Unit 2 – Electrochemistry (Lecture Review)

A comprehensive set of question-and-answer flashcards covering oxidation–reduction concepts, balancing methods, electrode potentials, electrolysis calculations, and identifying oxidising/reducing agents from Unit 2 Electrochemistry lecture notes.

What is liberated at an electrode during electrolysis determined by?

The nature of the electrolyte, the position of the species in the redox series, and the concentration of the ions present.

State the mathematical relationship that links the Faraday constant (F), Avogadro’s constant (NA), and the elementary charge (e).

F = NA × e (≈ 96 500 C mol⁻¹).

Which equation lets you calculate the quantity of charge passed (Q) during electrolysis?

Q = I × t, where I is current (A) and t is time (s).

How can the Avogadro constant be deduced electrochemically?

By measuring the charge required to deposit or liberate a known amount of substance and using Faraday’s laws to relate charge to moles of electrons.

Define standard electrode potential (E°).

The potential of a half-cell relative to the standard hydrogen electrode when all species are at unit activity: 1 mol dm⁻³ for solutions, 1 atm for gases, and pure solids or liquids at 298 K.

What is a standard cell potential (E°cell)?

The potential difference between two standard electrodes combined to form a cell under standard conditions: E°cell = E°(cathode) – E°(anode).

Describe the standard hydrogen electrode (SHE).

A platinum electrode in contact with 1 mol dm⁻³ H⁺ solution and hydrogen gas at 1 atm, assigned a potential of 0.00 V.

How do you predict whether a redox reaction is feasible using E° values?

Combine the two half-reactions; if E°cell is positive, the reaction is thermodynamically feasible and electrons flow from the anode (more negative E°) to the cathode (more positive E°).

What is indicated when E°(A) > E°(B) for two half-cells?

The species in half-cell A is a stronger oxidising agent (or weaker reducing agent) than the species in half-cell B.

Give the Nernst equation for a half-cell at temperature T.

E = E° – (RT / nF) ln Q, where Q is the reaction quotient, n is electrons transferred, R is gas constant, and F is Faraday constant.

In a voltaic (galvanic) cell, at which electrode does oxidation occur?

Oxidation occurs at the anode (negative electrode in a galvanic cell).

Using a Cu–Zn cell, which metal acts as the anode and why?

Zinc acts as the anode because it has the more negative E° value and undergoes oxidation (Zn → Zn²⁺ + 2e⁻).

Define oxidation in terms of electrons.

Oxidation is the loss of electrons by a species.

Define reduction in terms of electrons.

Reduction is the gain of electrons by a species.

What is an oxidation number (oxidation state)?

The apparent positive or negative charge an atom would have in a compound if all bonds were ionic.

How is oxidation recognised using oxidation numbers?

By an increase in the oxidation number of an element.

Write the two half-equations for the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

Oxidation: Zn(s) → Zn²⁺ + 2e⁻ ; Reduction: Cu²⁺ + 2e⁻ → Cu(s).

In CH₄ + 2O₂ → CO₂ + 2H₂O, which element is oxidised and which is reduced?

Carbon is oxidised (oxidation state −4 to +4); oxygen is reduced (0 to −2).

What is a disproportionation reaction?

A redox reaction in which the same element is simultaneously oxidised and reduced.

Give an example of disproportionation discussed in the lecture.

Decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂, where O changes from −1 to both −2 and 0.

State the core principle behind the oxidation-number method of balancing redox equations.

Total electrons lost in oxidation must equal total electrons gained in reduction.

During balancing by the oxidation-number method, why are bridging lines drawn between atoms?

To show which atoms change oxidation state and the number of electrons transferred.

What are the three main steps of the ion-electron (half-reaction) method?

(1) Balance atoms other than O and H; (2) Balance O with H₂O and H with H⁺ (or OH⁻ in base); (3) Balance charge with electrons and combine half-reactions ensuring electron cancellation.

In acidic medium, how are oxygen and hydrogen balanced in half-reactions?

Add H₂O to balance O atoms and H⁺ to balance H atoms.

Identify the oxidising agent in MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂.

MnO₂ is the oxidising agent because Mn is reduced from +4 to +2.

What defines an oxidising agent?

A substance that accepts electrons (is reduced) and thus causes another substance to be oxidised.

What defines a reducing agent?

A substance that donates electrons (is oxidised) and thus causes another substance to be reduced.

In the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂, which reactant is the reducing agent?

CO is the reducing agent because carbon is oxidised from +2 to +4.

Why is the whole molecule (e.g., CO) considered the reducing agent rather than just the element (C)?

Because the entire species donates electrons and undergoes the chemical change, not merely the atom inside it.

What experimental observation indicates that FeSO₄ reduces KMnO₄ in acidic solution?

The purple colour of KMnO₄ is discharged (fades) as it is reduced, while Fe²⁺ is oxidised to Fe³⁺.

What is a galvanic cell and what are it’s 2 other names??

A galvanic cell is also called a denial cell and volatic cell. It is a cell that converts chemical energy into electrical energy.

What is the purpose of salt bridge?

The purpose of the salt bridge in a galvanic cell is to maintain electrical neutrality by allowing the flow of ions between the two half-cells, completing the circuit and preventing charge buildup.

How does a salt bridge works

The salt bridge has both kinds of ions.

When one side needs positive ions, the salt bridge sends positive ions there.

When the other side needs negative ions, it sends negative ions there.

This balance lets the electrons keep flowing through the wire — and that's how the battery keeps

What are the 2 ways to balance a redox reaction equation??

1. Redox method/ ion oxidation method

2. Ion electron method

Which reaction is on anode and which is on cathode??

Oxidation always happen on anode and reduction on cathode

[An ox….Red Cat]