Electrochemical Cells (SQA National 5 Chemistry): Revision Note

Building electrochemical cells

• An electrochemical cell (or just a 'cell') is a device that uses a chemical reaction to produce electricity.

• It is a way of converting chemical energy into electrical energy

• A cell is created by connecting two different conducting materials in a solution

• For National 5 Chemistry, there are three cells you need to know about:

  1. A simple cell

  2. The half cell-system

  3. Non-metal half-cells

A simple cell

• The simplest type of cell can be made by placing two different metals into an electrolyte

  • An electrolyte is a solution that can conduct electricity because it contains free-moving ions

  • Ionic solutions, like salt water or ammonium chloride solution, are electrolytes

• The two different metals are connected by a wire through a voltmeter

• When the metals are dipped into the electrolyte, a voltage is produced and a current flows

Example simple cell

The half cell-system

• To study the reactions happening in a cell more closely, chemists often set it up using two separate beakers, called half-cells

• A half-cell consists of a metal rod placed in a solution containing its own ions

  • For example, a copper rod in a solution of copper(II) sulfate

• To make a full cell, two different half-cells are connected:

  • A wire and voltmeter connects the two metal rods, allowing electrons to flow

  • An ion bridge (or salt bridge) connects the two solutions

The ion bridge

• The ion bridge is essential for the cell to work

  • It is usually a piece of filter paper or a tube filled with an ionic solution (like potassium nitrate)

• The ion bridge completes the circuit by allowing ions to move between the two half-cells

• This prevents a build-up of charge in either beaker, which would stop the flow of electricity

Non-metal half-cells

• Electricity can be produced even if one of the half-cells does not contain a metal

  • For example, a half-cell could be made with a solution of iodine and iodide ions

• Since there is no metal rod to act as an electrical conductor, a different material must be used for the electrode.

• In half-cells that do not contain a metal, a graphite rod is used as the electrode.

  • Graphite is chosen because it is unreactive and conducts electricity

A cell can be constructed using a zinc half-cell and an iodine half-cell

• Half-cell 1:

  • A zinc metal rod placed in a solution of zinc sulfate (containing Zn²⁺ ions)

• Half-cell 2:

  • A carbon rod placed in a solution containing both iodine and potassium iodide (containing I₂ molecules and I^- ions)

• The two half-cells are connected by a wire and voltmeter, and an ion bridge

How to determine the overall reaction

1. Find them in the electrochemical series (data booklet, page 10):

   • Zinc (Zn) is very high up the series

   • Iodine (I₂) is much lower down, below copper

2. Determine electron flow:

   • Electrons always flow from the species higher up the series to the one lower down

   • Therefore, electrons flow from the zinc rod to the carbon rod

3. Write the ion-electron equations:

• At the zinc electrode (oxidation):

  • Zinc is higher, so its equation is reversed

  • The zinc metal loses electrons and the rod will get smaller

Zn (s) → Zn²⁺ (aq) + 2e^-

• At the carbon electrode (reduction):

  • Iodine is lower, so its equation is used as written.

  • The carbon electrode simply provides a surface for the iodine molecules to gain electrons and turn into iodide ions

I₂ (aq) + 2e^- → 2I^- (aq)

• This complete circuit produces a voltage, demonstrating that a cell can function perfectly well with a non-metal half-cell