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# Question on redox equilibria?? Watch

1. I have attached a past paper question here and the markscheme. But I really don't understand the highlighted part of the markscheme

Why is there zero current in the cell? I thought the whole point of this was to create a current
Attached Images

2. Electrode potential is the one measured for an open circuit, that means zero current.

When the circuit is closed potential difference drops down by IR.
3. (Original post by Borek)
Electrode potential is the one measured for an open circuit, that means zero current.

When the circuit is closed potential difference drops down by IR.
Sorry I just don't understand. If the circuit is open then how are the electrons flowing?
4. Hydrogen is 0 by definition, so they can compare everything else to it
5. (Original post by h.pocock1)
Hydrogen is 0 by definition, so they can compare everything else to it
So you would use an experiment like one in the image? But there is a voltmeter connected so that you can measure the electrode potential. What you are saying is that there is no voltmeter...so how would you measure the potential difference across it?
Attached Images

6. (Original post by lorobolorolo)
Sorry I just don't understand. If the circuit is open then how are the electrons flowing?
They don't flow. You don't need current for the potential difference. When you take an AA battery, even if it is not shorted (not closed circuit) the potential difference between + and - ends exists.

All you need for potential difference is two, separated, different charges. That's the physical definition.
7. (Original post by Borek)
They don't flow. You don't need current for the potential difference. When you take an AA battery, even if it is not shorted (not closed circuit) the potential difference between + and - ends exists.

All you need for potential difference is two, separated, different charges. That's the physical definition.
Thank you. That clears up my confusions a bit. I never liked the physics part of chemistry
8. In simpler terms the standard hydrogen electrode is used as a reference electrode. This is because the voltmeter is a two way connection, and so with the SHE the overall cell potential could be calculated.
9. (Original post by Adex william)
In simpler terms the standard hydrogen electrode is used as a reference electrode. This is because the voltmeter is a two way connection, and so with the SHE the overall cell potential could be calculated.
But if the voltmeter is a two way connection aren't you saying that it's also connected to the other cell?
10. The voltmeter is often labelled as a "high resistance voltmeter". If resistance were high enough (i.e. infinite), current would be zero.
11. The higher the input resistance, the better the voltmeter.

Technically all voltmeters measure current that flows through them, so the circuit is never really open. But the higher the resistance, the better the result.
12. What I mean is that if you want to the determine the electrode potential of one specific element the SHE is used as a reference electrode as the value of the voltage cant be calculated using one cell.
13. Obviously if you were the measure the overall cell potential of two half cells then you wouldn't use hydrogen as a half cell.
14. Remember electrode potential is the measure of voltage of a half cell when its attached to SHE under standard conditions

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