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F2.6 · Predict redox spontaneity from standard reduction potentials

Learn to predict redox spontaneity from standard reduction potentials through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Electrochemistry

Read standard reduction potentials consistently

A table of standard reduction potentials lists half-reactions as reductions. To predict whether a proposed redox reaction is spontaneous under standard conditions, identify which half-reaction runs as reduction at the cathode and which runs in reverse as oxidation at the anode. Then combine their potentials without multiplying a potential by a stoichiometric coefficient.

What you will learn

  • Identify cathode and anode from a proposed galvanic reaction.
  • Calculate standard cell potential from tabulated reduction potentials.
  • Interpret what a positive or negative result says under standard conditions.

Use one convention for the table

Both values copied from a standard reduction-potential table are reduction potentials, even though the anode reaction proceeds in reverse. The cathode is where reduction occurs and the anode is where oxidation occurs. Electron flow through the external wire goes from anode to cathode.
Choose the half-reactions that match the proposed overall reaction. If the table lists an anode half-reaction as a reduction, subtract that tabulated value. Equivalently, reverse the anode reaction and change the sign of its potential before adding. These are the same calculation; do not do both sign changes.
E°_{cell\mathrm{cell}}=E°_{cathode,red\mathrm{cathode,red}}-E°_{anode,red\mathrm{anode,red}}

Interpret the sign carefully

A positive standard cell potential supports a spontaneous forward reaction under standard conditions. A negative result means the proposed forward direction is not spontaneous under those conditions; the reverse direction is favoured. A zero result corresponds to no standard driving potential.
Balancing the electron count is essential when writing the overall equation, but voltage is energy per unit charge. Therefore multiplying a half-reaction to cancel electrons does not multiply its tabulated potential. Also, standard-potential predictions assume standard conditions; a real cell with different concentrations can behave differently.
  • Balance electrons in the equations, but never scale a potential by the balancing coefficient.
  • State that the conclusion is for standard conditions.

Worked example

A zinc–copper galvanic cell

For a cell with zinc oxidized and copper(II) reduced, the tabulated reduction potentials are +0.34 V for copper(II)/copper and −0.76 V for zinc(II)/zinc. Find the standard cell potential and predict the forward direction.
  1. Assign electrodes
    Copper(II) gains electrons at the cathode; zinc loses electrons at the anode.
    \ce{Cu^{2+} + 2e- -> Cu}, \ce{Zn -> Zn^{2+} + 2e-}
  2. Use both tabulated reduction values
    Subtract the anode reduction value rather than changing its sign twice.
    E°_{cell\mathrm{cell}}=(+0.34\ \mathrm V)-(-0.76\ \mathrm V)=+1.10\ \mathrm V
  3. Interpret
    The positive result predicts that the written forward reaction is spontaneous under standard conditions.
Answer: The standard cell potential is +1.10 V for zinc oxidation coupled to copper(II) reduction.
Check: The overall equation ceZn+Cu2+−>Zn2++Cuce{Zn + Cu^{2+} -> Zn^{2+} + Cu} conserves atoms and charge.

Common mistakes and how to avoid them

Multiplying the voltage when a half-reaction is doubled to balance electrons.
Correction: Scale the chemical equation only. Standard electrode potential does not scale with stoichiometric coefficients.
Negating the anode value and then also subtracting it.
Correction: Either subtract the tabulated anode reduction potential or add its sign-reversed oxidation potential, not both.

Lesson summary

  • All table entries are written as reductions.
  • Use Ecircmathrmcell=Ecircmathrmcathode,red−Ecircmathrmanode,redE^circ_{mathrm{cell}}=E^circ_{mathrm{cathode,red}}-E^circ_{mathrm{anode,red}}.
  • A positive value predicts the proposed forward redox reaction is spontaneous under standard conditions.

Check your understanding

Question 1

If the cathode reduction potential is +0.20 V and the anode reduction potential is +0.50 V, what is the standard cell potential for the proposed direction?
  1. +0.70 V
  2. −0.30 V
  3. +0.30 V
Show answer and explanation
−0.30 V
Subtract the tabulated anode reduction value: +0.20 V − (+0.50 V) = −0.30 V. The proposed forward direction is not spontaneous under standard conditions.

Key terms

Cathode
Electrode where reduction occurs.
Anode
Electrode where oxidation occurs.
Standard reduction potential
Electrode potential tabulated for a reduction half-reaction under standard conditions.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation F2.6. It is a study resource, not an official curriculum publication.

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