AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Cells are used to measure electrode potentials by reference to the…
Full specification wording
Electrode potentials and cells (A-level only): Cells are used to measure electrode potentials by reference to the standard hydrogen electrode.
An electrode potential cannot be measured alone, because a measurement needs a complete circuit and therefore a second half-cell.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An electrode potential cannot be measured alone, because a measurement needs a complete circuit and therefore a second half-cell. The standard hydrogen electrode is the agreed reference and is defined as exactly 0.00 V. It is platinum foil coated in platinum black, dipping into 1.00 mol dm⁻³ H⁺(aq) with hydrogen at 100 kPa bubbled over it at 298 K. Connect the half-cell under test to it through a salt bridge and the voltmeter reading is that half-cell's electrode potential. The salt bridge, usually saturated KNO₃, completes the circuit by letting ions move between the two solutions and keeps each electrically neutral; it does not carry electrons. A high-resistance voltmeter is used so that almost no current flows and neither equilibrium is disturbed.
What earns the marks
- Stating that the standard hydrogen electrode is assigned a potential of exactly 0.00 V.
- The conditions 1.00 mol dm⁻³ H⁺(aq), 100 kPa H₂ and 298 K.
- Stating that the salt bridge allows ions to move so that the circuit is complete.
- Naming a suitable salt bridge electrolyte such as KNO₃ that does not react with the ions present.
- Using a high-resistance voltmeter so that negligible current flows.
Common mistakes and how to put them right
Writing that the salt bridge allows electrons to flow, which examiners specifically refuse.
Choosing potassium chloride for the salt bridge in a cell containing silver or lead ions, where a chloride precipitates.
Giving the hydrogen ion concentration but leaving out the gas pressure or the temperature.
Leaving the platinum out of contact with either the hydrogen or the acid in a labelled diagram.
Exam tips
- Learn the three standard conditions as a set; a question asking for the conditions usually wants all three.
- Say 'allows ions to move' or 'completes the circuit' for the salt bridge, never 'allows electrons to pass'.
Try the lesson
The start of the Electrode potentials and cells (A-level only): Cells are used to measure electrode potentials by reference to the standard hydrogen electrode. lesson, made from checked teaching notes for this specification point.
Have a go first
In a standard hydrogen electrode, what is the role of the platinum electrode?
Listen
An electrode potential cannot be measured alone, because a measurement needs a complete circuit and therefore a second half-cell. The standard hydrogen electrode is the agreed reference and is defined as exactly 0.00 V. It is platinum foil coated in platinum black, dipping into 1.00 mol dm⁻³ H⁺(aq) with hydrogen at 100 kPa bubbled over it at 298 K. Connect the half-cell under test to it through a salt bridge and the voltmeter reading is that half-cell's electrode potential. The salt bridge, usually saturated KNO₃, completes the circuit by letting ions move between the two solutions and keeps each electrically neutral; it does not carry electrons. A high-resistance voltmeter is used so that almost no current flows and neither equilibrium is disturbed. A strong answer earns marks for these points. stating that the standard hydrogen electrode is assigned a potential of exactly 0.00 V. the conditions 1.00 mol dm⁻³ H⁺(aq), 100 kPa H₂ and 298 K. stating that the salt bridge allows ions to move so that the circuit is complete. naming a suitable salt bridge electrolyte such as KNO₃ that does not react with the ions present. Exam tip: Learn the three standard conditions as a set; a question asking for the conditions usually wants all three.
Key words
Say what each word means before you turn the card.
standard hydrogen electrode
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salt bridge
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high-resistance voltmeter
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See every definition
- standard hydrogen electrode
- The agreed reference to which electrode potentials are measured, assigned a potential of exactly 0.00 V.
- salt bridge
- Usually saturated KNO₃, it completes the circuit by letting ions move between the two solutions and keeps each electrically neutral; it does not carry electrons.
- high-resistance voltmeter
- Used so that almost no current flows and neither equilibrium is disturbed.
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