Define oxidising agent

An oxidising agent is a substance that oxidises another species by causing it to lose electrons, while itself gaining electrons and being reduced . Its oxidation number decreases in the reaction.

Define reducing agent

A reducing agent is a substance that reduces another species by causing it to gain electrons, while itself losing electrons and being oxidised . Its oxidation number increases in the reaction.

Why does the oxidation number of an oxidising agent decrease during a reaction?

The oxidising agent gains electrons from the reducing agent, which raises the electron count around the atom and lowers its oxidation state .

In a redox reaction, the oxidising agent is .......... and the reducing agent is .......... .

In a redox reaction, the oxidising agent is reduced and the reducing agent is oxidised .

In the reaction of MnO 4 - with Fe 2+ ions, which species is the reducing agent and why?

Fe 2+ is the reducing agent as it loses electrons , being oxidised to Fe 3+ , which enables the MnO 4 - ions to gain those electrons.

True or False? A substance cannot act as both an oxidising agent and a reducing agent.

False. Some substances can act as both an oxidising agent and a reducing agent , depending on what they are reacting with and the reaction conditions .

When predicting a missing product in a redox equation, why is H 2 O often the first suggestion?

Most redox reactions occur in solution , where water, H + and OH - are always available to account for any oxygen atoms not balanced elsewhere in the equation.

Define redox titration

A redox titration is a titration in which an oxidising agent is titrated against a reducing agent , with electrons transferred between species to determine the concentration of an unknown solution.

Why is dilute sulfuric acid used to acidify the solution in a potassium manganate(VII) titration, rather than hydrochloric acid?

Hydrochloric acid would be oxidised by the manganate(VII) ions, introducing error; dilute sulfuric acid does not react with MnO 4 - and does not interfere with the titration.

In the iodine-thiosulfate titration, iodine reacts with .......... ions and is converted to colourless .......... ions.

In the iodine-thiosulfate titration, iodine reacts with thiosulfate ions and is converted to colourless iodide ions.

What colour change indicates the endpoint in a potassium manganate(VII) titration with Fe 2+ ions?

The solution changes from colourless to a permanent pale pink tinge, caused by a slight excess of MnO 4 - ions remaining unreacted in the flask.

True or False? Potassium manganate(VII) acts as its own indicator in a redox titration.

True. The deep purple colour of MnO 4 - ions turns the solution a pale pink at the endpoint , making a separate indicator unnecessary.

Why is starch added near the endpoint in an iodine-thiosulfate titration rather than at the start?

Adding starch too early causes it to bind strongly to iodine, making the blue-black colour difficult to discharge; adding it when the solution is a pale straw colour gives a sharper colour change at the endpoint.

In an iodine-thiosulfate titration, what is the molar ratio of S 2 O 3 2- to I 2 ?

The ratio is 2:1 : two moles of thiosulfate ions react with one mole of iodine, as shown by the equation 2S 2 O 3 2- (aq) + I 2 (aq) → 2I - (aq) + S 4 O 6 2- (aq).

Define standard electrode potential (E θ )

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

Why must standard conditions be used when comparing electrode potentials of different half-cells?

Electrode potential depends on temperature , pressure and concentration , so fixing these to standard values ensures all measurements are made on the same basis for a valid comparison .

Define standard hydrogen electrode

A standard hydrogen electrode is a reference half-cell consisting of H 2 gas at 100 kPa in equilibrium with H + ions at 1.00 mol dm -3 in contact with an inert platinum electrode, assigned a value of 0.00 V .

In an electrochemical cell, .......... takes place at the negative electrode and .......... takes place at the positive electrode.

In an electrochemical cell, oxidation takes place at the negative electrode and reduction takes place at the positive electrode.

Why is platinum used as the electrode in a non-metal/non-metal ion half-cell?

Platinum is inert and does not participate in the redox reaction, allowing the equilibrium to be established on its surface without introducing additional redox chemistry.

True or False? Oxidation occurs at the positive electrode in an electrochemical cell.

False. Oxidation (loss of electrons) occurs at the negative electrode ; reduction occurs at the positive electrode.

In the conventional cell diagram Zn(s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu(s), which half-cell is the positive pole and why?

The Cu 2+ /Cu half-cell is the positive pole because its E θ value (+0.34 V) is more positive than that of the Zn 2+ /Zn half-cell (-0.76 V).

What is the formula used to calculate standard cell potential from two half-cell E θ values?

E cell θ = E right θ - E left θ , which is equivalent to E reduction θ - E oxidation θ , placing the more positive value on the right.

Define standard cell potential (E cell θ )

The standard cell potential (E cell θ ) is the potential difference between two half-cells under standard conditions, calculated as E right θ minus E left θ , where the more positive half-cell is placed on the right.

Why does a positive E cell θ value indicate that a reaction is feasible?

A positive E cell θ means the forward reaction is thermodynamically spontaneous , releasing energy as electrons flow from the less positive to the more positive half-cell.

By convention, the half-cell with the .......... E θ value is placed on the right of a cell diagram and acts as the .......... pole.

By convention, the half-cell with the more positive E θ value is placed on the right of a cell diagram and acts as the positive pole.

Why does a positive E cell θ value not guarantee that a reaction will actually occur?

E cell θ gives information about thermodynamic feasibility only and provides no information about the rate of reaction , which may be too slow to be observed.

True or False? A reaction with a negative standard cell potential is feasible in the forward direction.

False. A negative E cell θ value means the forward reaction is not feasible ; it is the reverse reaction that would be spontaneous.

How does increasing the concentration of the oxidised species in a half-cell affect the electrode potential?

Increasing the concentration of the oxidised species shifts the equilibrium to the right by Le Chatelier's principle, making the electrode potential less negative (more positive).

A primary cell is an electrochemical cell that is non-rechargeable because the cell reaction is irreversible ; once the reactants are consumed the cell cannot be restored.

Why are lead-acid batteries considered disadvantageous despite their high current output?

They are very heavy , contain toxic lead and lead(IV) oxide, and use corrosive sulfuric acid as the electrolyte, creating challenges for safe disposal.

Define secondary cell

A secondary cell is a rechargeable electrochemical cell in which the cell reaction can be reversed by applying a voltage greater than the cell voltage, regenerating the original reactants.

In a hydrogen-oxygen fuel cell, the only reaction product is .......... and the cell operates continuously as long as there is a supply of .......... and oxygen.

In a hydrogen-oxygen fuel cell, the only reaction product is water and the cell operates continuously as long as there is a supply of hydrogen and oxygen.

Why do hydrogen-oxygen fuel cells offer environmental advantages over combustion-based engines?

Water is the only product , so no harmful oxides of nitrogen or carbon dioxide are released. The reaction occurs at room temperature , avoiding the high-temperature formation of pollutants.

True or False? Lithium-ion cells suffer from the memory effect after repeated charging cycles.

False. Lithium-ion cells do not exhibit the memory effect , so they can be topped up at any point without any loss of total charge capacity .

What does the term battery correctly refer to, and how does a car battery illustrate this?

A battery is correctly a collection of individual cells joined together; a car battery is six lead-acid cells in series, giving a combined voltage of approximately 12 V.

A LevelChemistryOCRFlashcards5. Physical Chemistry & Transition ElementsRedox & Electrochemistry

Redox & Electrochemistry (OCR A Level Chemistry A): Flashcards

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Define oxidising agent

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Define oxidising agent
An oxidising agent is a substance that oxidises another species by causing it to lose electrons, while itself gaining electrons and being reduced. Its oxidation number decreases in the reaction.
Define reducing agent
A reducing agent is a substance that reduces another species by causing it to gain electrons, while itself losing electrons and being oxidised. Its oxidation number increases in the reaction.
Why does the oxidation number of an oxidising agent decrease during a reaction?
The oxidising agent gains electrons from the reducing agent, which raises the electron count around the atom and lowers its oxidation state.
In a redox reaction, the oxidising agent is .......... and the reducing agent is .......... .
In a redox reaction, the oxidising agent is reduced and the reducing agent is oxidised.
In the reaction of MnO₄^- with Fe²⁺ ions, which species is the reducing agent and why?
Fe²⁺ is the reducing agent as it loses electrons, being oxidised to Fe³⁺, which enables the MnO₄^- ions to gain those electrons.
True or False?
A substance cannot act as both an oxidising agent and a reducing agent.
False.
Some substances can act as both an oxidising agent and a reducing agent, depending on what they are reacting with and the reaction conditions.
When predicting a missing product in a redox equation, why is H₂O often the first suggestion?
Most redox reactions occur in solution, where water, H⁺ and OH^- are always available to account for any oxygen atoms not balanced elsewhere in the equation.
Define redox titration
A redox titration is a titration in which an oxidising agent is titrated against a reducing agent, with electrons transferred between species to determine the concentration of an unknown solution.
Why is dilute sulfuric acid used to acidify the solution in a potassium manganate(VII) titration, rather than hydrochloric acid?
Hydrochloric acid would be oxidised by the manganate(VII) ions, introducing error; dilute sulfuric acid does not react with MnO₄^- and does not interfere with the titration.
In the iodine-thiosulfate titration, iodine reacts with .......... ions and is converted to colourless .......... ions.
In the iodine-thiosulfate titration, iodine reacts with thiosulfate ions and is converted to colourless iodide ions.
What colour change indicates the endpoint in a potassium manganate(VII) titration with Fe²⁺ ions?
The solution changes from colourless to a permanent pale pink tinge, caused by a slight excess of MnO₄^- ions remaining unreacted in the flask.
True or False?
Potassium manganate(VII) acts as its own indicator in a redox titration.
True.
The deep purple colour of MnO₄^- ions turns the solution a pale pink at the endpoint, making a separate indicator unnecessary.
Why is starch added near the endpoint in an iodine-thiosulfate titration rather than at the start?
Adding starch too early causes it to bind strongly to iodine, making the blue-black colour difficult to discharge; adding it when the solution is a pale straw colour gives a sharper colour change at the endpoint.
In an iodine-thiosulfate titration, what is the molar ratio of S₂O₃^2- to I₂?
The ratio is 2:1: two moles of thiosulfate ions react with one mole of iodine, as shown by the equation 2S₂O₃^2-(aq) + I₂(aq) → 2I^-(aq) + S₄O₆^2-(aq).
Define standard electrode potential (E^θ)
The standard electrode potential (E^θ) is the potential difference produced when a standard half-cell is connected to a standard hydrogen electrode under standard conditions (1.00 mol dm^-3, 298 K, 100 kPa).
Why must standard conditions be used when comparing electrode potentials of different half-cells?
Electrode potential depends on temperature, pressure and concentration, so fixing these to standard values ensures all measurements are made on the same basis for a valid comparison.
Define standard hydrogen electrode
A standard hydrogen electrode is a reference half-cell consisting of H₂ gas at 100 kPa in equilibrium with H⁺ ions at 1.00 mol dm^-3 in contact with an inert platinum electrode, assigned a value of 0.00 V.
In an electrochemical cell, .......... takes place at the negative electrode and .......... takes place at the positive electrode.
In an electrochemical cell, oxidation takes place at the negative electrode and reduction takes place at the positive electrode.
Why is platinum used as the electrode in a non-metal/non-metal ion half-cell?
Platinum is inert and does not participate in the redox reaction, allowing the equilibrium to be established on its surface without introducing additional redox chemistry.
True or False?
Oxidation occurs at the positive electrode in an electrochemical cell.
False.
Oxidation (loss of electrons) occurs at the negative electrode; reduction occurs at the positive electrode.
In the conventional cell diagram Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), which half-cell is the positive pole and why?
The Cu²⁺/Cu half-cell is the positive pole because its E^θ value (+0.34 V) is more positive than that of the Zn²⁺/Zn half-cell (-0.76 V).
What is the formula used to calculate standard cell potential from two half-cell E^θ values?
E_cell^θ = E_right^θ - E_left^θ, which is equivalent to E_reduction^θ - E_oxidation^θ, placing the more positive value on the right.
Define standard cell potential (E_cell^θ)
The standard cell potential (E_cell^θ) is the potential difference between two half-cells under standard conditions, calculated as E_right^θ minus E_left^θ, where the more positive half-cell is placed on the right.
Why does a positive E_cell^θ value indicate that a reaction is feasible?
A positive E_cell^θ means the forward reaction is thermodynamically spontaneous, releasing energy as electrons flow from the less positive to the more positive half-cell.
By convention, the half-cell with the .......... E^θ value is placed on the right of a cell diagram and acts as the .......... pole.
By convention, the half-cell with the more positive E^θ value is placed on the right of a cell diagram and acts as the positive pole.
Why does a positive E_cell^θ value not guarantee that a reaction will actually occur?
E_cell^θ gives information about thermodynamic feasibility only and provides no information about the rate of reaction, which may be too slow to be observed.
True or False?
A reaction with a negative standard cell potential is feasible in the forward direction.
False.
A negative E_cell^θ value means the forward reaction is not feasible; it is the reverse reaction that would be spontaneous.
How does increasing the concentration of the oxidised species in a half-cell affect the electrode potential?
Increasing the concentration of the oxidised species shifts the equilibrium to the right by Le Chatelier's principle, making the electrode potential less negative (more positive).
Define primary cell
A primary cell is an electrochemical cell that is non-rechargeable because the cell reaction is irreversible; once the reactants are consumed the cell cannot be restored.
Why are lead-acid batteries considered disadvantageous despite their high current output?
They are very heavy, contain toxic lead and lead(IV) oxide, and use corrosive sulfuric acid as the electrolyte, creating challenges for safe disposal.
Define secondary cell
A secondary cell is a rechargeable electrochemical cell in which the cell reaction can be reversed by applying a voltage greater than the cell voltage, regenerating the original reactants.
In a hydrogen-oxygen fuel cell, the only reaction product is .......... and the cell operates continuously as long as there is a supply of .......... and oxygen.
In a hydrogen-oxygen fuel cell, the only reaction product is water and the cell operates continuously as long as there is a supply of hydrogen and oxygen.
Why do hydrogen-oxygen fuel cells offer environmental advantages over combustion-based engines?
Water is the only product, so no harmful oxides of nitrogen or carbon dioxide are released. The reaction occurs at room temperature, avoiding the high-temperature formation of pollutants.
True or False?
Lithium-ion cells suffer from the memory effect after repeated charging cycles.
False.
Lithium-ion cells do not exhibit the memory effect, so they can be topped up at any point without any loss of total charge capacity.
What does the term battery correctly refer to, and how does a car battery illustrate this?
A battery is correctly a collection of individual cells joined together; a car battery is six lead-acid cells in series, giving a combined voltage of approximately 12 V.

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