Electrolysis Using Copper Sulphate And Copper Electrodes

To calculate how much copper will deposit on the cathode, first of all the electrical charge must be calculated by the formula: Q = I x t. The time would be varied in the experiment and the current kept constant so for each time the charge can be calculated. Gallagher and Ingram say, “1 mole of electrons has a charge of 96500 coulombs” therefore, 1/96500 multiplied by the number of coulombs would show how many moles of electrons are used. After writing an equation for the reaction at the cathode, which would be: Cu + 2e- → Cu, the amount of grams of copper deposited can be calculated.

The Experiment

Variables

In an experiment a fair test is needed. A fair test is a test where only one variable is changed and all the others are kept constant. The variable changed will have an influence on another bringing about a result. The independent variable, the only variable changed, will in this experiment be the charge. The charge will be found by multiplying the current by the time. So that the independent variable can be controlled, the current will be kept constant at 0.2 amps and the time will be varied.

The dependant variable is the variable that is affected by the independent variable. It is also the variable that I will use to measure the results and in this experiment it will be the loss of mass of the anode in grams. To do this the copper anode will be measured before and after electrolysis using electronic balances.

The fixed variables, the variables kept constant so that the results are not affected, will be the temperature, volume and concentration of the electrolyte and the current. The temperature be kept at room temperature and be measured to check it stays constant. The current will be kept constant by using a variable resistor and the resistance needed will be found in the preliminary experiment. 200cm3 of 0.5M of copper sulphate will be used in the experiment. The volume and concentration will be kept the same

Prediction

The following prediction are made for the electrolysis of copper sulphate using copper electrodes:

1. The anode will dissolve and decrease in mass. This will occur because the copper anode will form copper ions and move towards the cathode causing the anode to become smaller and, therefore, loose mass. 

2. The cathode will become larger. This will occur because the copper ions from the anode will be attracted to the cathode. When they gain electrons at the cathode they will become neutral atoms attached to the cathode. This will be especially noticeable towards the end of the experiment as the cathode will be thicker. However, the mass of the cathode will not be measured because not all of the copper ions from the anode will stick firmly onto the cathode so it would be hard to produce accurate results.

3. As the number of coulombs is increased the more mass will be lost. The shape of the graph of loss of mass against number of coulombs will form a straight line from the origin. This will be because there are no other factors affecting the loss of mass so the ratio at which the loss of mass increases will not change.

4. The loss of mass of the anode for different times can be calculated:

0 minutes:         the loss of mass will be 0g

 2.5 minutes:

1. The charge is found:

  Q = I x t

        = 0.2 x 2.5 x 60

        = 30 coulombs

2. The charge is converted into moles of electrons

1/96500 x 30

        = 3.11 x 10-4 moles of electrons

3. The equation for the reaction at the cathode is used to find how much copper is deposited.

Cu2+ + 2e- → Cu

2 moles of electrons give 1 mole of copper atoms

                         gives 64g of copper

1 mole of electrons  gives 32g of copper

3.11 x 10-4 x 32

        = 9.9 x 10-3 g

5 minutes:

 Q                = 60 coulombs

Moles                = 6.22 x 10-4 moles of electrons

Mass                = 0.0199 g

7.5 minutes:

Q                = 90 coulombs

Moles                = 9.33 x 10-4 moles of electrons

Mass                = 0.0298 g

10 minutes:

Q                = 120 coulombs

Moles                = 1.23 x 10-3 moles of electrons

Mass                = 0.0398 g

12.5 minutes:

Q                = 150 coulombs

Moles                = 1.55 x 10-3 moles of electrons

Mass                = 0.0497 g

15 minutes:

Q                = 180 coulombs

Moles                = 1.87 x 10-3 moles of electrons

Mass                = 0.0597 g

17.5 minutes:

Q                = 210 coulombs

Moles                = 2.18 x 10-3 moles of electrons

Mass                = 0.0696 g

20 minutes:

Q                = 240 coulombs

Moles                = 2.49 x 10-3 moles of electrons

Mass                = 0.0796 g

22.5 minutes:

Q                = 270 coulombs

Moles                = 2.80 x 10-3 moles of electrons

Mass                = 0.0895 g

25 minutes:

Q                = 300 coulombs

Moles                = 3.11 x 10-3 moles of electrons

Mass                = 0.0995 g                        

Method

Apparatus

Power supply

Variable resistor

Ammeter

Wires

Beaker

200cm3 of 0.5 M CuSO45H2O

Copper electrodes

Stopwatch

Thermometer

Balance

Radiator

Distilled water

Preliminary Experiment

1. A circuit will be set up as shown below.

2. The variable resistor will be used to change the resistance and the ammeter to measure the current. First of all the resistor will be set a 10 ohms, then at 20, then at 30 and so on to until 0.2 amps is seen on the ammeter. The variable resistor may need to be changed to between two numbers, for example, 20 and 30 ohms.

3. When the resistance needed is found it will be recorded and the variable resistor left at that number.

Experiment

1. The copper anode will be weighed in grams using an electronic pair of balances. The mass will be recorded.

2. The same circuit as above will be set up. The variable resistor will be set at the figure found in the Preliminary Experiment and the temperature of the copper sulphate will be measured and recorded.

3. The power supply will be turned on and after two and a half minutes it will be turned off.

4. Before the copper anode is weighed it must be washed with distilled water so that there is no copper sulphate solution on the electrode. Then the copper anode must be placed on a hot radiator so that the distilled water is evaporated off the anode.

5. The copper anode will be weighed and the mass recorded.

6. This will be repeated and each time the time will increase by two and a half minutes so that the anode will be measured first after two and a half minutes, then five minutes and so on.

7. The results will be plotted on a graph of loss of mass against charge (number of coulombs). The resistance will be found by multiplying the time by the resistance found in the preliminary experiment.

Precautions

• Wear a laboratory coat to protect your clothing from any spillages.

•  Keep water away from the circuit to avoid electrocution. If water is spilt turn the switch off before dealing with the water.

• Do not short the circuit by there being no resistance such as the copper sulphate or a resistor.

• Always turn off the variable resistor before changing the resistance.

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