Assess how changing the electric current in the electrolysis of acidified water affects the rate at which hydrogen gas is produced.

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WILLLIAM WEBSTER 5SK CHEMISTRY SC1

Electrolysis - Planning

In this investigation, I will assess how changing the electric current in the electrolysis of acidified water affects the rate at which hydrogen gas is produced. The solution to be electrolysed is made up using acid and water. It is of little consequence what acid is used however in this case I will use Sulphuric acid (H2SO4).

When H2SO4 is put in water it is dissociated and forms ions:

H2SO4 › 2H (2+) + SO4 (2-)

Ions are also present from the water in the solution:

H2O › H (+) + OH (-)

During the electrolysis process, the positive hydrogen ions move towards the cathode and the negative hydroxide and sulphate ions move towards the anode.

At the cathode the hydrogen ions gain an electron. They are discharged and are converted into hydrogen gas:

2H (+) + 2e (-) › H2

At the anode, the hydroxide, not the sulphate ions are discharged. Water and oxygen gas are formed:

4OH (-) › 2 H2O + O2 + 4e (-)

The hydrogen gas can be collected and measured. The greater the volume of hydrogen gas formed over a set period of time, the faster electrolysis is occurring.

In the experiment there are several possible ways of changing the electric current such as changing the voltage or the position of the electrodes within the electrolysis cell. However, it was found from preliminary work that the most effective way to change the current was to change the concentration of the acid solution. The preliminary work showed that the greater the concentration of the acid, the greater the current. Ohm's law states that R(resistance) = V(voltage)

I (current)

Therefore, I = V

R

Therefore, if V is constant, in order to increase I, R must decrease. Inversely, to decrease I, R must increase. The value of R depends on how easy it is for the electric charge to pass through the conductor. Therefore to decrease R, the passage must be made easier and to increase R, the passage must be made more difficult. This can be achieved by making the solution a better or poorer conductor. The preliminary work showed that the greater the acid concentration, the greater the current, therefore an increase in acid concentration will make the solution a better conductor and result in a decrease in resistance.

Method

. Set up the apparatus as shown in the diagram. Make sure that the distance between the electrodes is equal to the diameter of the reaction vessel.

2. Turn on the power supply (D.C. 7V) for 120 seconds (measure the time using the stopwatch) and record the volume of gas produced after 120 seconds and the current (ammeter reading) in the results table.

3. Taking care to ensure a fair test by taking all the possible variables (detailed below) into account repeat the experiment using the different acid concentrations specified in the results table.

4. If a result appears anomalous repeat that particular experiment.

Acid Volume (ml)

Water Volume (ml)

Current (A)

Gas Volume (cm³)

5

245

0

240

5

235

20

230

25

225

30

220

35

215

40

210

45

205

50

200

It will be possible to calculate the rate of hydrogen production per minute by dividing the final volume of gas produced by the total number of minutes that the experiment runs for (2).
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Current (A)

Gas volume/2 (rate of H2 production per min)

In order to ensure a fair and accurate test, it is necessary to make sure that the current is the only variable. All other possible variables must be controlled. These include:

* Position of the electrodes within the solution - moving the electrodes closer together is a way of varying the current. It was found in the preliminary work that moving the electrodes closer together caused an increase in the current. As changing the acid concentration, not changing the position of the electrodes is my ...

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