1 Faraday (F) = 96500 coulombs (approximately)
Amount of electricity passed = current (A) x time (s)
96500 (1 F) F
It is found that the amount of electricity passed determines the mass of products formed.
(2) The masses of different elements liberated by the same amount of electricity form simple whole number ratio’s when divided by their relative atomic masses.
One Faraday of electricity contains 1 mole of electrons
An example for Faraday’s second law of electrolysis is during two different electrolysis experiment 1.08g of silver was deposited and 0.32g of copper was deposited. The atomic masses (Ar) of silver and copper are 108 and 64. Therefore dividing the mass deposited by the right atomic mass gives us: silver copper
1.08 = 0.01 0.32 = 0.005
108 64
The answers obtained have a simple ratio of 2:1. This is what Faraday’s second law of electrolysis is suggesting.
It is also found that the discharge of one mole of atoms of an element from an ion with a single positive or negative charge requires 1 F of electricity.
Therefore, 1 F produces 108g of silver from Ag+ ions
Or 35.5g of chlorine from Cl- ions.
In the same way, the discharge of one mole of atoms of an element from an ion with a double positive or negative charge requires 2 F of electricity.
Therefore, 2 F produces 64g of copper from Cu2+ ions
Or 16g of copper from O2- ions
A sample of the calculation.
Calculate the mass of calcium atoms produced when a current of 5A which is passed for 32 min 10 s through molten calcium bromide
(Ca2+ .2Br-) (Ar (Ca) = 40).
Quantity of electricity passed= 5 x 1930 C
5 x 1930
= 96500 F
Since calcium ions are Ca2+, when 2 F are passed, the number of moles of calcium atoms formed =1
Therefore, 0.1 Faraday produces 0.1 mol of calcium atoms.
2
= 0.1 x 40g of calcium
2
= 2g of calcium
As you can see the positive ions from the anode are moving towards the negative cathode and the impurities are bring deposited at the bottom. The reaction that has taken place at the anode is 4OH- = 2H2 O + O2 +4e-The reaction that has taken place at the cathode is Cu + 2e Cu
APPARATUS: Here is a list of apparatus I used in this experiment.
- Copper II Sulphate (150 ml), which will be used for placing the copper electrodes in.
- Variable Resistor also known as a rheostat, which will be used to control the amount of current that will be passed thought the experiment.
- Copper Electrodes, one labelled as anode and one marked as cathode. These two electrodes will be placed into the copper sulphate solution and observed.
- Hair dryer, which will be used to dry the electrodes so that the water of the copper sulphate solution does not get weighed as well as the electrodes leading to an unfair test.
- Glass beaker, which will be used to place the copper sulphate solution and the electrodes in.
- Wires, black and red, which will be used to connect the power pack, ammeter, rheostat and the electrode holder together.
- Power pack, which will be used to connect into the experiment so that the amount of voltage that is applied can also be controlled.
- Ammeter, which will be used to measure the amount of current that will pass through the experiment.
- Stop watch that will be used to time the experiment
- Top pan balance (1 d.p), which will be used weighing the electrodes
- Electrode holder used to hold the electrodes into the copper sulphate solution.
Here is a diagram of the apparatus.
SAFETY: During this experiment I will make sure that everything I do will be safe.
- I will wear a pair of goggles to protect my eyes from the solution.
- I will also make sure that the electrodes do not touch each other or the sides of the beaker so that no spark occurs and that no copper oxide is formed.
- I will also wear protective footwear so that if there is an accidental spill of the copper sulphate solution, it does not harm my feet.
- I will also be careful using electrical appliances so that I do not harm myself or anyone around me in anyway.
FAIR TESTING: During the investigation, I will make sure to keep both of the variables, the size of the current and the time (which I will be checking after ten minutes) are the same throughout both experiments. The only thing that I will be changing is the size of the current by 0.1 amps after ten minutes, using the ammeter and the rheostat. I will also make sure that the conditions around both of the experiments are the same and that the electrodes are not left underneath the hair dryer for to long so that the electrons are not blow off.
BASIC METHOD: The basic method that I used in my preliminary work was that I first got two copper electrodes and labelled one as the cathode and the other as the anode. I then weighed both these electrodes and made a note of the mass of them. I then poured 150ml of Copper II Sulphate solution into a beaker. I attached both of the electrodes together and placed them into the solution.
Electrical wires (and crocodile clips) were used to make the connection between the electrodes and the power pack, which was then plugged into a socket. The variable that I had chosen was the size of the current that was being applied into the experiment and observing the time, the electrodes were left inside the solution. To do this, an ammeter was connected into the circuit to measure the current that was to be passed into the solution. I then switched on the voltage to 6 volts (which displayed a current of around 0.4 to 0.5 amps) and timed the experiment for five minutes.
After the five minutes, I took the electrodes out of the copper sulphate solution and dried off the liquid off the electrodes using a hair dryer. The reason I used a hair dryer to dry the liquid off the electrodes is that when I will weigh the electrodes, I did not want to weigh the liquid to be weighed as well. I followed this same method until there had been 30 minutes worth of results.
My preliminary work enabled me to make many changes to my final method. The first change I made was that I used an electrode holder to hold the electrodes because before I had noticed that the electrodes touched and produced some black substance which I think was copper oxide. The electrode holder also enabled me to attach the electrical wires in an easier way without using crocodile clips. The electrodes also produced sparks when they touched each other. Using the electrode holder prevented these problems to occur.
I also decided to use a rheostat along with the ammeter because I noticed that before the current would not stay at a stable position and would keep moving continuously which was not a fair test. The rheostat allowed me to choose the size of the current of my choice and also stay at the same position throughout the experiment. I also decided to change the size of the current that was applied and start it from at least 0.5 amps because previously I noticed that when I started the experiment from around 0.4 amps that not much reaction was taking place inside the solution.
From the previous experiment, I noticed that when I placed the electrodes beneath the hair dryer for a long time, the electrons blew off the electrodes. This time I was sure to apply less heat onto the electrode and to stop applying heat to them as soon as the water is not visible so that the electrons could not be blown off the electrodes enabling me to gain inaccurate results.
I also decided to increase the amount of time that the electrodes were in the solution from five minutes to ten minutes because previously I noticed that very little reaction was taking place inside the solution. I also decided to increase the time of the experiment from thirty minutes to fifty minutes because now that I have decided to check the electrodes after ten minutes, I will get fewer results than before, which is why I have decided to increase the time of the whole experiment. All these changes helped me to gain more accurate results than previously.
RESULTS: Here are my results for both experiments.
To find out the average mass of the cathode :
Experiment 1 + Experiment 2
2
GRAPHS: The graph below shows the average results of both experiments. The graph shows the average mass of the cathode after both experiments.
A GRAPH TO SHOW HOW THE SIZE OF THE CURRENT AFFECTS THE AVERAGE MASS OF COPPER GAINED AT THE CATHODE
A GRAPH TO SHOW HOW THE AMOUNT OF TIME AFFECTS THE AVERAGE MASS OF COPPER GAINED AT THE CATHODE
OBSERVATIONS: We used five different amounts of currents. For the first ten minutes, we used 0.5 amps as the current. (We started from 0.5 amps because from my preliminary work I noticed that there was less reaction at the cathode when the current was less than 0.5 amps so that is why we started from 0.5amps). After ten minutes, we increased the current to 0.6 amps. We kept on increasing the amount of current by 0.1 amps after ten minutes until we had gained five results. The current did not stay the same due to the ammeter, which kept changing the current every time we put our hands onto the table. This caused the results to be different from what was expected.
As I observed the experiment, I noticed the edges of the anode wearing away. Bronzy and dark reddish flakes (positive ions) fell off from the edges of the anode. Also as the size of the current increased as well as the time, the anode got increasingly thinner whilst the cathode gained the positive ions, which deposited copper onto the cathode. The cathode also had small lumps of copper, which was attached around the edges of the cathode. The solar coating of the copper was visible on the cathode.
I also noticed that as the current increased, the solution got warmer and warmer because the ions in the solution had gained more current, which means more kinetic energy for them to move. At the beginning of this experiment, I had identified a variable that affected this experiment. That variable was the temperature of the experiment, which would affect it by increasing the reactivity at the cathode, as the temperature would rise. This prevented this experiment from becoming a fair test because I did not monitor the temperature in the solution but only the time and current. A solution to this experiment to this experiment could be to use a water bath, which would control the temperature of the experiment. Our solution did not turn pale by a large amount but it turned pale by a very small amount by the end of the experiment because as the current increased so did the current allowing the copper ions in solution to used up as well therefore the colour of the solution changed colour. This proves that the copper ions in the solution were used during this experiment.
There were not many copper ions floating at the top of my solution but there were a few copper ions floating at the top of the copper sulphate solution. I also noticed when the experiment was taking place; many copper ions were visible in the middle of the beaker meaning that the ions were depositing the copper ions to the cathode. A small pile of the copper impurities were also left at the bottom of the solution, which was in the glass beaker.
ANALYSIS: As you can see above I have displayed my table of results as well as my graphs. The two variables that I monitored were the size of the current and the time of the experiment. The two graphs that I have displayed are both similar except the first graph shows how the mass of copper increases at the cathode as the size of the current increases. The other graph shows how the amount of copper increases at the cathode as the amount of time increases.
Both graphs show that the fourth result was odd. The graph should have been a straight line because the time and size of the current were both directly proportional to the mass of copper gained at the cathode. As the current and time increased so did the amount of copper that was being produced. The first three results of both graphs, the size of the current is directly proportional to the mass of copper gained at the cathode. The fourth result is very odd seen as what should have happened is because the size of the current had increased than before, the cathode should have gained more copper than before.
I think the reason the fourth result was not accurate because every time the table where the apparatus was placed on moved, the ammeter would move as well. Even though we used a rheostat to control the amount of current that was applied into the experiment, moved and I remember there being a point at which the ammeter dropped from 0.8 amps to around 0.6 amps. This allowed less current to be applied into the solution therefore there was less activity in the solution at the cathode. I had to readjust the rheostat to 0.8 amps.
In addition, the last result that was taken should have been higher than what we gained before because if the amount of current was the highest amount, why did the cathode not gain on the most mass so far? According to my prediction the amount of reactivity, taking place should have increased as the current or time did but this did not happen. As you can see from my graph above, the red circles show where the graph points should have been. In my experiment the current and time where only directly proportional to the average mass of copper gained at the cathode, for the first thirty minutes after that the results did not go according to plan.
This may be because the current was not always stable or maybe because the copper might have been blown away when I dried the electrodes. If the fourth result was accurate, I think there would have been a graph that would be curved. I think this is because I used the same two sets of electrodes throughout both of the experiments. This allowed the anode to loose a lot of mass at the beginning, and when there starts to be a curve in the graph, that is the point where the anode has a very few grams of copper left so it cannot loose its positive ions. Therefore, the graph curves at the point where the anode cannot loose any more ions because it will all vanish.
If I had used a heavier anode, I may have gained accurate results because the anode could loose its ions for a longer time than before so that more copper would have been formed at the cathode.
CONCLUSION: Overall, my prediction and hypothesis were both correct. It is true that when you apply a direct current between two copper electrodes, the positive ions from the anode move towards the cathode and the negative ions move from the cathode to the anode. I also found out that the positive copper ions combined with cathode and produced pure copper whereas the anode lost all of its mass that the cathode used to gain its pure copper.
As the current increased the size, the amount of copper also increased at the cathode. The activity inside the solution did increase to a point where an error was made by me (which was the ammeter not being stable). I also noticed that as the mass of the cathode increases the mass of the anode decreased. To link this relationship we can say that the anode is inversely proportional to the cathode. As the anode loses its mass, that cathode gains it.
If the cathode-increased mass as the current and time increased, then I should have got a straight-line graph where all the results should have been passed by the line of best fit. Only three of my results were in line with the line of best fit. The other two results were odd seen as one dropped by 0.05g and then the other increased by 0.05g. This error occurred because the rheostat did not control the current as it should have and therefore the graph did not turn out as it was expected. If the variable resistor had not moved then maybe I may have gained accurate results, which would have matched my prediction by 100%.
EVALUATION: Our investigation went well but it was not conducted to the best of my ability in the conditions provided. I could have done much better. The variables that I had chosen to monitor where the time the electrodes where left in the solution for, and the size of the current applied in the solution. One way in which I could have improved this investigation is by using a larger range of the size of the current and by carrying this investigation on for a longer time to see what happens next. Maybe if I had checked the electrodes after fifteen minutes as well as using a much larger current, I could have gained better results. My graph did not support my hypothesis by 100% but I thought it was reasonably good.
Previously, for my basic method that I used in my preliminary work, I had made a few changes. I decided to use an electrode holder so that the electrodes did not touch each other and cause a spark. I also used a larger size current because I had noticed that previously I did gain enough copper on the cathode when the current was around 0.3amps. Therefore, I started from 0.5 amps and increased it by 0.1 amps after each ten minutes. I also decided to leave the electrodes in solution for a longer time. Before I left it for five minutes then I increased the time in my final experiments to ten minutes.
In addition, I used a variable resistor (rheostat) as well as an ammeter to control the size of the current. I had also decided to not leave the electrodes beneath the hair dryer for too long because before I discovered that the copper gained at the cathode was blown away. After these changes, my plan was even easier and fairer than before.
Even though I had made these improvements, I still noticed that I had gained some odd results after I had completed my experiment. As shown on my graphs in the results section, you can see two odd results on the average results graph. When I start the experiment, I notice that my hypothesis agreed with my results, that as the time or current increases, the rate of reaction that is taking place in the solution also increases. When I come to towards the end of the experiment, suddenly the rate of reactivity dropped. It should have increased more than before because more current was being applied so there should have been more reactivity taking place.
I think the main reason for this was that when I attached an ammeter and a variable resistor, to monitor the current, the current was not stable. I once noticed that even though the variable resistor should have helped to control the amount of current applied in the solution, it kept altering every time someone would move their hands on the table where the experiment was taking place. I should have made sure earlier that no one was sitting around the experiment whilst it was taking place so that the current would have not altered.
The amount of copper that was lost from the anode and the copper that was gained from the cathode was constant. (The amount that was lost from the anode was directly proportional to the amount of copper that was gained at the cathode). In the end, if I had conducted this experiment accurately I should have got a graph that should have looked something like this.
I also noticed during this experiment, that the temperature was changing during this experiment. This should not have happened because originally, I started this experiment only to monitor the current and time and not the temperature. I did not consider that the temperature would change as the current increased. I should have controlled the temperature from the beginning so that it would have been a fair test. To control the temperature of the solution I may have considered using a water bath to keep the temperature constant during the experiment.
The reason I chose the copper electrodes are that I wanted to keep the concentration of copper the same. If I had used carbon electrodes, then I would see the cathode gaining the copper ions from the solution and I would see no change in weight at the anode because it could not dissolve and deposit any atoms at the cathode. I would only see the ions from the Copper Sulphate solution being used to increase the mass of the copper. The anode would not take part in the reaction. The copper anode that was used in this experiment allowed there to be a balance in the solution. Therefore, when the anode lost the mass, the cathode should have gained the same amount of mass that the anode had lost. In addition, if I had used carbon electrodes then I would have also noticed that the colour of the solution would have faded because the copper ions in the solution would be used in this process and not the carbon anode because it would not have dissolved.
If I have to carry this investigation out once again, I would make sure that I make the following improvements. I would use a device to make sure that the temperature of the solution is constant all the way through because previously I noticed that the temperature was not constant and increased as more current was applied, which was not a fair test. I would also use bigger electrodes so that I can continue this experiment on for a longer period allowing me to gain a larger number of results as well as results that are more accurate.
I would also use a top pan balance that was maybe up to three decimal places instead of one decimal place. This is because every time that I weighed the electrodes, I sometimes noticed that the mass had not changed much as it should have or sometimes the mass did not change at all. If there were a three decimal place balance, I would gain more accurate results, graphs as well as for a more precise conclusion. I would also increase the time of the electrodes that are left in the solution so I could investigate further. I would also investigate the other our variables to see if there is any difference in the results.
Overall, I thought that my experiment went fairly well. I tried to keep to the safety aspects as well as trying to lead a fair test. The majority of the experiment was fair and I gained reliable results that proved the majority of my hypothesis. Also my hypothesis and prediction was correct and did prove the majority of the results gained except for a few which I have accounted for already. From this experiment, I learnt about electrolysis in more depth as well as learning more about Michael Faraday and his two laws of electrolysis (stated above in the hypothesis). I have also learnt were I have to improve in carrying out this experiment as a fair test which I have already explained earlier on.