Cell Potential and Free Energy
Introduced| English |
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| cell potential/sel pəˈtenʃl/ |
The push behind the electrons
- A battery's voltage measures how hard it drives electrons.
- That voltage is really a measure of the reaction's power.
- A positive voltage means the reaction runs on its own.
- One equation ties voltage to free energy.
Cell potential
- The cell potential 电池电势 $E°$ is the voltage a cell produces.
- A positive $E°$ means a spontaneous (galvanic) reaction.
- A negative $E°$ needs an outside push (electrolytic).
A cell with a positive $E°$ has a reaction that is...
Positive $E°$ gives negative $\Delta G°$ -- spontaneous.
Voltage and free energy
- The link is $\Delta G° = -nFE°$.
- Here $n$ is the moles of electrons and $F$ is Faraday's constant.
- A positive $E°$ gives a negative $\Delta G°$, so the reaction is favourable.
The equation linking free energy and cell potential is...
$\Delta G° = -nFE°$, with the minus sign flipping the sign.
In $\Delta G° = -nFE°$, the symbol $n$ is the moles of ____ transferred.
$n$ counts the electrons transferred in the balanced redox reaction.
Reading the sign
- Positive $E°$ matches negative $\Delta G°$ matches spontaneous.
- Negative $E°$ matches positive $\Delta G°$ matches not spontaneous.
- A larger positive $E°$ means a more strongly favourable reaction.
Cell potential and free energy
A positive cell potential means a negative ΔG - a spontaneous cell. They are proportional.
A cell with a negative $E°$ describes a reaction that...
Negative $E°$ means positive $\Delta G°$ -- electrolytic.
A larger positive cell potential corresponds to a more favourable reaction.
More positive $E°$ means more negative $\Delta G°$.
A cell has $E° = +1.1\ \text{V}$. Is the reaction spontaneous?
- A positive $E°$ gives a negative $\Delta G°$.
- So yes, the reaction is spontaneous.
Doubling the amount of reactants doubles the cell potential $E°$.
$E°$ is intensive (a voltage), so it does not scale with amount.
A positive $E°$ means spontaneous (negative $\Delta G°$) -- the sign flips because of the minus in $\Delta G° = -nFE°$. Do not forget $n$ (moles of electrons) when converting. And $E°$ is an intensive property (a voltage), so it does not scale with amount, unlike $\Delta G$.
The cell potential $E°$ is a cell's voltage: positive for a spontaneous (galvanic) reaction, negative for a forced one. It links to free energy by $\Delta G° = -nFE°$, so a positive $E°$ gives a negative $\Delta G°$. A larger positive $E°$ is more strongly favourable.