Standard Cell Potential and Predicting Spontaneity HL Only

Full Notes

What Is Standard Cell Potential?

Standard cell potential (E°_cell) is the overall potential difference produced by a voltaic cell under standard conditions.

It can be used to tell us whether a redox reaction is spontaneous.

When two different half-cells are connected together, the reaction that occurs overall will be the one that gives a positive E_cell.

How to Calculate E°cell

Note:

• The cathode is the half-cell where reduction happens (the half-cell with the more positive standard electrode potential).
• The anode is the half-cell where oxidation happens (the half-cell with the more negative standard electrode potential).

Meaning you can also write this as:

Matt’s exam tip

In a spontaneous electrochemical cell, the half-cell with the more positive E° undergoes reduction, and the half-cell with the more negative E° undergoes oxidation. But be careful – in non-spontaneous processes (like electrolysis), this is reversed. Rather than relying on E° signs alone, always check which species is gaining electrons (reduction) and which is losing electrons (oxidation) to avoid mistakes.

Example: Zn and Cu Cell

Half-equations and their standard electrode potential:

• Zn²⁺ + 2e⁻ ⇌ Zn (E° = –0.76 V)
• Cu²⁺ + 2e⁻ ⇌ Cu (E° = +0.34 V)

E°_cell = +0.34 V – (–0.76 V) = +1.10 V

This means the spontaneous reaction is: Zn + Cu²⁺ → Zn²⁺ + Cu

Predicting Spontaneity

Spontaneous nature or feasibility of process:

• If E°_cell is positive, the overall reaction is feasible.
• If E°_cell is negative, the reverse reaction is favoured.
• No spontaneity at E°_cell = 0 (equilibrium).

Electron Flow:
Electrons flow from the more negative half-cell (anode) to the more positive half-cell (cathode).

Matt’s exam tip

A spontaneous reaction is one that can happen on its own, without energy input — but that doesn’t mean it will happen. If the activation energy is high, the reaction might be so slow that it appears not to occur at all. So spontaneity doesn't guarantee it will actually occur.

Reversibility and Spontaneity Explained

If a redox reaction has a negative E°_cell, the forward reaction is not spontaneous.

However, the reverse reaction will be spontaneous, because the electrons would now flow in the opposite direction — from the now more negative to the more positive half-cell.

Confirming the above:

• If E°_cell > 0 then forward reaction is spontaneous
• If E°_cell < 0 then reverse reaction is spontaneous

This is useful when predicting whether a proposed redox reaction will proceed as written, or in reverse.

Summary

• E°_cell = E°(cathode) – E°(anode)
• Positive E°_cell = spontaneous forward reaction
• Negative E°_cell = spontaneous reverse reaction
• Spontaneity does not guarantee fast reaction rate