Cells, redox ratios and prediction limits
Introduced| English | Meaning |
|---|---|
| standard electrode potential/ˈstændəd ɪˈlektrəʊd pəˈtenʃl/ | The potential of an electrode relative to the standard hydrogen electrode, measured under standard conditions. |
| potential/pəˈtenʃl/ | Electric potential energy per unit positive charge at a point, relative to a chosen reference. |
| feasibility/ˌfiːzɪˈbɪlɪti/ | Whether a reaction is thermodynamically possible under specified conditions, without establishing how quickly it occurs. |
| activation barrier | An energy barrier along a reaction pathway that helps determine reaction rate, even when the overall reaction is thermodynamically favoured. |
Which way do electrons move?
- In a spontaneous zinc–copper cell, zinc loses electrons and copper(II) ions gain them. The negative electrode is zinc.
- Electrons move through the external wire. Salt-bridge ions carry charge inside the connecting bridge; electrons do not flow through the solutions.
The salt bridge carries charge through moving _____.
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Ions move through the bridge. Electrons move through the external conductor.
Compare standard reductions
- A standard electrode potential 标准电极电势 is measured relative to the standard hydrogen electrode at 298 K, 100 kPa gas pressure and 1.00 mol per cubic decimetre solutions.
- With reduction potentials +0.80 and +0.34 V, the more positive couple is reduced. Cell voltage is 0.80 − 0.34 = +0.46 V.
Which couple is reduced in a spontaneous cell made from +0.54 V and −0.25 V reduction potentials?
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The more positive reduction potential is the positive-electrode reduction.
Find standard cell voltage from +0.54 V and −0.25 V reduction potentials.
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0.54 − (−0.25) = 0.79 V.
Balance electrons without scaling voltage
- Copper oxidation gives two electrons; each silver ion needs one. The net equation is Cu + 2Ag⁺ → Cu²⁺ + 2Ag.
- Multiplying a half-equation changes amount and total transferred charge together. It does not multiply the voltage.
Electrons move in the wire, while ions carry current in the salt bridge.
Doubling a reduction half-equation doubles its standard electrode potential.
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Potential is per unit charge; scaling the amount does not change it.
Use the actual redox ratio
- Acidified manganate(VII) gains five electrons per MnO₄⁻, so its ratio with one-electron Fe²⁺ is 1:5.
- 22.40 cubic centimetres of 0.0200 moles per cubic decimetre titrant is 0.000448 mol. It reacts with 0.00224 mol Fe²⁺.
State the prediction limits
- Positive standard voltage predicts thermodynamic feasibility 可行性 under standard conditions. It does not prove an observable reaction rate.
- A high activation barrier 活化能垒 can make reaction slow. Non-standard concentrations can change the potential 电势 and the predicted direction.
At standard conditions, a positive cell voltage is compatible with a very slow reaction.
Balance electron transfer before using a titration ratio. Compare standard electrode potentials for a standard prediction, then state concentration and rate limits.
Which two conditions limit a prediction based only on standard potentials?
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Rate barriers and concentration changes limit what the standard thermodynamic calculation predicts.