5.5 Redox & Electrode Potentials for A-Level Chemistry (OCR)

What electrode is used as the reference when measuring standard electrode potentials?

Standard hydrogen electrode

All electrode potentials are measured relative to this reference electrode.

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What symbol represents the standard electrode potential of a half-cell?

E°

It indicates the electrode potential measured under standard conditions.

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Fill in the blank:

Standard electrode potentials are measured at a temperature of _______ K.

298

Standard conditions also include 100 kPa pressure and 1.00 mol dm^-3 solutions.

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True or False:

Standard electrode potentials are measured with ion concentrations of 1.00 mol dm^-3.

True

This ensures all measurements are comparable.

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What name is given to the ordered list of standard electrode potentials?

Electrochemical series

It ranks half-cells according to their tendency to be reduced.

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True or False:

A more positive E° value indicates a greater tendency to gain electrons.

True

Species with more positive E° values are more easily reduced.

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Fill in the blank:

The maximum voltage produced by an electrochemical cell is called the _______.

EMF

EMF stands for electromotive force.

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How is the EMF of a cell calculated using standard electrode potentials?

E°cell = E°(reduction) − E°(oxidation)

Alternatively: E°cell = E°(right) − E°(left) from the cell diagram.

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True or False:

A positive EMF indicates that the redox reaction is feasible.

True

A positive E°cell suggests the reaction is thermodynamically favourable.

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In the conventional cell representation Zn|Zn²⁺||Cu²⁺|Cu, which electrode is the negative electrode?

Zinc

Zinc is oxidised and acts as the anode.

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Fill in the blank:

In a conventional cell diagram, a double vertical line represents the _______ bridge.

salt

The salt bridge allows ions to move and complete the circuit.

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What process occurs at the anode in an electrochemical cell?

Oxidation

The anode is where electrons are released.

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What type of device converts chemical energy directly into electrical energy using redox reactions?

Electrochemical cell

These cells allow electron transfer through an external circuit to generate current.

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What type of electrochemical cell cannot be recharged once the reactants are used up?

Non-rechargeable cell

These are also called primary cells and operate via irreversible reactions.

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True or False:

Fuel cells must be recharged after their reactants are used up.

False

Fuel cells continuously produce electricity as long as fuel and oxidant are supplied.

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In a lithium cell, what process occurs at the negative electrode involving lithium atoms?

Oxidation

Lithium atoms lose electrons to form Li⁺ ions.

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True or False:

Give the half equation for the oxidation of lithium at the negative electrode

Li → Li⁺ + e^-.

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Fill in the blank:

In a hydrogen–oxygen fuel cell, hydrogen is _______ at the electrode.

oxidised

Hydrogen loses electrons to form water through reaction with oxygen.

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What is the overall product formed in a hydrogen–oxygen fuel cell?

Water

Hydrogen and oxygen combine electrochemically to form water.

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True or False:

Fuel cells generate electricity through redox reactions.

True

Oxidation and reduction occur at separate electrodes.

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Fill in the blank:

The movement of _______ through the external circuit generates electric current in electrode reactions.

electrons

Electrons flow from the negative electrode to the positive electrode.

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What advantage do hydrogen fuel cells offer compared with fossil fuel power sources?

Low emissions

The main product is water, reducing pollution compared with combustion.

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Give the half equation for the reduction reaction that occurs at the positive electrode of the lithium cell

Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻

Remember Lithium cobalt oxide (LiCoO₂) is the positive electrode in a lithium ion cell

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What is the half equation at the negative electrode in an alkaline hydrogen–oxygen fuel cell?

H₂ + 2OH⁻ → 2H₂O + 2e⁻

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What is the **half equation** at the **positive electrode** in an **alkaline hydrogen–oxygen fuel cell**?

O₂ + 2H₂O + 4e⁻ → 4OH⁻

What is the **overall reaction** in an **alkaline hydrogen–oxygen fuel cell**

2H₂ + O₂ → 2H₂O ## Footnote This is why the emissions are so low as when we combine the two half equations to make the overall equation. Only water is released.

Why are **fuel cells** useful for **continuous energy supply**?

They can operate continuously as long as fuel (H₂ and O₂) is supplied

What is a **major issue with hydrogen production in alkaline hydrogen-oxygen fuel cells**?

Hydrogen is often produced from fossil fuels, which releases CO₂

Why is **hydrogen storage a challenge**?

Hydrogen is highly flammable and requires high pressure or low temperatures to store

What is a key **advantage of rechargeable cells**?

They can be reused many times, reducing waste

Why are **rechargeable cells cost-effective**?

hey can be recharged, so fewer need to be bought over time

Why are **rechargeable cells better for the environment**?

Less disposal → reduced environmental pollution

What is a **disadvantage of rechargeable cells**?

They are more expensive initially

What **safety risk** is associated with **rechargeable cells**?

Risk of overheating or explosion if damaged or overcharged

What is an **advantage of non-rechargeable cells**?

They are cheap and readily available

What is the main **environmental issue** with **non-rechargeable cells**?

They must be disposed of after use, causing waste

What **electrode** is used when **no solid substance is present**?

An inert electrode (e.g. platinum)

Why is an **inert electrode** used when **no solid is present**?

To provide a surface for electron transfer without reacting

What is the **electrode potential** of the **SHE**?

0.00V

What are the **standard conditions** for the **SHE**?

Temperature = 298 K Pressure = 100 kPa (H₂ gas) [H⁺] = 1.0 mol dm⁻³

What is the **structure** of the **SHE**?

Platinum electrode in contact with H₂ gas and H⁺ solution

How is a **conventional cell representation written**?

Oxidation (anode) on the left, reduction (cathode) on the right

What does a **single vertical line ( | )** represent in a conventional cell representation?

A phase boundary (e.g. solid/liquid or liquid/gas)

What must be included when writing the conventional cell representation?

States of substances (s, aq, g) salt bridge phase boundaries most oxidised/ reduced closest to the salt bridge

How is an **inert electrode** shown in the **conventional cell representation** ?

As Pt(s) or C(s) at the appropriate end

What do **commas** represent in **conventional cell notation**?

Different species in the same phase

What type of reaction involves the **transfer of electrons** between substances?

Redox reaction ## Footnote Redox reactions involve oxidation and reduction occurring together via electron transfer.

What is the **loss of electrons** from a species called?

Oxidation ## Footnote Remember the mnemonic **OIL RIG**: Oxidation Is Loss, Reduction Is Gain.

What is the **gain of electrons** by a species called?

Reduction ## Footnote Reduction occurs when a species gains one or more electrons.

# True or False: **Oxidising agents** donate **electrons** during a redox reaction.

False ## Footnote Oxidising agents accept electrons and are themselves reduced.

What name is given to a **substance that accepts electrons** in a redox reaction?

Oxidising agent ## Footnote Because it causes another substance to be oxidised.

# True or False: A **reducing agent** donates **electrons** to another species.

True ## Footnote The reducing agent loses electrons and therefore becomes oxidised.

# Fill in the blank: The **numerical value** assigned to an atom to show **electron loss or gain** in compounds is its oxidation _\_\_\_\_\_\_.

state ## Footnote Oxidation states help track electron transfer in reactions.

What happens to the **oxidation state** of an element when it is **oxidised**?

Increases ## Footnote Losing electrons makes the oxidation state more positive.

# True or False: **Reduction** results in a **decrease in oxidation state**.

True ## Footnote Gaining electrons lowers the oxidation number.

What type of equation shows either the **oxidation** or **reduction process** separately in a redox reaction?

Half-equation ## Footnote Half-equations show electron transfer clearly.

# Fill in the blank: When combining **half-equations** to form an overall redox equation, the number of _\_\_\_\_\_\_ must be equal on both sides.

electrons ## Footnote Electrons lost in oxidation must equal electrons gained in reduction. They cancel out when the half-equations are combined, ensuring the overall equation is balanced for charge.

In an **oxidation half-equation**, are electrons on the **left** or **right** side?

Right side ## Footnote If a species is being oxidised it is releasing electrons, and anything released in a chemical equation is written on the right-hand side (products side).

In a **reduction half-equation**, are electrons on the **left** or **right** side?

Left side ## Footnote Reduction = Gain of electrons (RIG) So the species is taking in electrons, and anything that is taken in is written on the left-hand side (reactants side)

What is used to **balance oxygen atoms** in a half-equation?

Water (H₂O)

What is used to **balance hydrogen atoms** in a half equation?

Hydrogen ions (H⁺)

What is used to **balance charge** in a half-equation?

Electrons (e⁻)

What is the correct **order** for **balancing half-equations**?

Balance non-O/H atoms Balance O with H₂O Balance H with H⁺ Balance charge with e⁻

5.5 Redox & Electrode Potentials Flashcards

Predict feasibility of redox reactions using electrode potentials and electrochemical cells. (63 cards)