AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: use given electrode data to deduce the…
Full specification wording
Commercial applications of electrochemical cells (A-level only): Students should be able to: use given electrode data to deduce the reactions occurring in non-rechargeable and rechargeable cells deduce the EMF of a cell explain how the electrode reactions can be used to generate an electric current.
Given two half-equations with their E⦵ values, the more negative one is reversed and becomes the oxidation at the negative electrode; the more positive one runs as written and is the reduction at…
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
Given two half-equations with their E⦵ values, the more negative one is reversed and becomes the oxidation at the negative electrode; the more positive one runs as written and is the reduction at the positive electrode. Scale them so the electrons cancel, add them, and you have the discharge reaction. The EMF is the difference between the two potentials. Electrons are pushed out of the negative electrode, through the external circuit and into the positive electrode, and it is that flow that does the useful work. Inside the cell the circuit is completed by ions moving through the electrolyte or porous separator, never by electrons. Recharging applies a larger external voltage, which drives both electrode reactions backwards and regenerates the original reactants.
What earns the marks
- Identifying the half-equation with the more negative E⦵ as the one that is reversed.
- Stating that electrons flow through the external circuit from the negative to the positive electrode.
- The EMF as the difference between the two electrode potentials.
- Stating that the electrolyte or porous separator allows ions to move and complete the circuit.
- Writing the recharging reactions as the reverse of the discharge reactions.
Common mistakes and how to put them right
Saying the electrolyte or separator allows electrons to move, which examiners refuse.
Reversing the wrong half-equation, so the EMF comes out negative.
Routing electrons through the electrolyte instead of the external wire.
Reversing only one electrode reaction when describing recharging.
Exam tips
- Underline the more negative E⦵ on the question paper; that electrode is negative and its equation is the one you flip.
- For a 'how does this generate a current' mark, you must mention electrons in the external circuit and ions inside the cell.
Try the lesson
The start of the Commercial applications of electrochemical cells (A-level only): Students should be able to: use given electrode data to deduce the reactions occurring in non-rechargeable and rechargeable cells deduce the EMF of a cell explain how the electrode reactions can be used to generate an electric current. lesson, made from checked teaching notes for this specification point.
Have a go first
A cell is made from a Mg²⁺/Mg electrode (E = -2.37 V) and an Ag⁺/Ag electrode (E = +0.80 V). What is the EMF of this cell?
Listen
Given two half-equations with their E⦵ values, the more negative one is reversed and becomes the oxidation at the negative electrode; the more positive one runs as written and is the reduction at the positive electrode. Scale them so the electrons cancel, add them, and you have the discharge reaction. The EMF is the difference between the two potentials. Electrons are pushed out of the negative electrode, through the external circuit and into the positive electrode, and it is that flow that does the useful work. Inside the cell the circuit is completed by ions moving through the electrolyte or porous separator, never by electrons. Recharging applies a larger external voltage, which drives both electrode reactions backwards and regenerates the original reactants. A strong answer earns marks for these points. identifying the half-equation with the more negative E⦵ as the one that is reversed. stating that electrons flow through the external circuit from the negative to the positive electrode. the EMF as the difference between the two electrode potentials. stating that the electrolyte or porous separator allows ions to move and complete the circuit. Exam tip: Underline the more negative E⦵ on the question paper; that electrode is negative and its equation is the one you flip.
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Test your understanding of Commercial applications of electrochemical cells (A-level only): Students should be able to: use given electrode data to deduce the reactions occurring in non-rechargeable and rechargeable cells deduce the EMF of a cell explain how the electrode reactions can be used to generate an electric current.
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