medium difficulty9 min

Standard Electrode Potential and Cell EMF

Standard electrode potential is the potential of an electrode measured under standard conditions relative to the standard hydrogen electrode, whose potential is assigned zero volts.

In one line: Cell EMF is right only when the reduction winner is placed at the cathode side of the formula.

Practice This Concept

Understand it

Electrode potential values in the CBSE table are usually standard reduction potentials.
A more positive reduction potential means the species has a greater tendency to get reduced.
For a galvanic cell, the cathode has the higher reduction potential and the anode has the lower reduction potential.
The standard EMF of the cell is calculated as the standard reduction potential of cathode minus that of anode.
The sign of EMF indicates whether the cell reaction is spontaneous as written.

Example

If E degree of Cu2+/Cu = +0.34 V and E degree of Zn2+/Zn = -0.76 V, then E degree cell = 0.34 - (-0.76) = 1.10 V for the Daniell cell.

Rule to remember

Key formula: E degree cell = E degree cathode - E degree anode.

Under standard conditions, solute concentration is 1 mol L-1, gas pressure is 1 bar, and temperature is usually taken as 298 K in numerical questions unless stated otherwise.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Given Mg2+/Mg = -2.37 V and Ag+/Ag = +0.80 V, silver ion has the higher reduction potential, so Ag electrode is cathode and Mg is anode.

1E degree cell = 0.80 - (-2.37) = 3.17 V.
2The positive value shows that the reaction is spontaneous in this direction.

Method you can reuse

1List the given reduction potentials.
2Choose the larger value as cathode.
3Choose the smaller value as anode.
4Substitute directly into E degree cell = E degree cathode - E degree anode.
5Interpret a positive value as feasible for a galvanic cell.

Visual tip

A separate diagram is not essential here because the concept is mainly sign and formula based. A small comparison table of anode, cathode, oxidation, reduction, and potential sign is useful for revision.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

For a cell with E degree cathode = +0.80 V and E degree anode = -0.44 V, what is E degree cell?

Think first, then show answer

E degree cell = +0.80 - (-0.44) = +1.24 V.

Common wrong answer

A realistic error is calculating -2.37 - 0.80 = -3.17 V for Mg-Ag cell by subtracting in the wrong order.

Don't mix this up

Students often reverse the formula or change the sign of the anode value twice. Use reduction potentials directly in E degree cell = E degree cathode - E degree anode.

Exam tip

Before substitution, identify cathode as the half-cell with higher reduction potential. Do not mechanically take left minus right unless the cell notation has already been confirmed.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

CBSE questions may ask for EMF calculation, feasibility of a redox reaction, strongest oxidising agent, strongest reducing agent, or correct cell representation using electrode potential data.

1
Enough for 1 mark
Standard electrode potential is the potential of an electrode measured under standard conditions relative to the standard hydrogen electrode, whose potential is assigned zero volts.
2
Add for 2 marks
Example: If E degree of Cu2+/Cu = +0.34 V and E degree of Zn2+/Zn = -0.76 V, then E degree cell = 0.34 - (-0.76) = 1.10 V for the Daniell cell.
3
Add for 3 marks
A more positive reduction potential means the species has a greater tendency to get reduced.

MCQ Quiz

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