Quantitative electrolysis concerns the amount of product obtained in an electrolysis, and the various calculations to find the mass of a product using the different variables.

Rates of reactions

   With catalyst

   Without catalyst                                                        

                                                                               

                                                                

If mass α current then a graph plotted of these would be a straight line passing through the origin.

Quantity of Charge: (Q).

The quantity of charge is the product of current and time. The unit of current being Amps and that of time being Seconds, then the unit of quantity of charge (Q), has a unit of Coulombs, (C).

Q   =   I   x   t

Coulombs (C)   =  Amps (A)  x   Seconds (S)

One mole of electrons is a quantity of charge equivalent to 96,500 Coulombs.

So, e- = 96,500 C

Also 96,500 C = 1 Faraday (F)

So 1 F = 96,500 C = 1 mole of e-.

This quantity of charge can be applied to ionic half equations, to work out the mass of product obtained when a certain amount of charge is passed during an electrolysis.

Consider the following;

Cu2+(aq) + 2e- → Cu(s)        [Cu = 63.5]

⇒ This equation shows that ( 2x 96,500 = 193,000 ) C or 2 Faradays of charge will produce 63.5g of Copper. If a certain current is passed for a particular amount of time, it is possible to calculate the mass of Copper deposited.

        Oxidation and Reduction

Oxidation is the process which involves:

a- Gain of oxygen

b- Loss of hydrogen

c- Loss of electrons

Reduction is the opposite process to oxidation and it involves:

a- Loss of oxygen

b- Gain of hydrogen

c- Gain of electrons

These two processes always occur in parallel. Because of this, any reaction where they take place is called a redox reaction. Any substance that causes oxidation is called an oxidising agent. Similarly, any substance that causes reduction is called a reducing agent.

Electroplating

This is one  of the various uses of electrolysis, it is used when trying to coat a cheap metal with a more expensive one (like silver eating utensils or gold medals). In order for this process to be effective, it must be done while taking into account the fact that the aim is to produce a metal which looks nice and does so for a long time, (i.e. that doesn’t wear down). So in order for this the “coating metal” (i.e. Gold)  will have to stick very well and evenly. This can only happen under the following conditions:

1. The metal to be coated must be very clean and free of any grease.
2. It should also be sanded down to make little groves into which the coating metal will be able to stick.
3. The metal to be coated will have to be spun while it is being coated so that this happens evenly. (N.B. This isn’t very relevant to this experiment).

During electrolysis:

 

Anions go to the anode, and cations go to the cathode, then they discharge their products.

Apparatus

• Copper Sulphate Solution  0.5M
• Two Copper electrodes
• Acitone
• Distilled water
• 100ml beaker
• Sand paper
• Balance (that measures to 0.01 grams)
• Wires
• Power pack (power supply at 12 Volts)
• Multimeter (set for reading current)
• Rubber gloves
• Fume cupboard
• Stop clock
• Rheostat, (used to vary current intensity)
• Tissues for drying electrodes
• Metal claws

Plan

The apparatus shall be set out as shown in the diagram below. It shall be done in two parts. Firstly, the electric circuit shall be set up and then the rest of the apparatus (i.e. chemicals etc…) shall be put into place. Then the electrodes shall be measured, (i.e. their surface area) and they shall then be sanded down thoroughly and cleaned in order to prevent the copper deposited from falling off them, and dried (using acetone). They shall be clipped on to the circuit using the metal claws, making sure that their position is noted in order to clip them in exactly the same way once they have been removed for the next test. This will keep the depth of immersion the same and therefore the surface area immersed of the electrodes will also remain the same. The electrodes shall then be massed before the first test. This shall be recorded.

Then the beaker shall be filled with some copper sulphate solution (exactly 75 ml). The electrodes shall be introduced into the solution while keeping note of the distance separating them. Then the current shall be switched on and the rheostat shall be moved so that the reading on the ammeter is at

0.4A. As soon as it is stable at this reading, the stop clock shall be started.

The starting temperature shall be recorded. The current shall be allowed to flow through the solution for 5 minutes during which the temperature shall be recorded every 30 seconds as it cannot be kept constant.

After five minutes, the current shall be switched off and the electrodes shall be carefully removed using the rubber gloves, and dipped in the acitone. They shall be left to dry and then massed on the balance individually. Their weight shall be recorded. Then they shall be cleaned, re-sanded and the whole process shall be repeated at the same ampage. (N.B. It is important to keep looking at the ammeter during the course of the test because the ampage changes on its own.)

Then once the weights of the electrodes have been recorded the test shall be done twice for each of the following ampages:

• 0.7A
• 1.0A
• 1.3A
• 1.6A
• 1.9A

I have chosen these values because I think they will give me good results for the following reasons:

1. I have already conducted this experiment and when the current is below 2.0A, it causes less heat.
2. Obviously if the current is 0.0A I shall not get a reading at all, so I have spaced out the different currents between 0.4 and 2.0A.
3. I have chosen to take six readings testing each one twice to get a good average and a good spread of results.
4. The time was spread over five minutes because as I have already carried out this experiment. I found that if the current was not passed through for long enough, the change in mass at the electrodes was barely noticeable.

Diagram

NB: It doesn’t matter which way around the circuit is set up providing all the components are installed. It is important though to keep track of which electrode is the anode and which is the cathode.

Safety Precautions

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As a safety precaution, the acitone was placed in a fume cupboard as it is highly flammable. Rubber gloves were available to any who wished to use them. Coats were hung up on the coat hangers and stool were placed under benches. The bags were left at the front of the classroom.

Hypothesis and Predictions

My hypothesises are:

1. Mass α Current x Time
2. Mass lost at the anode = mass gained at the cathode
3. The higher the current, the higher the temperature rise.

 

OBTAINING EVIDENCE:

During the experiment

Current intensity:  Independent Variable

How long the current was passed for: 5.0 minutes… Controlled Variable

Concentration of CuSO4 solution: 0.5 M… Controlled Variable

Potential difference: 12 Volts… Controlled Variable

Volume of CuSO4 solution: 75 ml… Controlled Variable

Temperature of solution: As constant as possible… Monitored

Size of electrodes: 5 x 4 cm.. 20 cm2

Distance between electrodes: 2 cm… Controlled Variable

Depth of immersion: Constant.

Method

The apparatus was set out as shown in the diagram. The electrodes were sanded thoroughly and cleaned using distilled water; then dried using acetone . They were massed and weight recorded. They were clipped into the circuit be using the metal claws.

Then the beaker was filled with 75 ml of CuSO4 solution. The electrodes were placed in the solution. The current was switched on and the rheostat was quickly adjusted so that the ampage was set to 0.4A. The current was then passed through the solution for five minutes, and the temperature was recorded every thirty seconds. After five minutes the current was switched off. The electrodes were removed, dried and massed. The test was repeated at a current reading of 0.4A again, then with the following currents twice each time:

• 0.7
• 1.0
• 1.3
• 1.7
• 2.0

The results were taken as accurately as possible and the weights were taken correct to two decimal places.

Secondary knowledge

The following table is a set of standard results which I used to help determine the range of currents to be used.

Results

See graph 1

Temperatures: (All units given in degrees Celcius)

ANALYSING EVIDENCE AND DRAWING CONCLUSIONS

Graph Mass/current by hand,

Line of best fit

Anode + cathode results on same graph.

Prediction accuracy

1. Mass α Current x Time

As we can se from my results mass is α to current, and because time is constant then mass is therefore α to current x time.

2. Mass lost at the anode = mass gained at the cathode

From graph 1 we can see that the shape of the best lines are like a triangle on it’s side, like this:

The graph indicates that as the loss in mass at the anode increases, so does the gain in mass at the cathode. These increase at the same time so,

Mass lost at the anode = mass gained at the cathode

3. The higher the current, the higher the temperature rise.

The following table shows how the temperature rises in 300 seconds at different ampages.

As you can see the graph does not seem to indicate that temperature rises as current does. I think that this is due to experimental error for the following reasons:

• By the time I got to take a reading for the temperature for the experiment at 1.9A, the solution was already warm and it was acted upon by the surroundings making it harder for the electrodes to heat up the CuSO4.

• By the end of the experiment I was still using the same electrodes because there were none more available and by this time they were considerably smaller. This would mean that they would have less heating power and also that they were passing less current through the solution. ( c.f. evaluating evidence)

What has been found

Current α Mass

Current α Mass x Time

Mass lost at the anode = mass gained at the cathode

EVALUATING EVIDENCE

I think that this evidence is fairly reliable but some improvements could be made. This is because we are dealing substances at a molecular level and it is practically impossible to get perfect results.

Patterns

In terms of emerging patterns in my results there are a few that are quite obvious.

Graph 1 of Current against change in mass shows something peculiar.

In order to get a straight line passing through the origin when drawing a line of best fit I had to put the first few points and compensate by putting the last few points under it. This means that the points for the last test were lower than average, and those for the first tests are higher than average. I think that this is because the electrodes were wearing down during the experiment and

therefore had a small surface area exposed under water and would have been pumping less current through the solution. This also affected temperature rise, it didn’t go up quite as expected, (graph 2).

Going back to the first graph you may have noticed that I have given each point a name. I think that the graph is not what it should have been. There are several possible alternatives:

1. From first looking at the graph it would seem obvious that all the points concerning the anode are correct except for A5 which is a little out of place. But I don’t think so:
2. After looking more carefully at the graph while keeping in mind the fact that “Mass lost at the anode = mass gained at the cathode” we can start to see which points are correct. It becomes apparent the A1, A2, A3 and A4, and so are C1, C2, C3, and C4 because they are exactly lined up. (C1 is a bit out of place it should be a bit lower, (i.e. 0.4, -0.030). I think that A3, A5, C3 and C5 Should be move closer to the x axis. The reason for them being to low shall be investigated later.

During the experiment my attention was not always glued to the ammeter as it should have been because I was busy taking the readings for temperatures every 30 seconds. A few times I found that it had shot up and because it is hard (impossible) to tell for how long it had done so I was unable to compensate. I simply brought the reading back to what it was supposed to be.

Potential sources of error

The fact that the ammeter reading had to be adjusted at the start of each test meant that it was running at the wrong ampage for a few seconds.

The electrodes may not have been perfectly sanded down and they wouldn’t have gripped the copper so well.

The electrodes shrunk making the experiment more inaccurate.

Energy loss through heat which I couldn’t do anything about.

Suggested Amendments

Find a better way of preparing the electrodes.

Start with new electrodes when the old ones start to wear down. ( N.B. this would not make the experiment entirely accurate any way because their size would be changing although it might do some good.

Keep constant control of the rheostat.