Investigate the effect of the amount of sodium chloride, i.e. concentration gradient, in the aqueous solution on the electric current during electrolysis.

Planning

Background

When an electric current passes through sodium chloride solution, chemical reactions take place at both cathode and anode. If one passes through sodium chloride solution, there will be passage of ions moving through this solution. This results in positively charged sodium ions, which have been dissolved into the solution, moving towards the cathode and deposited there. At the same time, negatively charged chloride ions will be moving towards the anode and discharged at the anode. This is called electrolysis.

Aim of experiment

My aim is to investigate the effect of the amount of sodium chloride, i.e. concentration gradient, in the aqueous solution on the electric current during electrolysis.

Key Factors/ variables that affect the results of the experiment

The experiment carried out aimed to monitor the current during electrolysis when the amount of sodium chloride was changed. To ensure a fair test, only one of the listed key variables is allowed to change at a time with the rest of the variables are kept constant.”

This will give me an accurate set of results, which, I hope, enables me to make a decent conclusion. If we do not control the factors apart from the concentration we are testing, you can turn around and say that it was the other factors that had caused the difference and that it had nothing to do with the concentration. By keeping the factors controlled and equal, you can prove it is the concentration.

The following factors/variables must be controlled or monitored during the experiment:

Temperature
Quantity of solution
Voltage
Size of electrodes
Distance between electrodes
Surface on the electrodes
Distilled water

I plan to finish the experiment in one day, so the temperature won’t change drastically and use a stop-clock to maintain the time duration for applying voltage to 10s. By using a volumetric flask and pipette, I can measure and control the quantity of solution for each test batch more accurately. To make it fair, I will keep the voltage the same at 5V all throughout the experiment. I will use a variable voltage power pack to control the voltage at a certain setting during the experiment. I will use fresh new electrodes from the same pack and use the same electrode holder to fix the electrodes so the electrodes’ distance and size won’t change significantly. Finally, tap water contains magnesium and other impurities, therefore I will use distilled water throughout the experiment.

Apparatus and Materials

Electrode holder
scale
crocodile clip
graphite rods
volumetric flask
pipettes 50ml/ holder
beakers
4 wires
filter
voltmeter
variable voltage power pack
access to distilled water
access to NaCl
electric supply
marker pens
spatula

Procedure

First, I checked in the textbook and found that the maximum amount of salt can dissolve in 1000ml water is 360g at room temperature.

Also, from my preliminary work, I found that the current and voltage for solution with10g NaCl is:

And the current and voltage for solution with 30g NaCl is:

Therefore, I decided to use batches of solution with 5g, 10g, 15g, 20g and 30g NaCl.

This is a preview of the whole essay

Also, from my preliminary work, I found that the current and voltage for solution with10g NaCl is:

And the current and voltage for solution with 30g NaCl is:

Therefore, I decided to use batches of solution with 5g, 10g, 15g, 20g and 30g NaCl.

I have to clear the desk first, so I can have more room to do the experiment. I will also check if the apparatus I am going to use are clean, to prevent contamination. I will dissolve the 5 different measures of NaCl into 100ml of water using a volumetric flask. I will stand them on the bench for 2 days before doing the experiment to ensure that they have completely dissolved. I will then add distilled water up to 100ml, because from preliminary work I have noticed that water will become less than before salt has been dissolved. After that, I will use a burette to measure exactly 30ml of each NaCl solution into 3 beakers. So altogether there are 15 beakers. I will use marker pens to label them carefully to distinguish them easier and prevent mixing later. Then, I will set up the electric current as below.

I will make my testing fair by doing multiple tests using the same concentration and keeping the others constant. At the end, I can pool the results together to get an average. This will reduce the probability of biased results. I will also record up to 2 d.p. for more accurate results. I think the range of NaCl concentration that I have chosen will give me a reasonable set of results for me to analyse. It will be adequate enough to plot graphs. Also, if I can spot any anomalous results, I plan to repeat them.

Predictions

I predict that there will be a relationship/correlation between the current and the mass of sodium chloride. I think that the higher the concentration of sodium chloride solution the higher the current will be. This is because the electrolysis of an aqueous solution of sodium chloride using graphite electrodes will cause transfer of sodium from anode to cathode and ionization of the sodium chloride is ionised in the aqueous solution. The higher the concentration, the more delocalised ions, the quicker they can move around, which results a higher current.

Obtaining Evidence

Collecting measurements

This experiment is for finding whether my prediction, ‘The higher the concentration of sodium chloride solution the higher the current will be’ is correct. During the process of collecting measurements, I made sure that all factors listed in the plan, such as temperature, voltage and distance between electrodes, were kept constant and only the variable varied. I connected the beaker of sodium chloride solution with the electric current as in the plan and passed through electricity.

Safety Precautions

There are some safety precautions I have to beware of, or I will get myself into potential danger. I realize that chlorine gas is poisonous; I should not smell the gas for a long period of time. I am also aware that short circuit might also occur and affect my results. I have to make sure that the electrodes, wires; crocodile clips etc do not touch each other. Also, a voltage of 10A is a high voltage. I have to make sure through out the experiment; the voltage should be less than 10A. I don’t think it’s a large problem for me because a chose 5A for my experiment, unless I accidentally turned it the wrong way, I should not think that it will happen. Finally as this experiment involves electricity, electric shocks might occur. I will be careful and avoid possibilities of getting the shock, for example, using wet hands to touch the wires and before using the wire, check if it is safe, i.e. no holes. Sodium Chloride, i.e. salt, is not believed to present a significant hazard to health. Therefore, I do not need to be so cautious, but that doesn’t mean being careless. I will wear safety goggles, if the electrolysis reaction is too vigorous.

Evidence Collected

My measurements of the experiment are shown in the table below:

I believe that these results to be accurate and reliable as measurements in the same tests are quite similar and they all go in sequence. All of the experiments follow the trend that the higher the concentration, the larger the current. E.g. At 5g, the current is 0.22A and at 30g, the current is 0.58A.

My equipment was used to precision as the current and volume was record to the nearest d.p. I used the apparatus I listed in the plan, for example, top-pan balance, it reads to the nearest 2d.p. I made sure the reading with the container 0g before adding sugar and I weighed them twice, to check if the measurement was right. I also used volumetric flask and burettes to measure the volumes. They gave me a more accurate reading than measuring cylinders, therefore gives a fairer result. And I also started timing the experiment when the power current was switched on, to make it fair to all experiments.

I think I have a large enough range of results at different concentrations to make accurate observations.

However, I noticed that in my second and third test, the measurements of current with the concentration of 15g and 20g was slightly lower than my first test. I would like to have repeated it again, but due to the time, I decided to leave it.

Analysing Evidence and Concluding

Analysing Evidence

By doing the experiment and getting the results, I can state that the higher the concentration, the higher the current. This is because there are more particles inside the solution, so when electricity was inserted, they have more energy and move faster. As there are more particles at higher concentration, they have less space to move around. They have more chance of colliding with other particles and reacting.

This graph shows that the higher the concentration, the higher the current. There are no measurements out of place so I believe I have completed the experiments accurately.

This graph increased less in the concentration of 15-20g. However, it also agrees generally that it increases.

The graph above also shows that the overall trend is increasing. It levels out earlier than the ones above. The reason it levels out is because too many ions are in the solution this then results that the space for every ion is small, therefore they block each other. It is difficult to transfer anions to anode or cations to cathodes. Conductivity slows down.

Refer to separate sheet for the graph showing average results.

The graph shows the trend is a reasonably steady upward sloping line. It is reliable as there are no measurements which are miss fitting to the line, though there is a slight dip in the concentration of 15g, it is not big enough to be considered out of place and therefore can be ignored.

Conclusion

All the graphs show that there is a trend that the current increases with the concentration. Also, the current will finally level out as the concentration is high.

Sodium chloride is an ionic compound. It is made up from positive and negative ions arranged in a giant ionic lattice hold together by strong electrostatic attraction by opposite charged ion. Each positive ion (Na+) is surrounded by negative ions and each negative ion (Cl-) is surrounded by positive ions. Ionic compounds can conduct electricity when dissolved in water or melted.

The cathode is the electrode connected to the negative of the power supply. The positive ions are attracted to the cathode, and these ions gain electrons to become uncharged sodium atoms.

Cathode: Na+(l) + e- Na (l)

2H+(g) + 2e- H2 (g)

At the anode, which is attracted to the + electrode, the chloride ion is attracted. The chloride ions react, giving up their electrons, to form bubbles of chlorine.

Anode: Cl-(l) 1/2Cl2 (g) + e-

4OH- – 4e- 2H2O + 02

The amount of water contents in the solution does not change after electrolysis. There are more sodium chlorides in higher concentrations. This is because more electrons are released and so conductivity increase.

My prediction is largely correct. However, through the experiment, I now recognize that the speed of conductivity will go down if concentration is too high.

Evaluating Evidence

My investigation was overall good enough to make a conclusion because I gathered a suitable range of results and kept the variables constant by introducing suitable measures (refer to planning). Most of my measurements fit in with my prediction: the higher the concentration, the higher the current. The only result which I believe to be out of place is the second test, but as I mentioned before (refer to analyzing evidence), it doesn’t affect my overall result.

When I had finished my experiments, I discovered that the reason why there is a slight inaccuracy is because that after each experiment the electrodes were not always put back in the same position this may have affected the results because the ions may have shorter or longer distance to travel, resulting more or less ions to be deposited or detached. However, this might not have a great effect, because we have the electrode holder to hold it. What I am more worried about is that I used the same pairs of electrode throughout the experiment. The electrodes contained a different concentration before putting into another concentration. This might contaminate the solution. I think I should use a new pair of electrodes when I do the repetitions. Also, I think I should have used the same top pan balance when weighing, as there may have been slight differences between the two balances. The above reasons are what could explain the anomaly in the graph.

To further improve my experiment and add additional conclusive data, I would test a larger range of concentrations, for example, from 0g of sodium chloride to 35g of sodium chloride. This would give a larger range of data for me to analyse and draw a conclusion from. I can also, use a larger voltage, e.g. 8V. I used 5V in my experiment, but found that it did not give me a wide range of current. I gathered from other fellow students’ investigations, that 8V has a better result.

I am also quite interested in seeing what time has to do with electrolysis. I am considering this factor for further work. Actually, the result I gave in the observation part was my second attempt of getting the results. At first, I made a mistake by using the same solution 3 times to get the result. The results decreased after each attempt, therefore I realize that time must have something to do with the current. I think this is because if there is more time, the sodium ions will have more time to deposit. I can also investigate other variables such as temperature and surface area.