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Help needed .... Electrochemistry

Discussion in 'Science' started by Jeroniymo, Mar 25, 2019.

  1. Jeroniymo

    Jeroniymo New commenter

    Hi

    Can anyone explain WHY the surface area of the electrode has no effect on the voltage of an electrochemical cell?

    I am running a course which requires investigating such factors and cannot find any secondary references to the situation - Unlike temperetaure and concentration and electrode metal which are all covered by Nernst.

    As I understand it an equilibrium is set up in each half cell by atoms becoming ions or visa versa. This produces an unmeasurable internal emf in the half cell. When we connect it to another cell however we can measure in relation to that.

    so my theory is that when we increase the surface area of the electrode this would increase both the electrons left on the metal and the ions produced …. so overall the difference in potential in the half cell would be the same.

    Thoughts?

    This is driving me mad.
     
  2. averagedan

    averagedan Occasional commenter

    There are a few misunderstandings in your post, the one that's getting in the way is the difference between current and voltage. Voltage (emf is the better term here) is the push on the electron and current is the rate at which electrons flow.

    The voltage is determined by the potential difference in energy levels, i.e. the difference in energy between where the electrons end up and where they start in each cell. Altering the surface area doesn't affect this energy change.

    The current however IS affected by the surface area of the electrode and this is used in batteries. One of the reasons that rechargeable batteries break over time is the decrease in the surface area of the electrode. So increasing the surface area should increase the current.

    This is only required knowledge for an A-level course, are you on the Salters course by any chance?
     
    phlogiston likes this.
  3. Jeroniymo

    Jeroniymo New commenter

    No i teach the applied general in Science from AQA its level 3 but generalised.

    Do you know of any sources / books that describe what you have explained for A level students?

    There is a distinct lack of any explanations online that i can find.
     
  4. averagedan

    averagedan Occasional commenter

    Any year 13 A-level textbook will contain a detailed explanation. There's also a good explanation on Chemguide:

    https://www.chemguide.co.uk/physical/redoxeqiamenu.html#top

    This is a complex topic and I'd suggest looking at topics such as reactivity, atomic structure, electricity and electron affinity. It builds on a wide range of topics.
     
  5. gainly

    gainly Established commenter

    The EMF is the voltage when no current is flowing and (as averagedan says) this is not affected by the area of the electrodes. However it does affect the internal resistance of the cell and hence the voltage measured when a current is flowing. The internal resistance is inversely proportional to the area of the electrodes dipping into the electrolyte.

    The voltage when a current is flowing is given by

    V = E - Ir

    V is the voltage
    E is the EMF
    I is the current
    r is the internal resistance

    Ir is often referred to as "lost volts" i.e. the volts lost in overcoming the resistance inside the cell.
     
  6. gainly

    gainly Established commenter

    For the situation normally shown in chemistry textbooks, where the voltmeter is connected directly between the two half cells, the voltage measured will be essentially equal to the EMF and so wouldn't be affected by the area of the electrodes. This is because a voltmeter has a very high resistance so there is virtually no current. If the voltmeter was replaced with a lamp there would be a current and the equation above would apply.
     

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