Increasing current intensity or voltage
Increasing the current or voltage causes the ions to move through the electrolyte faster which increases the frequency of effective collisions. Doubling the current or voltage doubles the frequency of energetic collisions between the ions and electrodes. This means that the mass of product will also double. So mass is directly proportional to the current intensity or voltage.
Increasing the temperature
At any temperature the molecules possess a range of different energies. Those with more energy move faster than those with low energy and some molecules have activation energy. This means that at any particular temperature only a proportion of the molecules are activated.
Increasing the temperature has two effects:
- All the molecules gain energy even those which are already activated gain as much as those which don’t possess activation energy.
- Some molecules become activated as they pass the activation energy. This results in an increase in the number of the activated molecules and an increase in their velocity. The frequency of effective collisions between the activated molecules therefore increases resulting in an increase in the rate of the reaction.
A rise in temperature increases the proportion of activated molecules in a certain volume of a substance since molecules attain activation energy. It is true that a10 degree rise in temperature doubles the number of activated molecules and hence also the rate of the reaction. This is the Q10 x 2 rule.
Increasing the concentration
Increasing the concentration increases the rate of reaction because this means there will be a greater number of reactant molecules in the same volume of substance. These molecules will be closer together and so they will collide more often. There will be an increased chance of reaction taking place and the doubling the concentration doubles the rate of reaction: so rate of reaction is directly proportional to concentration.
The difference here is that the proportion of activated molecules doesn’t get bigger it is only the number of activated molecules which increases since no new molecules obtain activation energy.
E.PREDICTIONS:
I will make a number of predictions about what I expect the experiment to show from his background knowledge:
- The mass of Copper deposited will be directly proportional to the current used. This will produce a straight line graph through the origin when mass is plotted against current as shown below:
-
The mass of copper deposited at the cathode will be the same as the mass of copper lost from the anode because it is true that gain in mass at the cathode is equal to the loss in mass at the anode (all the cations of Copper which pass into the electrolyte mixture will discharge at the cathode).
- Larger currents will produce larger unwanted temperature increases
F. PRELIMINARY EXPERIMENT:
Before starting the actual experiment a preliminary test was conducted and a current was passed through the Copper (II) Sulphate solution using untreated plates (not sanded because this is just to determine the current range) and the upper and lower limits of current that this rheostat gave were determined to see if there was an adequate range of amp measurements to conduct the experiment without a large temperature increase.
A preliminary or trial experiment was conducted to choose an adequate range of amperages and a good time period. This means a range of amperages and a time period which won’t cause the electrolyte to heat up too much so as to have a considerable affect on the results. Values of current were chosen for a particular amount of time that will give a reasonable mass of copper but with a minimal temperature rise.
The range that the rheostat provided was from 0.1A to about 2A this is enough for a one-amp range to be used but what minimum and maximum ranges to use were determined by passing differing current values for differing time periods as the mass increase and temperature rises were measured in a trial and error process.
At 0.1A three experiments were carried out after two, six and ten minutes.
At all the time periods at this amperage the temperature rise wasn’t large so they were all suitable in that criteria. However after two minutes the mass increase was so small that the balance couldn’t record it, after five minutes the mass increase was still very small and not suitable. After ten minutes the mass increase was reasonable but the time is much too long. Since the temperature increases were very small at this amperage I tried at a slightly higher one.
At 0.3A two experiments were carried out after two and five minutes because ten minutes was too long:
At both amperages the temperature increase is very small. However the increase in mass after two minutes wasn’t large enough to be recorded accurately. After five minutes there was a reasonable increase in mass with little temperature rise so 0.3A was chosen as the first amperage and a five minute time period for the experiment was considered.
Now that a minimum current had been chosen a maximum current had to be found. At 2A an experiment was conducted to see the temperature increase up to the five minute period and mass increases at certain of these:
The mass increases were reasonable even after two minutes. However the temperature increases were much too large even after two minutes so only those under two minutes were suitable and since at 0.3A mass increase after two minutes was too small a 2A current was considered too large if a 0.3A current was to be used.
A lower amperage of 1.5A was tried. The temperature increases and mass increases were measured at times which were considered suitable for a 0.3A amperage:
In terms of mass and temperature increase all these time periods at this amperage are suitable.
It was found that current values of around two amps heated the solution up too much, too fast and that any time period between 4 and 6 minutes was large enough to get a measurable increase in mass but short enough not to heat up the solution too much (i.e.: an eight degree rise in temperature). A range of current values between 0.3 and 1.5 amps was chosen. Another experiment was conducted at 0.3A for a four minute time period and no mass increase was recorded so this time period was considered too small for this amperage. From this evidence a five minute time period was chosen since it was the shortest time which gave a reasonable mass increase at 0.3A.
This seemed to be the best possible choice.
3. ANALYSING EVIDENCE:
It is obvious from the results that higher currents produce larger unwanted temperature increases which can affect results.
The graph of mass in grams against current in Amps was plotted using the mean value calculated from the two experiments. The best line of fit was drawn. The graphs for just the anode and just the cathode were also plotted to see if there was any pattern.
The graph obtained of the mean results is essentially a straight-line graph through the origin which means that current and mass of Copper deposited are directly proportional. This can be explained in terms of movements of ions. Increasing the current causes the ions which constitute the current in the electrolyte to move faster. This means that the frequency of collisions and therefore effective collisions between the ions and the electrodes increases. The mass of Copper deposited would be expected to be larger during a certain period. Doubling the current doubles the frequency of energetic collisions of the ions and electrodes and so doubles the mass of Copper deposited. This is one explanation of the direct proportionality between the current and mass of Copper deposited.
The ionic half equations in this experiment are:Cu(s) – 2electrons → Cu2+ (aq)
Cu2+ + 2electrons → Cu
One Cu atom is turned into Cu2+ at the anode which then discharges at the cathode to become Cu yet again. So one Cu atom is lost at the anode and gained at the cathode this must mean that the number of atoms of Copper lost at the anode must be equal to that gained at the cathode. Therefore the mass of Copper lost at the anode should equal that gained at the cathode. The experiment doesn’t seem to show this, since in all cases there is a difference between the mass gained at the cathode and that lost at the anode. In fact the results show that in most cases the amount lost from the anode is marginally larger than that gained at the cathode.
It can also be seen clearly from the graphs that in general the mass of Copper gained at the cathode is smaller than the mass of Copper lost from the anode. In theory these should be equal because during all electrolyses the number of electrons lost at the anode must equal the number of electrons gained at the cathode. The fact that the graphs don’t show this doesn’t contradict this statement or disprove his second prediction; in fact these differences can be very easily explained in terms of experimental error. Although this amount was very small it seems as if it has affect the results significantly. At the anode Copper is lost so little can “fall off” during the cleaning and drying processes (only that which was half removed from the anode at removal of it from the Copper(II) Sulphate) but the reason that the mass of Copper lost is higher than that gained at the cathode is because there may have been some water left on the anode when massing it. But then one could say the same to be true for the cathode and it most likely was true that water increased the mass but at the cathode Copper is deposited, sometimes very loosely. This means that during the processes of cleaning by distilled water and drying in acetone some is likely to simply fall off reducing or over compensating for this effect of excess water which hasn’t evaporated from the electrode when massed. So the mass deposited at the cathode is increased by excess water but decreased by loss of Copper.
When examining the graph of the mean values extremely closely I saw that none of the mean results were on the best line of fit. The results at higher currents seemed to be above the best line of fit and the results at the lower currents below it. This means that the increase in mass is greater at higher currents in proportion to what would be expected. It is also true that the temperature increase was greater in the higher currents. Could there be any correlation between the two?
This correlation can be explained by collision theory. Larger currents cause the temperature of the electrolyte to rise more. So at higher currents the temperature increases more as shown by the recorded temperatures. Increasing the temperature of the electrolyte increases the proportion of activated ions attaining Ea (activation energy). This means that there will be an increase in the frequency of collisions of the activated ions with the electrodes and so the mass of product would be expected to increase. The higher the temperature the greater the increase in the frequency of collisions and so in mass of Copper deposited at the cathode. It is likely that the mass has increased more at the higher currents for this very reason and that at lower currents the effect of temperature is less substantial and loss of Copper deposits is more obvious.
The mass change of the plates in the experiment involving 0.60A being passed through the electrolyte the results were surprisingly far from the calculated results at that amperage. The calculations above show that, in terms of percentage difference these results are the worst: 4.52% off from the calculated change in mass; nearly double the average percentage error of the experiment. This is strange because the current involved is relatively small and the temperature rise is hence also relatively small so the results here would be expected to be more accurate than say those for the experiment where 1.35A were passed through the electrolyte because the effect that temperature would have on the rate of reaction would be smaller. This is not the case at all since the latter turned out to be the second most accurate set of results; the first was predictably that which involved the smallest amperages and smallest temperature rises and hence the smallest effect on rate of reaction. So this anomalous result can only be attributed to carelessness by me when carrying out the experiment (loss of mass when rinsing and drying or excess mass from water which hadn’t yet evaporated).
The graph of just the anode and cathode shows many anomalous results; all of the results at the cathode are smaller than they should be and at the anode they are larger than they should be. Strangely the worst results, those which are furthest from the average aren’t those at the highest currents but the anode reading 0.85A and the cathode readings at 0.6A and 0.35A. he only possible explanation is that theses experiments were done less accurately. The later experiments, at higher currents were done when we had the most experience, but the first ones were done probably less diligently.
Oddly the most accurate results were at 1.35A this is most likely to be because the effect of temperature rise instead of making results worse by making the anode lose more than it should and the cathode gain more mass actually counterbalanced the effect of Copper lost by drying and added mass of water thus making the results seem accurate.
The third graph shows that the results found by the experiment were very close to the actual values. It shows beyond doubt that mass is directly proportional to current since the graph obtained from the calculated results is a straight line graph through the origin. It also shows that the results found by the experiment are generally a fraction smaller than what they should be. This is probably due to the copper falling off the electrodes during the time between removing the plates from the Copper(II) Sulphate and massing them. The effect of Copper loss is more significant than that of excess water so it would be expected for the results to be slightly below what they should be.
The excess water and loss of Copper deposits seem to balance out nicely in the average change in mass.
I used Quantitative electrolysis to calculate what I should have found at the various currents:
A current of Y Amps was passed for five minutes through Copper(II) Sulphate solution using copper electrodes knowing the RMM of Copper to be 64:
Q = I x t
= Y x 5 x 60
= 300Y
From ionic half equation; Cu(s) – 2electrons → Cu2+ (aq)
193000C 64g
So; 193000C deposits 64g
300Y Coulombs deposits 64 x 300Y / 193000 =19200Y / 193000
= Y x 19200 / 193000
With a current of 0.35A:
Mass deposited = 0.35 x 19200 / 193000
= 0.0348g
With a current of 0.60A:
Mass deposited = 0.60 x 19200 / 193000
= 0.0597g
With a current of 0.85A:
Mass deposited = 0.85 x 19200 / 193000
= 0.085g
With a current of 1.09A:
Mass deposited = 1.09 x 19200 / 193000
= 0.108g
With a current of 1.35A:
Mass deposited = 1.35 x 19200 / 193000
= 0.134g
This table clearly shows that the results obtained from the experiment were roughly correct since they are very close to the calculated results.
4. EVALUATION
This experiment was successful in that the results were clear enough to derive a relationship between the mass of Copper deposited and the current proving my first hypothesis.
There was however a lot a room for improvement as there were several possibilities for error:
The most obvious consideration is that copper deposits may have fallen off the electrode at various times: as it was being removed from the electrolyte, as it was being rinsed by distilled water, as it was being dried in acetone or just when being moved. I was extremely careful in trying to limit this but it was almost always inevitable. A better method would have been to simply allow the Copper to dry in air because none would be lost in the acetone. This wasn’t possible because it would have taken a long time and the experiment was conducted over a limited period.
Another factor was that when massed there was usually water left on the electrode which hadn’t yet evaporated. This was because there wasn’t enough time to leave the electrode to dry completely and sometimes there were small amounts of Copper(II) Sulphate solution which increased the mass of the plate.
When the plates were placed in the solution and the power pack turned on the current was never immediately at the desired level and in all cases it had to be adjusted to the current value that had been decided. Usually this was very quick but sometimes it took longer and less or more current was passing through the electrolyte than wanted for a small time period. Also the current changed during the course of the experiment and the rheostat had to be adjusted to keep the current constant. This wasn’t usually a problem but it did mean that the current wasn’t always at the intensity wanted. Both these factors could have had an effect on the results obtained.
The same plates should be used for one whole set of results but this wasn’t possible. At the highest high currents the mass of copper lost from the anode is largest so eventually the plate will begin to disintegrate. After some time the anode become too small, because of electron loss, to be used but also the edges were flaked and sharper so not only was it harder to get an accurate reading for the original and final masses but it is also dangerous.
It is possible to calculate the percentage error at different points because I know what the mass deposited should be at a certain current intensity. I will divide the difference between the actual result and the calculated result by the exact result and multiply this by 100 to obtain this percentage error:
At current intensity 0.35A
0.0002/0.0348 x 100 = 0.57%
At current intensity 0.60A
0.0027/0.0597 x 100 =4.52%
At current intensity 0.85A
0.025/0.085 x 100 = 2.94%
At current intensity 1.09A
0.003/0.108 x 100 = 2.78%
At current intensity 1.35A
0.0015/0.134 x 100 = 1.12%
Average percentage error = (1.12 + 2.78 + 2.94 + 4.52 + 0.57)/5 = 2.386%
In most cases the margin of error is very small and so the graph drawn is precise and the experiment was successful.
When looking at the graph to show the change in mass at the anode and at the cathode it is obvious that the anode gained more mass than expected and that the cathode lost more mass than expected. They are above and below respectively the graph for quantitative electrolysis. This means that overall the anode must have lost more mass than expected this can be attributed to excess water that hadn’t yet evaporated at the time of massing and that overall the cathode gained less mass than expected, this can be attributed to Copper deposits lost in the drying process in acetone and in the rinsing prior or even when removing the electrodes from the electrolyte. This has all been explained in the analysis.
In fact the results at the anode were closer to the quantitative results than those at the cathode and I can conclude that the effect of excess water on the mass change isn’t as large as the effect of losing Copper deposits in the various processes.
The mass change of the plates in the experiment involving 0.60A being passed through the electrolyte the results were surprisingly far from the calculated results at that amperage. The calculations above show that, in terms of percentage difference (calculated in the evaluation) these results are the worst: 4.52% off from the calculated change in mass; nearly double the average percentage error of the experiment. This is strange because the current involved is relatively small and the temperature rise is hence also relatively small so the results here would be expected to be more accurate than say those for the experiment where 1.35A were passed through the electrolyte because the effect that temperature would have on the rate of reaction would be smaller. This is not the case at all since the latter turned out to be the second most accurate set of results; the first was predictably that which involved the smallest amperages and smallest temperature rises and hence the smallest effect on rate of reaction. So this anomalous result can only be attributed to carelessness by me when carrying out the experiment (loss of mass when rinsing and drying or excess mass from water which hadn’t yet evaporated).
The graph of just the anode and cathode shows many anomalous results; all of the results at the cathode are smaller than they should be and at the anode they are larger than they should be. Strangely the worst results, those which are furthest from the average aren’t those at the highest currents but the anode reading 0.85A and the cathode readings at 0.6A and 0.35A. The readings at the cathode stated above are below both the average and the best line of fit, they are both also low currents; this could mean that the heat rise was smaller and so the effect it had on the rate of reaction would have been smaller. This means that the loss in Copper deposits would be more evident as the increased rate of reaction wouldn’t compensate for this loss in mass. Also very small amounts of Copper were deposited and so even a small loss would be large as a percentage of the entire mass. The results at the cathode at 0.85A and 1.09A are closer but above the best line of fit this could mean that because the temperature rise was large at these currents the rate of reaction had a larger effect which helped to cancel out that of the loss of Copper from the cathode. This is even more evident at 1.35A where the point is closest to the best line of fit when it would be expected to be well above the true value due to the effect of heat on the rate of reaction. Here it is probable that the effects of temperature rise and excess loss in mass combine to nearly cancel each other out. The only other possible explanation is that theses experiments were done less accurately. The later experiments, at higher currents were done when we had the most experience, but the first ones were done probably less diligently.
The reading at the 0.85A anode reading is well off the actual calculated results. I can only propose the following explanation: either the experiment was done hastily and the water wasn’t allowed enough time to evaporate.
I thought that temperature rise would be a problem in affecting the accuracy of my results but it was in fact helpful, it counterbalanced, to some extent, the effect of extra water not yet evaporated and Copper deposit loss. So the results were affected by two factors which should have individually ruined the results but that combined had less overall effect.
One improvement would have been to have taken more readings this would reduce the effect of experimental error because averages would be taken.
The range of currents was merely one amp. Is this wide enough to draw a definite conclusion. I think that it is in this experiment because it is impossible to do the experiment at currents of over 2 or 3 Amps because the effect of temperature on the mass of Copper deposited would affect the results too much. However it could have been possible to cool the electrolyte using for example cold spray. Then the experiment could have been done at higher currents and even extreme currents. More experiments of the sort describe above would have to be conducted to conclude that the mass of Copper deposited is proportional to the current passed through the electrolyte in all cases.
Another problem was that the rheostat was tricky to handle in that maintaining the same current was difficult. A problem arises from the moment the experiment is begun: the current has to be adjusted to the chosen value. If this adjustment takes too long then the results will be affected. A ten second delay could be called minimal over a five minute period but this nevertheless adds to other inaccuracies. Sometimes during the course of the experiment in trying to maintain the current one could overcompensate momentarily. The importance of this upon the results is probably minimal but lots of little errors begin to add up to end up with large errors in the final results.
So this experiment has proved that for small currents at least the mass of Copper deposited is proportional to the current passed through the Copper(II) Sulphate. But is the same true in other electrolyses. Further work could be carried out to investigate this. The electrolysis using different electrolytes to prove that the relationship found in this experiment when using reactive electrodes is true generally for all electrolyses.
ELECTROLYSIS OF Cu(NO3)2 solution using Copper electrodes:
The ionic equations would be the same for this as for those in the main experiment:
At the anode no ions would discharge since it is easier for Copper electrodes to lose electrons which become Copper ions which pass into the solution:
Cu(s) – 2electrons → Cu2+ (aq)
AtthecathodeCu2+ discharges in preference to H+ since it is lower in the reactivity series:
Cu2+ + 2electrons → Cu
This electrolysis should also have the overall effect of transferring Copper from the anode to the cathode because for every Copper ion produced in the solution from the anode one Copper ion is discharged at the cathode; so mass lost at anode should equal mass gained at cathode without changing the concentration of the Cu(NO3)2.
This experiment could be carried out and should give exactly the same results.
ELECTROPLATING A METAL WITH SILVER:
The anode would be made of silver and the cathode would be the metal that we wanted to be plated. The electrolyte would have to be a solution of a soluble salt of silver such as Silver Nitrate (AgNO3).
Since the anode is made of silver which is an unreactive metal no ions actually discharge at it since it is easier for the anode to lose electrons which become Silver ions in the solution:
Ag(s) - electron → Ag+
At the cathode the Silver ions discharge in preference since they are lower than H+ ions:
Ag+ (aq) + electron → Ag(s)
It would be expected that the mass of Silver lost at the anode equals that gained at the cathode since one ion is passed into the solution at the anode and one is discharged at the cathode so gain in mass at the cathode should equal loss in mass from the anode without changing the concentration of the electrolyte solution.