GALVANIC AND ELECTROLYTIC CELLS

REDOX REACTIONS

OXIDATION REDUCTION

A reaction in which a substance loses electrons

A reaction in which a substance gains electrons

Mg → Mg2+ + 2e- oxidation reaction

reducing agent (donates electrons and so can cause reduction)

Cl2 + 2e- → 2Cl- reduction reaction

oxidising agent (accepts electrons and so can cause oxidation)

Mg → Mg2+ + 2e- Mg is oxidised (1)

Cl2 + 2e- → 2Cl- Cl2 is reduced (2)

Mg + Cl2 → MgCl2 Redox reaction

The electrons cancel each other out.

MgCl2 is an ionic compound (Mg2+ 2Cl-)

The equation shows 2 half reactions (1 and 2) that add to give the full redox reaction.

A coil of copper was placed in a silver nitrate solution

The solution became blue because copper ions were formed.

Solid silver deposited on the copper wire.

DIRECT ELECTRON TRANSFER

From the observations we can infer that:

Cu → Cu2+ + 2e-

Ag+ + e- → Ag

Electrons are transferred from the copper atoms on the piece of copper, to the silver ions in the silver nitrate solution.

This is a redox reaction.

This is a spontaneous reaction.

A GALVANIC CELL

THE ZINC-COPPER CELL

The following observations were made:

• the zinc plate decreases in mass.

• the copper plate increases in mass.

• the voltmeter reading indicates that electrons flow from the zinc plate to the copper plate.

From these observations we can infer that

The salt bridge:

• is a gel solution that connects the two electrolyte solutions that the metals dip into.

• acts as a transfer medium that allows ions to flow through but prevents the two solutions mixing.

• completes the circuit allowing electrons to flow through the connecting wire.

Zn(s) → Zn(aq)2+ + 2e-

Cu(aq)2+ + 2e- → Cu(s)

Zn(s) + Cu(aq)2+ → Zn (aq)

2+ + Cu(s)

The redox reaction for the cell:

The zinc dissolves – the mass of the zinc plate decreases.

The copper plate increases in mass because copper ions deposit as copper metal.

The electrodes:

Each metal strip is an electrode.

Each electrode with its associated electrolyte solution, is called a half cell.

The electrode where oxidation occurs is the ANODE. It has a negative charge.

The electrode where reduction occurs is the CATHODE. It has a positive charge.

Cell notation:

Zn / Zn2+ // Cu2+ / Cu

Zinc atoms are oxidised to Zn2+ ions (anode).

Cu2+ ions are reduced to copper atoms (cathode). Reduction written on the Right

Write the anode half reaction first – salt bridge (//) – then the

cathode half reaction

ELECTROLYTIC CELLS

Source of energy

Positive electrode Negative electrode

A battery is always required in this type of cell.

Chemical reactions occurring in the cell:

Na+ ions are attracted to the negative electrode, where they gain electrons.

Na+(aq) + e- → Na(s)

Reduction occurs at the negative electrode – the CATHODE.

Cl- ions are attracted to the positive electrode, where they lose electrons.

Cl-(aq) → Cl -(g) + e-

Oxidation occurs at the positive electrode – the ANODE.

Comparison of galvanic and electrolytic cells.

Galvanic cell Electrolytic cell

Produces an emf from a chemical reaction

Uses the emf of a cell to produce a chemical reaction

Chemical potential energy converted to electrical potential energy

Electrical potential energy converted to chemical potential energy

Positive terminal - cathode

Positive terminal - anode

Two containers each with an electrodes

One container with both electrodes dipping in

What should you be able to do?

• distinguish between galvanic and electrolytic cells.

• write equations for the reactions occurring in galvanic and electrolytic cells.

• explain the difference between oxidation and reduction and explain why the two types of reactions go together.

• label diagrams of galvanic and electrolytic cells.