8 (1.11) Measuring EMF of electrochemical cell

Question 1

Setting up an electrochemical cell: zinc and copper

Answer 1

1. Clean a piece of copper and a piece of zinc using emery paper or fine grade sandpaper.
2. Degrease the metal pieces using some cotton wool and propanone.
3. Place the copper into a 100 cm3​ ​ beaker with about 50 cm3​ ​ of 1.0 mol dm-​3 CuSO4​ ​ solution.
   This is a half-cell.
4. Place the zinc into a 100 cm3​ ​ beaker with about 50 cm3​ ​ of 1.0 mol dm-​3​ ZnSO4​ solution.
   This is a half-cell.
5. Lightly plug one end of a plastic U-tube with cotton wool and fill the tube with the solution of 2.0 mol dm-​3​ sodium chloride solution (NaCl), then plug other end.
   
   6. Join the two beakers with U-tube so that the plugged ends are in the separate beakers.
      ● Ensure the ends are fully submerged in the solutions.
   7. Connect the Cu(​ s)|​ Cu2​ +(​ aq)​ and Zn(​ s)|​ Zn2​ +(​ aq)​ half-cells by connecting the metals using crocodile clips and leads. Include a voltmeter in the circuit in order to read off the voltage.

Question 2

Why sandpaper?

Answer 2

Removes the oxide layer on the metal.

Question 3

Why degrease

Answer 3

Grease could prevent the cell working as efficiently.

Question 4

What do salt bridges do?

Answer 4

● Allow the movement of ions between electrodes and to complete the circuit.
● Must not react (inert) with the electrolyte or ions in solution and must conduct electricity.
● Platinum is often used.
● Alternatively, a strip of filter paper saturated with NaCl solution can be used instead of a U-tube.

Question 5

Comparing electrode potentials of different metals:

Answer 5

1. Clean a piece of copper using emery paper or fine grade sandpaper.
   ● Removes the oxide layer on the metal.
   
   2. Connect to the positive terminal of a voltmeter using a crocodile clip and one of the leads.
      
      3. Cut a piece of filter paper to about the same area as the copper and moisten with sodium chloride solution. Place it on top of the metal.
      4. Connect a second lead to the voltmeter and use the crocodile clip to connect the lead to a piece of a different metal.
      5. Hold the metal against the filter paper. Record the voltage reading, including the sign
2. Repeat with different metals and compare the electrode potentials.

Question 6

What is an electrochemical cell

Answer 6

Two different half-cells are connected by a salt bridge, with their electrodes connected to a voltmeter (measures EMF/cell potential). This allows the flow of electrons.

The electrical energy generated is from chemical redox reactions.

Question 7

What does half cell consist of

Answer 7

-Half-cells are usually metal/metal ion (metal
electrode in the metal ion solution) or ion/ion
(ions that are the same element but with
different oxidation states, in solution).

-Each half cell will contain the chemical species
from one half of a redox reaction (redox half
equation).