Feasibility and the Nernst equation
Introduced| English | Meaning |
|---|---|
| feasible/ˈfiːzɪbl/ | Thermodynamically possible under the stated conditions; this does not establish that a reaction is fast. |
| free energy/friː ˈenədʒi/ | In this chemical context, Gibbs energy: enthalpy minus absolute temperature times entropy. |
| Nernst equation/nɜːnst ɪˈkweɪʒn/ | An equation relating an electrode or cell potential to its standard potential, temperature and reaction quotient. |
| half-equation/hɑːf ɪˈkweɪʒn/ | An equation showing the electron gain or loss of one species in a redox reaction. |
| activation energy/ˌæktɪˈveɪʃn ˈenədʒi/ | The minimum energy that reacting particles need for a successful reaction. |
Predicting if a reaction goes
- A positive cell potential means a reaction is feasible 可行.
- Cell potential links to free energy 自由能.
- Non-standard concentrations are handled by the Nernst equation 能斯特方程.
Nernst shift lab
E shift index changes with concentration ratio
Change concentration ratio and see electrode potential shift.
A reaction is feasible if the cell potential (Ecell) is positive.
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Ecell = E(reduced) − E(oxidised).
Feasibility and redox equations
- A reaction is feasible when $E^{\ominus}_{\text{cell}}$ is positive.
- To build the full equation: take the two half-equations 半反应方程式, reverse the one that is oxidised, and add them so the electrons cancel.
- Link to free energy: $\Delta G^{\ominus} = -n E^{\ominus}_{\text{cell}} F$ (n = moles of electrons).
Electrolysis: ions move to the electrodes and react
A redox reaction is feasible when the cell potential is:
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A positive E°cell means the reaction can happen (analogous to a negative ΔG).
The link between cell potential and free energy is:
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ΔG° = −nE°cellF; a positive E°cell gives a negative ΔG° (feasible).
To build the overall redox equation from two half-equations, you:
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One half-equation runs in reverse (oxidation); adding them so electrons cancel gives the full equation.
Match each feasibility idea.
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Each item links the term to its correct meaning.
The Nernst equation
If concentrations aren't standard, $E$ changes:
- $z$ = number of electrons in the half-equation.
- Raising the oxidised species' concentration makes $E$ more positive.
According to the Nernst equation, increasing the concentration of the oxidised species makes E:
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The log term rises, so E becomes more positive when the oxidised species is more concentrated.
Feasible is not always fast
- A positive Ecell means a reaction is thermodynamically feasible, but it may still be very slow.
- A high activation energy 活化能 can make a feasible reaction not happen in practice.
You've got it
- a reaction is feasible when $E^{\ominus}_{\text{cell}} > 0$
- build the equation: reverse the oxidised half-equation and add so electrons cancel
- $\Delta G^{\ominus} = -n E^{\ominus}_{\text{cell}} F$
- Nernst: $E = E^{\ominus} + \dfrac{0.059}{z}\log\dfrac{[\text{oxidised}]}{[\text{reduced}]}$ — more oxidised species → more positive $E$