To find out how current affects the rate of electrolysis

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Electrolysis experiment.

Aim:

        

        To find out how current affects the rate of electrolysis.

Pre-test:

Pre-test Apparatus:

  1. Power Pack
  2. Stop watch
  3. 3 wires
  4. Amp meter
  5. A piece of Card
  6. stop watch
  7. Top pan balance

Pre-test Method:

  1. Check to see if all the apparatus work by forming a small circuit with a bulb
  2. Collect apparatus and set them out like the diagram below. Ensure that the positive wire leads to the anode and the negative leads to the cathode.
  3. Measure out 50 ml of copper sulphate
  4. In to a small class beaker
  5. Weigh cathode on top pan balance
  6. Slide anode and cathode in to piece of card and place in electrolyte
  7. Turn on the power pack
  8. Time until two minutes
  9. Weigh cathode make sure you do not scrape off any copper
  10. Repeat three times
  11. Repeat for each current

Pre-test Findings:

By doing this pre-test I have learnt that:

  • The analog amp meter is very unreliable it
  • To be very accurate I need to use a multi meter
  • To be careful not to scrape of copper when measuring weight
  • Make sure that the anode and the cathode do not touch during the experiment
  • Make sure wires and power packs are working before you start
  • To change the electrolyte after each experiment
  • To use a stop watch instead of a clock as it is more accurate
  • To use a variable resistor to get the exact current you need

Research:

        There are many factors that affect the rate of electrolysis:

  • The current
  • The dept of the electrodes
  • The distance between the electrodes
  • Time allowed for electrolysis
  • The concentration of the electrolyte

        The current effects electrolysis. This is a straightforward relationship. If you double the charge, then the rate of electrolysis will double. This is because electrolysis involves ions taking electrons from the cathode and depositing electrons at the anode. Charge is the number of electrons. If there are twice as many electrons available then twice as many ions can pick up electrons each second.

        The concentration of the solution does have a significant effect, since the concentration of ions do not make it any easier for the electrons to flow, nor does it affect any factors in the ionic equation of reaction, so it should not have an effect. However, at very low concentration, the resistivity of water increases due to the lack of dissolved ions, and this would have an effect on the current and therefore having an effect on the mass of copper deposited.

        The time allowed for the electrolysis to run would have an effect on the mass of copper deposited, since the deposition of copper is a continuous process, and providing there is a fixed current it happens at a fixed rate. Therefore, the longer the time, the more copper atoms would be allowed to deposit and the bigger the combined mass. The mass would be directly proportional to time since the rate is fixed.

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        If you increased the surface area by using larger pieces of electrode (eg copper sheet) then this would make it easier for the electricity to enter the electrolyte so there is more place for reactions to take place so this might accelerate the electrolysis.

        The shorter the distance between the two electrodes the faster the rate of electrolysis. This is because if they are closer together the current would flow through each electrode faster, therefore increasing the making the rate of electrolysis faster.

[http://www.bbc.co.uk/schools/ks3bitesize/sosteacher/science/45200.shtml]

Faraday’s Law:

That the number of moles of substance produced at an electrode during electrolysis is ...

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Here's what a star student thought of this essay

Although some concepts are misunderstood, the technical terms are used correctly most of the time. There are very few spelling and grammatical errors.

The student shows evidence of trying to control as many variables as possible to increase the validity and reliability of the experiment which included repetition of trails. Although there is no evidence of the student calculating averages from their results. The pre-test section was impressive as it showed that the student was considering all aspects of the experiment and modified their method, which was fairly well written. The student also includes a safety list which is very in depth. In the conclusion the whole essay is nicely summarised and does include the calculation of percentage accuracy to show the reliability of their results and allows them to be critical of them. However these calculations, although brilliant to include, should be placed elsewhere in the essay as the conclusion shouldn’t include any new evidence. The research section is very impressive as the student has tried to control all the variables in the experiment, even those not covered at GCSE. However the paragraph about how concentration affects the rate of electrolysis seems confused and is contradicting itself. An increase in the concentration of the solution should increase the rate of electrolysis as there are more ions available to accept electrons. Also the student fails to realise that the charge carriers are the ions in the solution and there is no flow of electrons between the electrodes. Furthermore, the resistivity of water is irrelevant as water does not conduct electricity without the presence of aqueous ions. In the paragraph below this the student makes another mistake by assuming that the rate is fixed when all the variables are constant. This is untrue as the more copper which precipitates out of the solution, the less ions available and the less surface available on the electrode. There are a few issues with the student’s knowledge of the behaviour and charge of ions, as the student states that the Cu2+ ion needs to lose electrons at the negative cathode where in fact it would gain them. This shows that the student doesn’t really understand the fundamentals behind the electrolysis process which is further emphasised with the final paragraph where they explain their prediction. Throughout the essay there is also an incorrect use of half equations. The section where the student tries to calculate something using faraday’s law is incorrect and completely irrelevant to the question.

The student has answered the question and drawn the correct conclusions from their data. The essay is very thorough and covers all variables which could affect the experiment. Some of the content included is above GCSE level and shows extended research around the subject to get as accurate results as the student can, although some sections included are irrelevant to the essay.