Define reduction in terms of electrons and oxidation number.

Gain of electrons; a decrease in oxidation number.

Define standard electrode potential, E°.

The e.m.f. of a half-cell measured against the standard hydrogen electrode under standard conditions.

State the standard conditions for E°.

298 K, 100 kPa pressure of gases, 1.00 mol dm⁻³ ion concentration.

Describe the standard hydrogen electrode.

H₂ gas at 100 kPa over a platinum electrode in 1.00 mol dm⁻³ H⁺(aq) at 298 K.

Why is a reference electrode needed?

A single half-cell potential cannot be measured, only the potential difference between two half-cells.

Why is platinum used in the SHE and in ion/ion half-cells?

It is inert, conducts electricity, and there is no solid metal to act as an electrode.

What is used as the salt bridge in electrode potential measurements?

Filter paper soaked in potassium nitrate solution.

Why is a high-resistance voltmeter used?

It draws almost no current, so it measures the e.m.f. of the cell.

How do you measure E° for Fe³⁺/Fe²⁺?

Platinum electrode in a solution of 1.00 mol dm⁻³ Fe³⁺ and Fe²⁺, connected to the SHE by a salt bridge, at 298 K.

What does a more positive E° mean?

The species on the left is more easily reduced (a stronger tendency to gain electrons).

Standard electrode potentialsEdexcel A-Level Chemistry: Flashcards

Card 1 of 140 of 14 known

Question

Define oxidation in terms of electrons and oxidation number.

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

Define oxidation in terms of electrons and oxidation number.: Loss of electrons; an increase in oxidation number.
Define reduction in terms of electrons and oxidation number.: Gain of electrons; a decrease in oxidation number.
What is the oxidation number of Mn in MnO₄⁻?: +7.
What is the oxidation number of Cr in Cr₂O₇²⁻?: +6.
Define standard electrode potential, E°.: The e.m.f. of a half-cell measured against the standard hydrogen electrode under standard conditions.
State the standard conditions for E°.: 298 K, 100 kPa pressure of gases, 1.00 mol dm⁻³ ion concentration.
Describe the standard hydrogen electrode.: H₂ gas at 100 kPa over a platinum electrode in 1.00 mol dm⁻³ H⁺(aq) at 298 K.
What is E° of the SHE?: 0.00 V by definition.
Why is a reference electrode needed?: A single half-cell potential cannot be measured, only the potential difference between two half-cells.
Why is platinum used in the SHE and in ion/ion half-cells?: It is inert, conducts electricity, and there is no solid metal to act as an electrode.
What is used as the salt bridge in electrode potential measurements?: Filter paper soaked in potassium nitrate solution.
Why is a high-resistance voltmeter used?: It draws almost no current, so it measures the e.m.f. of the cell.
How do you measure E° for Fe³⁺/Fe²⁺?: Platinum electrode in a solution of 1.00 mol dm⁻³ Fe³⁺ and Fe²⁺, connected to the SHE by a salt bridge, at 298 K.
What does a more positive E° mean?: The species on the left is more easily reduced (a stronger tendency to gain electrons).

Exam questions on Standard electrode potentials

A teacher demonstrates three redox changes. In the first, sodium burns in chlorine: 2Na + Cl₂ → 2NaCl. In the second, orange acidified potassium dichromate(VI) turns green as it is reduced. In the third, purple acidified potassium manganate(VII) is decolourised when it is added to iron(II) sulfate solution.
Use oxidation numbers and electron transfer to identify the species oxidised and the species reduced when sodium burns in chlorine.2 marks
To compare the tendency of different species to gain electrons, chemists measure electrode potentials against a single reference. The reference used is the standard hydrogen electrode, which is given an electrode potential of exactly 0.00 V.
Explain why a reference electrode is needed to measure the standard electrode potential of a half-cell.2 marks
In Core Practical 10, a student investigates electrochemical cells by measuring standard electrode potentials. Each half-cell is connected to a standard hydrogen electrode so that a voltmeter can give the electrode potential. The student has zinc metal, zinc sulfate solution, iron(II) sulfate and iron(III) sulfate solutions, a platinum electrode, filter paper and potassium nitrate solution.
Describe how the student sets up the zinc half-cell and connects it to the standard hydrogen electrode so that the standard electrode potential of Zn²⁺/Zn can be measured.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).