Electricity From Redox Reactions (HSC SSCE Chemistry): Revision Notes

Electricity From Redox Reactions

Introduction to electrochemistry

Chemistry and electricity are closely connected through a branch of science called electrochemistry. This field studies how chemical reactions can produce electrical energy and how electrical energy can drive chemical reactions.

In our everyday lives, we rely on portable sources of electricity called batteries. These power devices like torches, watches, toys, calculators, cameras, cordless shavers, car batteries, and mobile phones. All batteries share one important feature: they generate electricity through chemical reactions.

Understanding how batteries work requires us to explore the relationship between redox reactions and the production of electric current.

How redox reactions generate electricity

In redox reactions, electrons transfer from one reactant to another. An electric current is simply a flow of electrons moving through a conductor, such as a wire.

To generate electricity from a redox reaction, we need to set up the reaction in a special way. The key is to separate the oxidation and reduction processes so they occur at different locations. We then connect these locations with a wire, allowing electrons to flow through it. This arrangement creates an electric current. This is exactly how all batteries function.

The copper-silver galvanic cell

Let's examine a practical example that demonstrates how a redox reaction produces electricity. In this experiment, we create a simple galvanic cell using copper and silver.

Setup of the cell

The apparatus consists of:

• A strip of copper metal suspended in a beaker containing copper nitrate solution ($\text{Cu(NO}_3\text{)}_2$)
• A spiral of silver wire suspended in a separate beaker containing silver nitrate solution ($\text{AgNO}_3$)
• A U-tube connecting the two solutions, filled with potassium nitrate solution ($\text{KNO}_3$) and held in place by cotton wool plugs
• A voltmeter connected across the two metal pieces

The U-tube provides electrical contact between the two solutions, which is essential for the cell to operate properly.

Observations and measurements

When we connect a voltmeter across the copper and silver pieces, we observe:

• The silver wire measures approximately 0.5 V positive relative to the copper
• An ammeter shows that significant current flows through the circuit
• Electrons flow from the copper, through the external circuit, to the silver wire

If we allow the current to flow for some time, we can observe several chemical changes:

1. Metallic silver deposits on the silver wire (visible and confirmed by weighing)
2. The copper strip dissolves (confirmed by a decrease in its mass)
3. Silver ion concentration decreases in the silver nitrate solution
4. Copper ion concentration increases in the copper nitrate solution

These observations tell us that a chemical reaction is occurring, and this reaction is producing the electrical energy we measure.

Chemical reactions in the cell

Oxidation at the copper electrode

At the copper strip, copper atoms lose electrons and dissolve into solution as copper ions:

$\text{Cu(s)} \rightarrow \text{Cu}^{2+}\text{(aq)} + 2e^-$

This is an oxidation reaction because copper loses electrons.

Reduction at the silver electrode

At the silver wire, silver ions from the solution gain electrons and deposit as metallic silver:

$\text{Ag}^+\text{(aq)} + e^- \rightarrow \text{Ag(s)}$

This is a reduction reaction because silver ions gain electrons.

Overall cell reaction

To find the overall reaction, we combine these two half reactions. We need to balance the electrons, so we multiply the silver half reaction by two:

$\text{Cu(s)} + 2\text{Ag}^+\text{(aq)} \rightarrow \text{Cu}^{2+}\text{(aq)} + 2\text{Ag(s)}$