Problem 110 - General Chemistry: Principles and Modern Applications

Question

Explain the important distinctions between each pair of terms: (a) half- reaction and overall cell reaction; (b) voltaic cell and electrolytic cell; (c) primary battery and secondary battery; (d) \(E_{\text {cell }}\) and \(E_{\text {cell }}^{\circ}\).

Step-by-Step Solution

Answer

Half-reaction is a partial reaction, either oxidation or reduction, while overall cell reaction combines both. A voltaic cell generates current from a spontaneous reaction, while an electrolytic cell drives a non-spontaneous reaction. Primary batteries are non-rechargeable, converting chemical energy into electricity. Secondary batteries are rechargeable, with reversed chemical processes. \(E_{\text {cell}}\) is cell potential under non-standard conditions, while \(E_{\text {cell}}^{\circ}\) is under standard conditions.

Step 1: Half-reaction and Overall Cell Reaction

A half-reaction is either the oxidation or reduction that occurs in a cell, split into two parts for ease of analysis. In contrast, the overall cell reaction is the combination of the oxidation half-reaction and the reduction half-reaction. It represents the net chemical process in an electrochemical cell.

Step 2: Voltaic Cell and Electrolytic Cell

A voltaic cell, also known as a galvanic cell, generates an electrical current produced by spontaneous redox reactions. On the other hand, an electrolytic cell uses electrical energy from an external source to drive a non-spontaneous redox reaction.

Step 3: Primary Battery and Secondary Battery

A primary battery, also known as a disposable battery, can convert chemical energy into electrical energy directly until its reactants are exhausted. It’s not rechargeable. A secondary battery, on the other hand, is a rechargeable battery. It can be reused multiple times as the chemical processes inside them are reversible.

Step 4: \(E_{\text {cell}}\) and \(E_{\text {cell}}^{\circ}\)

\(\(E_{\text {cell}}\)\) is the cell potential under non-standard conditions. It can be calculated using the Nernst equation. \(\(E_{\text {cell}}^{\circ}\)\), on the other hand, is the cell potential under standard conditions, which means a temperature of 298K, a pressure of 1 atm for gases and concentrations of 1 M for solutions.

Key Concepts

Half-reaction

In the world of electrochemistry, understanding half-reactions is essential. Think of a half-reaction as part of a bigger picture, focusing on just one side of a chemical process. It represents either the oxidation part where electrons are lost, or the reduction part where electrons are gained. In cases of electrochemical cells, we split reactions into half-reactions to better analyze and understand the flow of electrons.
When combined, these half-reactions form the overall cell reaction, embodying the complete process happening in the cell. This complete reaction brings together the changes in both electrodes to illustrate the total chemical transformation as electrons move from one substance to another.

Voltaic cell

Voltaic cells, also called galvanic cells, are devices that transform chemical energy into electrical energy via spontaneous redox reactions. The beauty of these cells lies in their ability to harness natural chemical processes to produce power.
A simple example is the battery in your remote or flashlight. Inside a voltaic cell, reactions occur spontaneously to push electrons through an external circuit, providing the electric current that powers devices.

• Anode: The side where oxidation occurs, losing electrons
• Cathode: The side where reduction happens, gaining electrons

In nature, voltaic cells work on their own without needing additional energy input to function.

Primary and secondary batteries

Primary and secondary batteries make up the two major types of batteries we commonly use. A primary battery is for one-time use only. Once the chemical reactants are used up, the battery becomes dead and must be disposed of. Think of non-rechargeable alkaline batteries.
In contrast, secondary batteries are designed for reusability. These rechargeable batteries can have their chemical reactions reversed by applying electrical energy. This reversibility allows you to use them repeatedly, making them more environmentally friendly over the long term. Common examples include lithium-ion batteries in smartphones and laptops.

Cell potential

Cell potential, often denoted as \(E_{\text{cell}}\), is a measure of the voltage or electromotive force of an electrochemical cell. It tells us how much energy per unit charge is available from a cell to do work. It's important to differentiate between standard and non-standard conditions:

• Standard cell potential \(E_{\text{cell}}^{\circ}\): This is measured under ideal conditions—298K temperature, 1 atm pressure, and 1 M concentration for solutions. It's like measuring the cell's potential under best-case scenarios.
• Non-standard cell potential \(E_{\text{cell}}\): This takes real-life conditions into account, such as different temperatures and concentrations. The Nernst equation allows us to calculate this value, adapting it to specific conditions, which can help in predicting how a cell will perform in actual use.

Understanding cell potential is key for designing batteries and predicting their efficiency in powering devices.