At the anode (+), it is the chlorine that comes off:
2Cl (aq) - 2e Cl (g)
If there are more Chloride ions, more will be able to come off at the cathode. This increase in activity, I predict, will increase the current as they carry the charge to the electrodes. When an electrolyte is solid, the charged ions it is made up of are in a fixed position. They therefore cannot move to the electrodes. Once they are dissolved in water, they are free to move in the solution. It is this movement that allows a current to flow. During electrolysis, the electrons from the battery go to the negative electrode where they meet the positive ions and are changed into neutral atoms. This is the electric circuit that takes place during electrolysis and is why I think that as you increase the concentration, the current will also increase.
Planning
I planned to set up the equipment as shown below:
I am going to measure the current flowing through the circuit when I record my results. It makes no difference where I put the Ammeter in the circuit as the current in a series circuit is the same all the way round. Between measurements I will change the concentration of sodium chloride in the distilled water. I will do this by measuring its mass. I will increase the amount of salt in the solution by 2g for each reading. I will measure out the sodium chloride on a top pan balance to get it as accurate as possible. I will do this to get results ranging from 0g of sodium chloride (pure distilled water) to 14g of Sodium Chloride. This means that I will take 8 readings in total. To make sure that it is a fair test, I will use the same power pack, electrodes, amount of distilled water (80ml), length of time the electrodes are in the solution, the types of circuit, the temperature (approx. 20°C, room temperature) and the same ammeter. This will keep my results constant. I plan to make sure that my results are accurate and reliable by:
- Not letting the electrodes touch,
- Making sure that the electrodes stay the same distance apart,
- Getting an idea of possible results by doing some preliminary work,
- Taking the range of 8 readings 4 times.
One of the safety precautions I will take is to make sure that I wear safety glasses while doing the experiment.
Preliminary Work
I decided that, to get an idea of what kind of results I was aiming to get from my main investigation and to identify any problems I might encounter, I performed roughly the same experiment as described above, I set the equipment up as shown below:
The major difference between the main investigation and my preliminary work was that I measured out the amount of salt for the solution by spatulas. I measured the current with 0-12 spatulas of salt dissolved in distilled water. From this I decided that measuring by mass was more accurate, but here I only wanted to get a rough idea. The results of my preliminary work are shown below:
Method
I performed the experiment as outlined in the plan. Because in my preliminary work, I had already highlighted and found solutions for any possible problems, I did not encounter any during the final investigation. I had the power pack set up so that my circuit had a voltage of 6V. The temperature of the room was approx. 20°C throughout the experiment and all the equipment was kept at this temperature. I repeated the experiment 4 times to ensure that I got the most accurate results possible. Because during electrolysis, ions are displaced, given off and moved around, I didn’t use the same distilled water all the way through. I changed it after every 2 levels of salt were added, for example I used the same water to collect results for 0g, then I added 2g of salt and collected these results. After this I got another 80ml of distilled water and added 4g of salt to it. To this I added another 2 g of salt and then changed the water. I repeated this process for each trial.
Results
N.B. for graph see separate sheet of graph paper.
Conclusion
From my graph, it is clear that the results increase in a curve. Trials 1 and 4 are generally above average and trials 2 and 3 are generally below average. My results for 8g of sodium chloride appear to be anomalous, as they don’t fit into the curve. My results from my preliminary also indicate that the results should form a curve. My prediction is supported by my results because as the concentration increases, so does the current. My results also show that the increase in current between 0g and 2g of salt is substantially greater than the difference in current between 12g and 14g of salt. This would suggest that the solution is becoming saturated and more full of sodium and chloride ions, so the increase is reduced.
Evaluation
I think that my experiment was very effective and I proved my prediction to be true. Although I had some anomalous results, the general trend in my results was clear. The test could have been fairer if I had used the same distilled water for each trial, and just added 2g of salt for each level. I would like to try the experiment using the same water for each trial, to see whether it makes any difference.
