RQP 8: Measuring the EMF of an electrochemical cell

Question 1

Describe the practical method of measuring the EMF of a zinc-copper cell

Answer 1

• Clean the zinc and copper foils with emery before use.
• Degrease the metal using some cotton wool and propanone
• Place the copper strip into a 100cm^3 beaker with about 50cm^3 of 1 moldm^-3 CuSO4 solution.
• Place the zinc strip into a 100cm^3 beaker with about 50cm^3 of 1 moldm^-3 ZnSO4 solution.
• Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge
• Connect the Cu(s)|Cu2+(aq) and Zn(s)|Zn2+(aq) half-cells by connecting the metals using the crocodile clips and leads provided to the voltmeter.

Question 2

What is the purpose of the salt bridge

Answer 2

To connect the electrical circuit and allow ions to move between the solutions.

Question 3

What makes a salt bridge and what conditions must it satisfy

Answer 3

• A salt bridge is made from a piece of filter paper soaked in a salt soolution, usually potassium nitrate.
• It can also be a glass U tube containing a salt solution plugged with cotton wool.
• The salt should be unreactive with the electrode and electrode solutions.

Question 4

Why is a metal wirse not used as a salt bridge to connect the two electrical circuits

Answer 4

A wire is not used because the metal wirse would set up its own electrode system with the solutions.

Question 5

Why is a platinum electrode used when a half cell does not contain a conducting metal

Answer 5

Platinum is unreactive and can conduct electricity.