What is an electrode potential ?

The electrode potential ( E ) is a measure of how easily a species is reduced . It arises from the potential difference between a metal electrode and a solution of its ions at equilibrium.

True or False? In a conventional cell diagram, the substance with the highest oxidation state in each half-cell is written next to the salt bridge.

True. By convention, the most oxidised species in each half-cell is placed adjacent to the salt bridge (||). This puts species in the order: electrode | ion || ion | electrode.

In a conventional cell diagram, a single vertical line (|) represents a .......... boundary, while a double vertical line (||) represents a .......... .

In a conventional cell diagram, a single vertical line (|) represents a phase boundary, while a double vertical line (||) represents a salt bridge .

In an electrochemical cell, at which electrode does oxidation occur and at which does reduction occur?

Oxidation occurs at the negative electrode (anode). Reduction occurs at the positive electrode (cathode). Electrons flow through the external circuit from the negative pole to the positive pole.

The conventional cell diagram for a zinc-copper cell is: Zn (s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu (s). This tells us that electrons flow from .......... to .......... , and the .......... half-cell is the positive pole.

The conventional cell diagram for a zinc-copper cell is: Zn (s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu (s). This tells us that electrons flow from zinc to copper , and the copper half-cell is the positive pole.

What factors affect the size of an electrode potential?

The nature of the ions in solution The concentration of the ions The type of electrode used The temperature

Why is potassium nitrate or potassium chloride commonly used in a salt bridge?

Their ions (K + , NO 3 - , Cl - ) form soluble salts with most other ions, so no precipitate forms at the junction. A precipitate would block ion flow and disrupt the equilibrium position of the half-cells.

Define standard electrode potential ( E ꝋ ).

The standard electrode potential ( E ꝋ ) is the potential difference produced when a standard half-cell is connected to a standard hydrogen electrode under standard conditions (298 K, 100 kPa, ion concentration 1.00 mol dm -3 ).

True or False? A more positive E ꝋ value means the species is more easily oxidised.

False. A more positive E ꝋ means the species is more easily reduced — its equilibrium position lies further to the right in the reduction half-equation.

Standard conditions for measuring electrode potentials: Ion concentration: .......... mol dm −3 Temperature: .......... K Pressure: .......... kPa .......... voltmeter (prevents current flow)

Ion concentration: 1.00 mol dm −3 Temperature: 298 K Pressure: 100 kPa High-resistance voltmeter (prevents current flow)

What are the four critical conditions for a standard hydrogen electrode?

Hydrogen gas bubbled through at 100 kPa H + ion concentration of 1.0 mol dm -3 (HCl accepted) Temperature of 298 K Inert platinum electrode in contact with H 2 gas and H + ions

What is the standard hydrogen electrode (SHE) and what value is it assigned?

The standard hydrogen electrode is the reference half-cell against which all other electrode potentials are measured. It is assigned a value of 0.00 V by convention.

The formula for calculating E ꝋ cell is: E ꝋ cell = E ꝋ .......... − E ꝋ .......... The half-cell with the .......... positive E ꝋ is always placed on the right.

The formula for calculating E ꝋ cell is: E ꝋ cell = E ꝋ right − E ꝋ left The half-cell with the more positive E ꝋ is always placed on the right.

True or False? When calculating E ꝋ cell , the E ꝋ values must be multiplied by the number of electrons to balance the half-equations.

False. E ꝋ is an intensive property — it does not change with the amount of substance. Never multiply E ꝋ by the number of electrons. Subtract the two values directly.

Calculate E ꝋ cell for a cell with Cu 2+ /Cu ( E ꝋ = +0.34 V) and Zn 2+ /Zn ( E ꝋ = −0.76 V).

E ꝋ cell = E ꝋ right − E ꝋ left = (+0.34) − (−0.76) = +1.10 V The Cu 2+ /Cu half-cell is the positive pole (right); Zn 2+ /Zn is the negative pole (left).

What does it mean for a reaction to be feasible in terms of E ꝋ cell ?

A reaction is feasible (likely to proceed spontaneously) when the standard cell potential E ꝋ cell is positive . A negative E ꝋ cell indicates the reverse reaction is feasible instead.

True or False? Electrons flow from the half-cell with the more positive E ꝋ to the half-cell with the less positive E ꝋ .

False. Electrons flow from the less positive (more negative) half-cell to the more positive half-cell — that is, from the negative pole to the positive pole through the external circuit.

For two half-cells with E ꝋ = +1.36 V (Cl 2 /Cl - ) and E ꝋ = +0.34 V (Cu 2+ /Cu), the species that is .......... is Cl 2 , and the species that is .......... is Cu. The E ꝋ cell = .......... V.

For two half-cells with E ꝋ = +1.36 V (Cl 2 /Cl - ) and E ꝋ = +0.34 V (Cu 2+ /Cu), the species that is reduced is Cl 2 , and the species that is oxidised is Cu. The E ꝋ cell = +1.02 V.

Given that E ꝋ values are measured under standard conditions, what is a key limitation of using E ꝋ cell to predict feasibility in practice?

Non-standard conditions (temperature, concentration, pressure) can shift the actual cell potential away from the standard value. A reaction predicted as feasible under standard conditions may not proceed at a measurable rate in practice.

The species with the .......... positive E ꝋ value undergoes reduction (forward reaction). The species with the .......... positive E ꝋ value undergoes oxidation (reverse reaction).

The species with the more positive E ꝋ value undergoes reduction (forward reaction). The species with the less positive E ꝋ value undergoes oxidation (reverse reaction).

Why does a high-resistance voltmeter give the maximum potential difference in an electrochemical cell?

A high-resistance voltmeter prevents current from flowing through it. If current flowed, energy would be dissipated and the measured voltage would be lower than the true E ꝋ cell .

What distinguishes a rechargeable cell from a non-rechargeable cell?

A rechargeable cell uses chemical reactions that can be reversed by applying an external voltage greater than the cell voltage, driving electrons in the opposite direction. A non-rechargeable cell undergoes irreversible reactions.

True or False? The term 'battery' correctly refers to a single electrochemical cell.

False. A battery is correctly a collection of cells joined together. A car battery, for example, consists of six cells connected in series, giving approximately 12 V.

In a zinc-carbon cell, the .......... casing acts as the negative electrode and the .......... paste acts as the electrolyte. The cell generates an emf of approximately .......... V.

In a zinc-carbon cell, the zinc casing acts as the negative electrode and the ammonium chloride paste acts as the electrolyte. The cell generates an emf of approximately 1.50 V.

What are the electrodes and electrolyte in a lead-acid battery, and what emf does each cell produce?

Negative electrode: Pb (lead) Positive electrode: PbO 2 ( lead(IV) oxide ) Electrolyte: sulfuric acid Each cell produces approximately 2 V (six cells in series give ~12 V).

NiCad cells use .......... as the negative electrode and a nickel(II) hydroxide-oxide system as the .......... electrode. A key problem with NiCad cells is the .......... effect.

NiCad cells use cadmium as the negative electrode and a nickel(II) hydroxide-oxide system as the positive electrode. A key problem with NiCad cells is the memory effect.

State two disadvantages of lead-acid batteries.

Any two from: They are very heavy . They contain toxic materials (lead and lead(IV) oxide). The sulfuric acid electrolyte is corrosive , posing disposal challenges.

Why was the Daniell cell considered impractical as a portable device?

The Daniell cell contained hazardous liquids (zinc sulfate and copper sulfate solutions), making it unsuitable for portable use. Modern cells use paste or solid electrolytes to overcome this.

Electromotive force (emf)

An octahedral complex is a complex ion in which six ligands are arranged around the central metal ion at the corners of an octahedron, giving bond angles of 90°.

Why is lithium used as the basis for rechargeable cells in portable electronics?

Lithium is used because it has a very low density and a very negative electrode potential (E ꝋ = −3.04 V), allowing a lightweight cell to produce a large voltage (typically 3.5–4.0 V).

A lithium-ion cell consists of a positive .......... electrode, a negative .......... electrode and a .......... polymer membrane electrolyte.

A lithium-ion cell consists of a positive lithium cobalt oxide electrode, a negative carbon electrode and a porous polymer membrane electrolyte.

True or False? Lithium-ion cells suffer from the memory effect, meaning they gradually lose capacity if not fully discharged before recharging.

False. Lithium-ion cells do not suffer from the memory effect. They can be topped up at any state of charge without loss of capacity, unlike NiCad cells.

State two environmental or sustainability concerns associated with lithium-ion cells.

Any two from: A global shortage of lithium means demand already exceeds supply, making widespread use unsustainable. If cells are discarded in landfill , lithium is lost to future generations. Reports of fires raise safety concerns about reactive lithium in consumer devices.

During discharge of a lithium-ion cell, lithium ions flow from the .......... electrode through the solid electrolyte to the .......... electrode. The cell generates an emf of between .......... V and .......... V.

During discharge of a lithium-ion cell, lithium ions flow from the negative (carbon) electrode through the solid electrolyte to the positive (lithium cobalt oxide) electrode. The cell generates an emf of between 3.5 V and 4.0 V.

State one advantage of the polymer electrolyte in a lithium-ion cell over liquid or paste electrolytes.

The polymer electrolyte cannot leak because it is neither a liquid nor a paste. This improves safety and allows the cell to be sealed more easily in consumer devices.

The memory effect is a reduction in the usable capacity of a rechargeable cell that occurs when the cell is repeatedly recharged before it is fully discharged . It is a problem with NiCad cells but not with lithium-ion cells.

A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode, generating electricity continuously as long as fuel is supplied.

True or False? A hydrogen-oxygen fuel cell needs recharging in the same way as a rechargeable battery.

False. A fuel cell does not need recharging because the reactants (hydrogen and oxygen) are supplied continuously . Energy is not stored in the cell itself.

In an alkaline hydrogen-oxygen fuel cell, hydrogen is oxidised at the .......... electrode and oxygen is reduced at the .......... electrode. The only product is .......... .

In an alkaline hydrogen-oxygen fuel cell, hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode. The only product is water .

What is the overall reaction in a hydrogen-oxygen fuel cell and what is the E ꝋ cell ?

2H 2 (g) + O 2 (g) → 2H 2 O (l) E ꝋ cell = +1.23 V This is the same regardless of whether the electrolyte is acidic or alkaline.

The porous separator in a fuel cell allows .......... to flow to complete the circuit. It does NOT allow .......... to flow — these travel through the .......... circuit.

The porous separator in a fuel cell allows ions to flow to complete the circuit. It does NOT allow electrons to flow — these travel through the external circuit.

State two advantages of a hydrogen-oxygen fuel cell over a petrol engine.

Any two from: Water is the only product — no CO 2 or harmful emissions. A greater proportion of chemical energy is converted to electrical energy (less energy lost as heat). No oxides of nitrogen are produced because the reaction occurs at room temperature , not in high-temperature combustion.

State two problems or risks associated with the use of hydrogen as a fuel in fuel cells.

Any two from: Hydrogen is highly flammable , presenting storage and transport safety hazards. Current production relies mainly on steam reforming of natural gas — a non-renewable, finite resource. Hydrogen gas has low energy density per unit volume , requiring large containers compared to liquid fuels.

The anode is the electrode at which oxidation occurs. In a hydrogen fuel cell, hydrogen is oxidised at the anode (the negative electrode ), releasing electrons that flow through the external circuit.

A LevelChemistryAQAFlashcardsAdvanced Physical ChemistryElectrode Potentials & Electrochemical Cells

Electrode Potentials & Electrochemical Cells (AQA A Level Chemistry): Flashcards

Exam code: 7405

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FrontRepresenting Cells
What is an electrode potential?

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Cards in this collection (44)

What is an electrode potential?
The electrode potential (E) is a measure of how easily a species is reduced. It arises from the potential difference between a metal electrode and a solution of its ions at equilibrium.
True or False?
In a conventional cell diagram, the substance with the highest oxidation state in each half-cell is written next to the salt bridge.
True.
By convention, the most oxidised species in each half-cell is placed adjacent to the salt bridge (||). This puts species in the order: electrode | ion || ion | electrode.
In a conventional cell diagram, a single vertical line (|) represents a .......... boundary, while a double vertical line (||) represents a .......... .
In a conventional cell diagram, a single vertical line (|) represents a phase boundary, while a double vertical line (||) represents a salt bridge.
In an electrochemical cell, at which electrode does oxidation occur and at which does reduction occur?
Oxidation occurs at the negative electrode (anode). Reduction occurs at the positive electrode (cathode).
Electrons flow through the external circuit from the negative pole to the positive pole.
The conventional cell diagram for a zinc-copper cell is: Zn (s) | Zn²⁺ (aq) || Cu²⁺ (aq) | Cu (s). This tells us that electrons flow from .......... to .......... , and the .......... half-cell is the positive pole.
The conventional cell diagram for a zinc-copper cell is: Zn (s) | Zn²⁺ (aq) || Cu²⁺ (aq) | Cu (s). This tells us that electrons flow from zinc to copper, and the copper half-cell is the positive pole.
What factors affect the size of an electrode potential?
1. The nature of the ions in solution
2. The concentration of the ions
3. The type of electrode used
4. The temperature
Why is potassium nitrate or potassium chloride commonly used in a salt bridge?
Their ions (K⁺, NO₃^-, Cl^-) form soluble salts with most other ions, so no precipitate forms at the junction. A precipitate would block ion flow and disrupt the equilibrium position of the half-cells.
Define standard electrode potential (E^ꝋ).
The standard electrode potential (E^ꝋ) is the potential difference produced when a standard half-cell is connected to a standard hydrogen electrode under standard conditions (298 K, 100 kPa, ion concentration 1.00 mol dm^-3).
True or False?
A more positive E^ꝋ value means the species is more easily oxidised.
False.
A more positive E^ꝋ means the species is more easily reduced — its equilibrium position lies further to the right in the reduction half-equation.
Standard conditions for measuring electrode potentials:
- Ion concentration: .......... mol dm⁻³
- Temperature: .......... K
- Pressure: .......... kPa
- .......... voltmeter (prevents current flow)
- Ion concentration: 1.00 mol dm⁻³
- Temperature: 298 K
- Pressure: 100 kPa
- High-resistance voltmeter (prevents current flow)
What are the four critical conditions for a standard hydrogen electrode?
1. Hydrogen gas bubbled through at 100 kPa
2. H⁺ ion concentration of 1.0 mol dm^-3 (HCl accepted)
3. Temperature of 298 K
4. Inert platinum electrode in contact with H₂ gas and H⁺ ions
What is the standard hydrogen electrode (SHE) and what value is it assigned?
The standard hydrogen electrode is the reference half-cell against which all other electrode potentials are measured. It is assigned a value of 0.00 V by convention.
The formula for calculating E^ꝋ_cell is: E^ꝋ_cell = E^ꝋ_.......... − E^ꝋ_..........
The half-cell with the .......... positive E^ꝋ is always placed on the right.
The formula for calculating E^ꝋ_cell is: E^ꝋ_cell = E^ꝋ_right − E^ꝋ_left
The half-cell with the more positive E^ꝋ is always placed on the right.
True or False?
When calculating E^ꝋ_cell, the E^ꝋ values must be multiplied by the number of electrons to balance the half-equations.
False.
E^ꝋ is an intensive property — it does not change with the amount of substance. Never multiply E^ꝋ by the number of electrons. Subtract the two values directly.
Calculate E^ꝋ_cell for a cell with Cu²⁺/Cu (E^ꝋ = +0.34 V) and Zn²⁺/Zn (E^ꝋ = −0.76 V).
E^ꝋ_cell = E^ꝋ_right − E^ꝋ_left
= (+0.34) − (−0.76) = +1.10 V
The Cu²⁺/Cu half-cell is the positive pole (right); Zn²⁺/Zn is the negative pole (left).
What does it mean for a reaction to be feasible in terms of E^ꝋ_cell?
A reaction is feasible (likely to proceed spontaneously) when the standard cell potential E^ꝋ_cell is positive. A negative E^ꝋ_cell indicates the reverse reaction is feasible instead.
True or False?
Electrons flow from the half-cell with the more positive E^ꝋ to the half-cell with the less positive E^ꝋ.
False.
Electrons flow from the less positive (more negative) half-cell to the more positive half-cell — that is, from the negative pole to the positive pole through the external circuit.
For two half-cells with E^ꝋ = +1.36 V (Cl₂/Cl^-) and E^ꝋ = +0.34 V (Cu²⁺/Cu), the species that is .......... is Cl₂, and the species that is .......... is Cu. The E^ꝋ_cell = .......... V.
For two half-cells with E^ꝋ = +1.36 V (Cl₂/Cl^-) and E^ꝋ = +0.34 V (Cu²⁺/Cu), the species that is reduced is Cl₂, and the species that is oxidised is Cu. The E^ꝋ_cell = +1.02 V.
Given that E^ꝋ values are measured under standard conditions, what is a key limitation of using E^ꝋ_cell to predict feasibility in practice?
Non-standard conditions (temperature, concentration, pressure) can shift the actual cell potential away from the standard value. A reaction predicted as feasible under standard conditions may not proceed at a measurable rate in practice.
The species with the .......... positive E^ꝋ value undergoes reduction (forward reaction). The species with the .......... positive E^ꝋ value undergoes oxidation (reverse reaction).
The species with the more positive E^ꝋ value undergoes reduction (forward reaction). The species with the less positive E^ꝋ value undergoes oxidation (reverse reaction).
Why does a high-resistance voltmeter give the maximum potential difference in an electrochemical cell?
A high-resistance voltmeter prevents current from flowing through it. If current flowed, energy would be dissipated and the measured voltage would be lower than the true E^ꝋ_cell.
What distinguishes a rechargeable cell from a non-rechargeable cell?
A rechargeable cell uses chemical reactions that can be reversed by applying an external voltage greater than the cell voltage, driving electrons in the opposite direction. A non-rechargeable cell undergoes irreversible reactions.
True or False?
The term 'battery' correctly refers to a single electrochemical cell.
False.
A battery is correctly a collection of cells joined together. A car battery, for example, consists of six cells connected in series, giving approximately 12 V.
In a zinc-carbon cell, the .......... casing acts as the negative electrode and the .......... paste acts as the electrolyte. The cell generates an emf of approximately .......... V.
In a zinc-carbon cell, the zinc casing acts as the negative electrode and the ammonium chloride paste acts as the electrolyte. The cell generates an emf of approximately 1.50 V.
What are the electrodes and electrolyte in a lead-acid battery, and what emf does each cell produce?
Negative electrode: Pb (lead)
Positive electrode: PbO₂ (lead(IV) oxide)
Electrolyte: sulfuric acid
Each cell produces approximately 2 V (six cells in series give ~12 V).
NiCad cells use .......... as the negative electrode and a nickel(II) hydroxide-oxide system as the .......... electrode. A key problem with NiCad cells is the .......... effect.
NiCad cells use cadmium as the negative electrode and a nickel(II) hydroxide-oxide system as the positive electrode. A key problem with NiCad cells is the memory effect.
State two disadvantages of lead-acid batteries.
Any two from:
1. They are very heavy.
2. They contain toxic materials (lead and lead(IV) oxide).
3. The sulfuric acid electrolyte is corrosive, posing disposal challenges.
Why was the Daniell cell considered impractical as a portable device?
The Daniell cell contained hazardous liquids (zinc sulfate and copper sulfate solutions), making it unsuitable for portable use. Modern cells use paste or solid electrolytes to overcome this.
Electromotive force (emf)
An octahedral complex is a complex ion in which six ligands are arranged around the central metal ion at the corners of an octahedron, giving bond angles of 90°.
Why is lithium used as the basis for rechargeable cells in portable electronics?
Lithium is used because it has a very low density and a very negative electrode potential (E^ꝋ = −3.04 V), allowing a lightweight cell to produce a large voltage (typically 3.5–4.0 V).
A lithium-ion cell consists of a positive .......... electrode, a negative .......... electrode and a .......... polymer membrane electrolyte.
A lithium-ion cell consists of a positive lithium cobalt oxide electrode, a negative carbon electrode and a porous polymer membrane electrolyte.
True or False?
Lithium-ion cells suffer from the memory effect, meaning they gradually lose capacity if not fully discharged before recharging.
False.
Lithium-ion cells do not suffer from the memory effect. They can be topped up at any state of charge without loss of capacity, unlike NiCad cells.
State two environmental or sustainability concerns associated with lithium-ion cells.
Any two from:
1. A global shortage of lithium means demand already exceeds supply, making widespread use unsustainable.
2. If cells are discarded in landfill, lithium is lost to future generations.
3. Reports of fires raise safety concerns about reactive lithium in consumer devices.
During discharge of a lithium-ion cell, lithium ions flow from the .......... electrode through the solid electrolyte to the .......... electrode. The cell generates an emf of between .......... V and .......... V.
During discharge of a lithium-ion cell, lithium ions flow from the negative (carbon) electrode through the solid electrolyte to the positive (lithium cobalt oxide) electrode. The cell generates an emf of between 3.5 V and 4.0 V.
State one advantage of the polymer electrolyte in a lithium-ion cell over liquid or paste electrolytes.
The polymer electrolyte cannot leak because it is neither a liquid nor a paste. This improves safety and allows the cell to be sealed more easily in consumer devices.
Memory effect
The memory effect is a reduction in the usable capacity of a rechargeable cell that occurs when the cell is repeatedly recharged before it is fully discharged. It is a problem with NiCad cells but not with lithium-ion cells.
What is a fuel cell?
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode, generating electricity continuously as long as fuel is supplied.
True or False?
A hydrogen-oxygen fuel cell needs recharging in the same way as a rechargeable battery.
False.
A fuel cell does not need recharging because the reactants (hydrogen and oxygen) are supplied continuously. Energy is not stored in the cell itself.
In an alkaline hydrogen-oxygen fuel cell, hydrogen is oxidised at the .......... electrode and oxygen is reduced at the .......... electrode. The only product is .......... .
In an alkaline hydrogen-oxygen fuel cell, hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode. The only product is water.
What is the overall reaction in a hydrogen-oxygen fuel cell and what is the E^ꝋ_cell?
2H₂ (g) + O₂ (g) → 2H₂O (l)
E^ꝋ_cell = +1.23 V
This is the same regardless of whether the electrolyte is acidic or alkaline.
The porous separator in a fuel cell allows .......... to flow to complete the circuit. It does NOT allow .......... to flow — these travel through the .......... circuit.
The porous separator in a fuel cell allows ions to flow to complete the circuit. It does NOT allow electrons to flow — these travel through the external circuit.
State two advantages of a hydrogen-oxygen fuel cell over a petrol engine.
Any two from:
1. Water is the only product — no CO₂ or harmful emissions.
2. A greater proportion of chemical energy is converted to electrical energy (less energy lost as heat).
3. No oxides of nitrogen are produced because the reaction occurs at room temperature, not in high-temperature combustion.
State two problems or risks associated with the use of hydrogen as a fuel in fuel cells.
Any two from:
1. Hydrogen is highly flammable, presenting storage and transport safety hazards.
2. Current production relies mainly on steam reforming of natural gas — a non-renewable, finite resource.
3. Hydrogen gas has low energy density per unit volume, requiring large containers compared to liquid fuels.
Anode
The anode is the electrode at which oxidation occurs. In a hydrogen fuel cell, hydrogen is oxidised at the anode (the negative electrode), releasing electrons that flow through the external circuit.

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