9.2 Electrochemical Cells

Syllabus

What does this mean?

Electrochemical cells

-are either Voltaic or Electrolytic

Voltaic cells convert chemical energy --> electrical energy

Electrolytic cells convert electrical energy --> chemical energy

Both cells require 2 electrodes – a conductor to take charge into and out of the cell.

But the labelling of the electrodes depends on which type of cell we are studying.

Electrolysis – Electrolytic cells

British pupils will have studied at least some electrolysis

They should already know that an Anion is negative (A Negative ion).

Cations are therefore positive.

Since Anions are negative they are attracted to the positive electrode which is then called the Anode.

And since Cations are positive they are attracted to the negative electrode which is then called the Cathode.

At the anode the anions are converted to atoms

Eg 2O^2- --> O₂ + 4 e^-

Since electrons are lost, this must be Oxidation.

And, at the cathode the cations are converted to atoms

Eg Ca²⁺ + 2 e^- --> Ca

Since electrons are lost, this must be Reduction.

Questions

Write half equations for the electrolysis of the following compounds and state which process (oxidation/reduction) is happening.

1. Lead(II) Bromide

Cathode Pb²⁺ -->

Anode Br- -->

2. Iron(II) Chloride

Cathode Fe²⁺ -->

Anode Cl- -->

3. Iron(III) Iodide

Cathode -->

Anode -->

4. Copper(II) Oxide

Cathode -->

Anode -->

5. Aluminium Oxide

Cathode -->

Anode -->

Voltaic cells

Voltaic cells are what most people call batteries – don’t do that in an exam.

Learn OXIDATION always happens at the ANODE
Learn REDUCTION always happens at the CATHODE

The problem is that voltaic cells are carrying out the opposite processes to electrolytic ones – they are producing electrical energy from chemical energy, not using electrical energy to store chemical energy.

So unlike electrolytic cells – the Cathode is positive, the anode is negative!

Building Voltaic cells

If you place Copper in a solution of Copper ions, or Zinc in a solution of Zinc ions, nothing will happen.

It is possible that Copper atoms might become Copper ions

Cu_(s) --> Cu²⁺_(aq) + 2e^-

But the opposite process Cu²⁺_(aq) + 2e^- --> Cu_(s) would happen at the same speed and so there would be nothing to observe.

Even if we connect two half cells together we still see no change because the circuit is not complete.

Electrons can pass from one half-cell to the other from the metals but the current cannot return.

Another connection is needed to pass charge from one solution to the other.

Now a Voltage is measurable.

The Salt Bridge.

Physics (yawn!) says that an electric current is __________________________ ______________________________

The salt bridge allows the two solutions to be kept apart while allowing a current to flow as a movement of _________________________________

To make a quick cell we can use filter paper and a saturated solution of any very soluble salt – such as Potassium Chloride – so long as the ions don’t react with other ions in the solutions.

A typical set-up is sometimes called the Daniell Cell.

Notice electrons move one way, positive ions move the same way.

Negative ions move the opposite way.