant
The equilibrium constant (K) is a crucial value in chemical reactions, defining the ratio of product concentrations to reactant concentrations at equilibrium. It reflects the favorability of a reaction; a large K suggests a reaction favoring products, while a small K indicates reactants are favored. Standard cell potential (\(E^{\text{0}}_{\text{cell}}\)) can be linked to K via the Nernst equation by simplifying at equilibrium (\(E_{\text{cell}} = 0 = E^{0}_{\text{cell}} - \frac{RT}{nF} \times \text{ln}(K)\)) revealing \[E^{0}_{\text{cell}} = \frac{RT}{nF} \times \text{ln}(K)\]).Thus, knowing K allows us to calculate standard cell potential, fundamental in predicting reaction spontaneity and electrochemical cell behavior.
Gibbs Free Energy
Gibbs free energy (\(\text{ΔG}\)) is vital in predicting reaction spontaneity. It combines enthalpy and entropy to indicate if a process will occur spontaneously at constant temperature and pressure. Our key relationship: \[ \text{ΔG}^{0} = -nF \times E^{0}_{\text{cell}}\]allows converting the cell potential (\(E^{0}_{\text{cell}}\)) to ΔG, making it easier to gauge reaction feasibility. Positive ΔG: reaction non-spontaneous, negative ΔG: reaction spontaneous.Thus, electrochemistry provides a practical way to measure ΔG directly through cell potentials.
Electrochemistry
Electrochemistry studies the interplay of electricity and chemical changes. It covers processes where electrons transfer between substances, crucial in batteries, fuel cells, and electrolysis. Key elements include:
- Redox Reactions: Involving oxidation (loss of electrons) and reduction (gain of electrons)
- Electrochemical Cells: Devices converting chemical energy to electrical energy or vice versa.
- Electrodes: Anode (oxidation) and Cathode (reduction)
Faraday's Constant
Faraday's constant (F) links electric charge to the amount of substance undergoing redox reactions. Defined as approximately 96485 C/mol, F tells us how much electric charge is required per mole of electrons. It's essential in calculating electrochemical cell potentials and changes in Gibbs free energy. Key aspects:
- F simplifies calculations, connecting moles to electric charge.
- It aids in understanding the quantitative aspects of electrochemistry.