Variation of Cell Potential in Zn - Cu Cell

The mechanism of the generation of electricity in a Daniell Cell is remarkable. Students learn how the concentration of the electrolyte can also alter cell potential. It is to be measured by doing an experiment. In this experiment, the variation of cell potential in Zn - Cu cell will be determined by changing the concentration of the electrolytes. Every part of this experiment will be elaborated so that you can easily understand when the standard electrode potential for Daniell cell is 1.1 V and when it varies. Study every section of this article and understand this experiment properly.

Variation in the Potential Difference in a Galvanic Cell: How to Conduct the Experiment?

The voltaic or galvanic cell used in this experiment contains a zinc and copper electrode connected with a voltmeter. The electrodes will carry electrons from the positive end to the negative end and the voltmeter will measure the cell potential. The cell potential is actually the difference we find between the potential of both the electrodes dipped in an electrolytic solution. The potential difference is measured in volts. It is also called the electromotive force of a cell when no circuit is connected to draw a current.

Aim of the Experiment

The aim of this experiment is to measure the variation of cell potential in Zn - Cu cell when the concentration of the electrolyte is changed at room temperature.

Theory of the Experiment

The inter-conversion of different forms of energies into each other has been studied before. In this experiment, you will study how chemical energy is converted into electrical energy in a Zn - Cu cell and how its potential is affected when the electrolyte concentration is altered.

For this experiment, we will use a Daniell Cell where zinc present in the electrode will react with copper sulphate in the electrolyte solution. The same reaction gives the precipitation of copper in a test tube but when carried in a Daniell Cell, it generates electricity. It becomes an electrochemical reaction conducted in a cell where chemical energy is converted into electrical energy. A flow of current will be noticed in the conductor connecting the electrodes. The potential difference between the electrodes will be measured by the voltmeter connected.

The reactions taking place in the electrodes in this galvanic cell are:

Zn Electrode: Zn (s) → Zn2+ (aq) + 2e–

Cu Electrode: Cu2+ (aq) + 2e– → Cu (s)

As you can see, zinc from the anode goes into the solution in the form of cations due to oxidation whereas copper goes into the solution in the form of copper atoms and precipitates. If we summarize both the electrode reactions, we will get:

Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)

The standard electrode potential for Daniell cell is 1.1 V. When the EMF is less than 1.1 V, you will observe that the electrons flow from the zinc anode to the copper cathode. You will also witness the deposition of copper in the cathode and the dissolution of zinc in the anode.

When the standard electric potential is more than 1.1 V, the current flows from the copper to the zinc electrode. The resultant reaction will also be the opposite.

Things Required For the Experiment

•  Cu Zn anode cathode

•  1 M solution of Zinc Sulphate (ZnSO4) and Copper Sulphate (CuSO4)

•  Beakers

•  Voltmeter

•  Salt bridge

•  Conducting wires

How to Set the Apparatus and Conduct the Experiment?

1. Preparing the salt bridge

2. Take a U-shaped glass tube. Heat agar-agar gel (20 gm) with potassium chloride (5 gm of KCl) in a clean beaker. Introduce the solution into the U tube by sucking the solution and let it cool.

3. Prepare 0.1 M solution of Zinc Sulphate and Copper Sulphate and put them in two separate beakers.

4. Put the salt bridge in such a way that it connects the electrolytes in both beakers.

5. Dip the zinc electrode with the negative end of the voltmeter and the copper electrode with the positive end with the help of conducting wires

6. Dip the zinc electrode in the 1 M Zinc Sulphate solution and the copper electrode in the 1 M Copper Sulphate solution.

7. Check the reading shown in the voltmeter.

8. Draw 10 ml of 1 M ZnSO4 solution and dilute it to form a 0.1 M solution.

9. Dip the Zn electrode in this solution and note the reading in the voltmeter.

10. Dilute 1 M ZnSO4 solution to form a 0.01 M solution and do the same. Take the reading.

11. Do the same with the CuSO4 solution and take the readings.

12. Use the data of potential values of both the electrodes for different concentrations to plot a graph. You will find the trend of variation of cell potential in Zn Cu cell due to the changes in the concentration of electrolytes.

Follow these steps for calculating the data you will use in the graph paper.

ECell = E0Cell – log

E0Cell = E0 (cathode) – E0 (anode)

F = 96500C

T = 298K

R = 8.314

n = 2 (where n = electrons gained or lost)

By substituting the values, we get

ECell = E0Cell – log

Results of the Experiment

You will find that the ECell will decrease with the increase in the molar concentration of Zn+2 in the electrolyte. The ECell will increase with the increase in the concentration of Cu+2 in the electrolyte.