Electrochemistry is the branch of chemistry that deals with the interrelation of electrical and chemical changes caused by the passage of current. It can be sub-divided into two: Electrochemical and Electrolyis
We are going to be looking at the experiments involved in this concept
Experiment : Electrochemical reactions
Aim: To investigate the reactions that take place in a zinc-copper cell
Apparatus: zinc plate, copper plate, measuring balance, zinc sulphate (ZnSO4) solution (1 mol.dm−3), copper sulphate (CuSO4) solution (1 mol.dm−3), two 250 ml beakers, U-tube, Na2SO4 solution, cotton wool, ammeter, connecting wire.
- Measure the mass of the copper and zinc plates and record your ﬁndings.
- Pour about 200 ml of the zinc sulphate solution into a beaker and put the zinc plate into it.
- Pour about 200 ml of the copper sulphate solution into the second beaker and place the copper plate into it.
- Fill the U-tube with the Na2SO4 solution and seal the ends of the tubes with the cotton wool. This will stop the solution from ﬂowing out when the U-tube is turned upside down.
- Connect the zinc and copper plates to the ammeter and observe whether the ammeter records a reading.
- Place the U-tube so that one end is in the copper sulphate solution and the other end is in the zinc sulphate solution. Is there a reading on the ammeter? In which direction is the current ﬂowing?
- Take the ammeter away and connect the copper and zinc plates to each other directly using copper wire. Leave to stand for about one day.
- After a day, remove the two plates and rinse them ﬁrst with distilled water, then with alcohol and ﬁnally with ether. Dry the plates using a hair dryer.
- Weigh the zinc and copper plates and record their mass. Has the mass of the plates changed from the original measurements?
Note: A voltmeter can also be used in place of the ammeter. A voltmeter will measure the potential diﬀerence across the cell.
During the experiment, you should have noticed the following:
- When the U-tube containing the Na2SO4 solution was absent, there was no reading on the ammeter.
- When the U-tube was connected, a reading was recorded on the ammeter.
- After the plates had been connected directly to each other and left for a day, there was a change in their mass. The mass of the zinc plate decreased, while the mass of the copper plate increased.
- The direction of electron ﬂow is from the zinc plate towards the copper plate.
Conclusions: When a zinc sulphate solution containing a zinc plate is connected by a U-tube to a copper sulphate solution containing a copper plate, reactions occur in both solutions. The decrease in mass of the zinc plate suggests that the zinc metal has been oxidised. The increase in mass of the copper plate suggests that reduction has occurred here to produce more copper metal. This will be explained in detail below.
Activity 2 ::
Demonstration : The movement of coloured ions A piece of ﬁlter paper is soaked in an ammonia-ammonium chloride solution and placed on a microscope slide. The ﬁlter paper is then connected to a supply of electric current using crocodile clips and connecting wire as shown in the diagram below. A line of copper chromate solution is placed in the centre of the ﬁlter paper. The colour of this solution is initially green-brown.
The current is then switched on and allowed to run for about 20 minutes. After this time, the central coloured band disappears and is replaced by two bands, one yellow and the other blue, which seem to have separated out from the ﬁrst band of copper chromate.
- The cell that is used to supply an electric current sets up a potential diﬀerence across the circuit, so that one of the electrodes is positive and the other is negative.
- The chromate (CrO2− 4 ) ions in the copper chromate solution are attracted to the positive electrode, while the Cu2+ ions are attracted to the negative electrode.
Conclusion: The movement of ions occurs because the electric current in the outer circuit sets up a potential diﬀerence between the two electrodes.
Demonstration : The electrolysis of copper sulphate Two copper electrodes are placed in a solution of blue copper sulphate and are connected to a source of electrical current as shown in the diagram below. The current is turned on and the reaction is left for a period of time.
- The initial blue colour of the solution remains unchanged.
- It appears that copper has been deposited on one of the electrodes but dissolved from the other.
- At the negative cathode, positively charged Cu2+ ions are attracted to the negatively charged electrode. These ions gain electrons and are reduced to form copper metal, which is deposited on the electrode. The half-reaction that takes place is as follows: Cu2+(aq) + 2e− → Cu(s) (reduction half reaction)
- At the positive anode, copper metal is oxidised to form Cu2+ ions. This is why it appears that some of the copper has dissolved from the electrode. The half-reaction that takes place is as follows: Cu(s) → Cu2+(aq) + 2e− (oxidation half reaction)
- The amount of copper that is deposited at one electrode is approximately the same as the amount of copper that is dissolved from the other. The number of Cu2+ ions in the solution therefore remains almost the same and the blue colour of the solution is unchanged.
Conclusion: In this demonstration, an electric current was used to split CuSO4 into its component ions, Cu2+ and SO2− 4 . This process is called electrolysis.