PAG 08 – Electrochemical Cells: Module 1: Practical Skills in Chemistry: Chemistry OCR A A Level

What must a half cell contain?

An element in 2 oxidation states.

Define the terms oxidation and reduction.

Oxidation - loss of electrons

Reduction - gain of electrons

Why must metal electrodes be cleaned with sandpaper before creating an electrochemical cell?

To remove any metal oxide that has formed on the surface and improve electrical conductivity.

Describe the movement of electrons in an electrochemical cell.

Electrons flow through the wire from the positive electrode to the negative electrode.

In an standard Zn/Zn^2+ cell, what is the role of potassium nitrate solution?

Potassium nitrate is used to saturate a piece of filter paper so that it can be used as a salt bridge.

Why is a salt bridge used in an electrochemical cell?

To maintain the charge balance and complete the circuit.

This is because negative electrons are moving from one half-cell to another. Without the salt bridge, positive charge would build up in the half cell containing the anode and negative charge would build up in the half cell containing the cathode. This would cause the reaction to stop.

Why must an inert salt be used in the salt bridge?

So that the salt doesn’t react with the solutions and alter their concentrations. If a reactive salt was used, the cell potential would change.

What moves across the salt bridge?

Ions

In an electrochemical cell, each metal electrode is placed in a solution containing what?

Its ions

E.g. solid copper is placed in a solution of copper(II) sulfate

After setting up an electrochemical cell, the reading on the voltmeter is negative. What should be done?

Switch the connection of the wires so the electrodes are the right way round.

What is the formula for the electromotive force (EMF)?

EMF = E_R − E_L

where:

E_R - right hand cell, where the reduction occurs, and

E_L - left hand cell, where oxidation occurs.

For a reaction to be thermodynamically feasible, what must the value of the EMF be?

Positive