18.7 Cell Potential and Free Energy in Galvanic Cells

Summary

This transcript explains the principles of a galvanic cell and its relation to electrochemical processes. It covers the conversion of chemical energy into electrical energy, the concept of electro-motive force (EMF), and how electrons move between the anode and cathode. The video also touches on the relationship between the cell potential (voltage) and Gibbs free energy, highlighting the key equation linking them. Standard conditions and the role of the standard hydrogen electrode in measuring cell potential are also discussed. The overall goal is to provide a deeper understanding of the thermodynamics of electrochemical reactions.

Takeaways

• 😀 The beauty of a galvanic cell is that it converts the chemical energy difference between reactants and products into electrical energy, which drives electrons.
• 😀 The concept of electro-motive force (EMF) is used to describe the electrical energy difference in a galvanic cell, where electrons move from a region of higher energy (anode) to lower energy (cathode).
• 😀 In a galvanic cell, oxidation occurs at the anode, making it the negative region, while reduction occurs at the cathode, which is the positive region.
• 😀 The movement of electrons is driven by the difference in energy between the anode and the cathode. Electrons naturally flow from the higher energy anode to the lower energy cathode.
• 😀 Electro-motive force (EMF) or cell potential quantifies the force that drives electrons from the anode to the cathode, measured in volts.
• 😀 A volt is defined as one joule per coulomb, and the cell potential relates to the energy needed to move charge through the cell.
• 😀 The relationship between cell potential (EMF) and Gibbs free energy (Delta G) links the thermodynamic tendency for a reaction to occur with the electrical energy generated.
• 😀 Delta G, representing the energy change in a reaction, is related to the cell potential through the equation Delta G = -nFE, where n is the number of electrons transferred and F is Faraday's constant.
• 😀 Under standard conditions, the standard cell potential (E° cell) can be calculated from the standard reduction potentials for oxidation and reduction reactions.
• 😀 The standard hydrogen electrode is used as a reference to measure the cell potential of other half-reactions, establishing a consistent point of comparison for different galvanic cells.

Q & A

What is the main function of a galvanic cell?

The main function of a galvanic cell is to convert chemical energy, in the form of a free energy difference between reactants and products, into electrical energy that can be used to drive electrons.

What is the role of electro-motive force (EMF) in a galvanic cell?

Electro-motive force (EMF) is the force that drives electrons from the anode (higher energy, negative region) to the cathode (lower energy, positive region). It represents the electrical energy difference between the two regions in the cell.

How do electrons move in a galvanic cell?

Electrons move spontaneously from the anode, where oxidation occurs (higher energy, negative region), towards the cathode, where reduction occurs (lower energy, positive region), due to the attraction between opposite charges.

What is the relationship between cell potential and Gibbs free energy (ΔG)?

Cell potential and Gibbs free energy are related; as cell potential increases, the Gibbs free energy decreases, meaning the reaction becomes more spontaneous. The equation linking them shows that a positive cell potential corresponds to a negative ΔG.

What does the SI unit 'volt' measure in the context of galvanic cells?

The volt measures the potential difference in a galvanic cell. It is defined as one joule per coulomb and represents the energy required to push one coulomb of charge through one volt of potential difference.

How is Faraday's constant (F) related to the charge in a galvanic cell?

Faraday's constant (F) represents the charge on one mole of electrons, equal to approximately 96,485 coulombs per mole. It allows the conversion of moles of electrons into charge, which is crucial for calculating cell potential.

What is the difference between standard and non-standard conditions for a galvanic cell?

Standard conditions refer to specific concentrations and pressures (e.g., 1.0 M for aqueous solutions, 1 atm for gases), which allow for consistent measurements and comparisons of cell potentials. Non-standard conditions occur when these values deviate from the standard.

Why is Hess's law important when calculating cell potential?

Hess's law is important because it allows the addition of the standard potentials for oxidation and reduction half-reactions to calculate the overall cell potential, making it easier to compute the voltage of a full redox reaction.

What is the standard hydrogen electrode (SHE) and why is it used?

The standard hydrogen electrode (SHE) is a reference electrode used to measure the potential of other half-reactions. It is defined at 0 volts and allows the measurement of other cell potentials relative to it.

How do standard reduction potentials differ from standard oxidation potentials?

Standard reduction potentials measure the tendency of a species to gain electrons and undergo reduction, while standard oxidation potentials measure the reverse process—how likely a species is to lose electrons and undergo oxidation. The oxidation potential is the negative of the corresponding reduction potential.