Problem 31 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) What is the definition of the volt? (b) Do all voltaic cells produce a positive cell potential?

Step-by-Step Solution

Answer

(a) The volt is the unit of electric potential difference or electromotive force (emf), defined as \( V = \frac{W}{Q} \), where V represents the potential difference, W represents the work done in moving the charge, and Q represents the quantity of charge. (b) In general, voltaic cells have either positive or negative cell potential. However, only those with a positive cell potential will act as voltaic cells and produce electrical energy from spontaneous redox reactions. Cells with a negative cell potential will function as electrolytic cells, consuming electrical energy to drive non-spontaneous redox reactions.

Step 1: (a) Definition of the Volt

The volt is the unit of electric potential difference or electromotive force (emf). A potential difference of one volt means that one coulomb of charge gains or loses one joule of energy when moving between two points in an electric circuit. Mathematically, it can be defined as: \( V = \frac{W}{Q} \) Where, - V (in volts) represents the electric potential difference or emf - W (in joules) represents the work done in moving the charge - Q (in coulombs) represents the quantity of charge

Step 2: (b) Cell Potential of Voltaic Cells

Voltaic cells (also known as galvanic cells) are an electrochemical device that produces electrical energy from spontaneous redox reactions. The cell potential, or electrode potential, is emf that drives the flow of electrons from the anode (the electrode where oxidation occurs) to the cathode (the electrode where reduction occurs) through an external circuit. In general, voltaic cells' cell potentials can be both positive or negative. A positive cell potential implies that the redox reaction is spontaneous and proceeds in the forward direction, whereas a negative cell potential means that the reaction is non-spontaneous and does not happen naturally in the specified direction. However, it is essential to note that a cell with a negative cell potential will operate as an electrolytic cell instead of a voltaic cell, consuming electrical energy to drive a non-spontaneous redox reaction. So, while all voltaic cells may not have a positive cell potential, only those with a positive potential will act as voltaic cells and produce electrical energy from spontaneous reactions.

Key Concepts

Volt Definition

The volt is a fundamental unit in the realm of electrochemistry and electricity. It's the standard unit used to measure electric potential difference or electromotive force (emf). When we say the potential difference is one volt, it implies that a single coulomb of electric charge gains or loses one joule of energy as it moves between two points in an electrical circuit.
In mathematical terms, it is represented by the formula:

• \( V = \frac{W}{Q} \)

where:

• \( V \) is the electric potential difference measured in volts,
• \( W \) is the work done or energy exchanged measured in joules,
• \( Q \) is the charge in coulombs.

This explanation helps us understand how energy is exchanged in circuits and lays the groundwork for learning more complex electrochemical processes.

Voltaic Cells

Voltaic cells, or galvanic cells, are a fascinating topic within electrochemistry. These electrochemical devices convert the energy of spontaneous redox reactions into electrical energy. They consist of two different metals connected by a salt bridge or porous membrane. The cell's operation hinges on two crucial components:

• The **anode**, where oxidation takes place and electrons are released,
• The **cathode**, where reduction occurs and electrons are accepted.

These cells are significant because they are the building blocks of batteries, harnessing and providing energy in a controlled manner. Unlike electrolytic cells, voltaic cells do not need external power sources to operate; they generate power on their own through spontaneous reactions.

Cell Potential

Cell potential, also known as electrode potential, is an essential factor in identifying how effective a voltaic cell is at generating electricity. The potential can be thought of as the 'pressure' driving electrons from one part of the cell to another. It can be either positive or negative depending on the direction of the redox reaction.

• A **positive cell potential** indicates a spontaneous redox reaction, suggesting that the process naturally occurs in the desired direction without needing external energy input.
• A **negative cell potential**, on the other hand, means the reaction is non-spontaneous. In a voltaic cell context, a negative potential would indicate that the cell cannot generate electrical energy without external intervention.

The value of cell potential is crucial in determining the efficiency and feasibility of electrochemical reactions in voltaic cells.

Redox Reactions

Redox reactions, or oxidation-reduction reactions, are at the heart of electrochemical processes in voltaic cells. They involve the transfer of electrons between two substances. One species undergoes oxidation (loss of electrons), while the other undergoes reduction (gain of electrons). These reactions can be split up into two half-reactions:

• Oxidation occurs at the anode: the substance loses electrons.
• Reduction occurs at the cathode: the substance gains electrons.

This electron flow, from anode to cathode, is what generates electricity in voltaic cells. Redox reactions are thus fundamental not only in generating electrical power but also in various biological and chemical processes beyond electrochemistry.