Electrochemical Cells

What is an electrochemical half-cell?

A metal electrode dipped in a solution of its own ions (e.g. Zn in Zn²⁺), where a redox equilibrium is established

What is the function of a salt bridge?

It completes the circuit by allowing ions to flow, prevents solutions mixing, balances charge

What is a salt bridge usually made of?

Filter paper or a U-tube soaked in an inert salt solution such as KNO₃ potassium nitrate

What are the standard conditions for measuring electrode potentials (E°)?

298 K temperature, 1.00 mol dm⁻³ ion concentration, 100 kPa pressure for gases

What is the Standard Hydrogen Electrode (SHE)?

A reference electrode assigned a potential of 0.00 V, consisting of H₂ gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), and a platinum electrode

Why is platinum used in the standard hydrogen electrode?

It is inert and provides a surface for the equilibrium between H⁺ and H₂ to establish

What is the definition of standard electrode potential (E°)?

The potential of a half-cell under standard conditions measured against the Standard Hydrogen Electrode

What does a more positive E° value indicate?

The species is more likely to gain electrons (be reduced) and is a stronger oxidising agent

How is standard cell potential (E°cell) calculated?

E°cell = E° positive (cathode) − E° negative (anode)

Which way do electrons flow in the external circuit?

From the more negative electrode (anode) to the more positive electrode (cathode)

What is the conventional representation for a Zn/Cu cell?

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

What does a single vertical line (|) represent in cell notation?

A phase boundary (e.g. between solid electrode and aqueous solution)

What does a double vertical line (||) represent in cell notation?

The salt bridge

What are the three main types of commercial electrochemical cells?

Non-rechargeable (primary), rechargeable (secondary), and fuel cells

What is a non-rechargeable cell?

A single-use cell where the redox reactions are irreversible; once reactants are used up, the cell cannot be recharged

Give an example of a non-rechargeable cell.

Zinc-carbon cell or alkaline battery

What is a rechargeable cell?

A cell where the redox reactions are reversible; applying an external current reverses the reactions, reforming the original reactants

What happens during charging of a rechargeable cell?

An external voltage is applied to force the current in the opposite direction, reversing the redox reactions

When is lithium-ion battery used?

Used in portable electronics, known for being lightweight and its rechargeable

What is the negative electrode (anode) reaction in a lithium-ion battery during discharge?

Li → Li⁺ + e⁻

What is the positive electrode (cathode) reaction in a lithium-ion battery during discharge?

Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻

What is a fuel cell?

An electrochemical cell that continuously converts the chemical energy of a fuel (e.g. hydrogen) and oxidant (e.g. oxygen) into electrical energy, as long as fuel is supplied

How does a fuel cell differ from a battery?

A fuel cell requires a continuous supply of fuel and oxidant; a battery stores chemical energy internally

What is the only product of a hydrogen-oxygen fuel cell?

Water (H₂O)

What is the overall equation for a hydrogen-oxygen fuel cell?

2H₂(g) + O₂(g) → 2H₂O(l)

What are the half-equations for a hydrogen-oxygen fuel cell in alkaline conditions?

Anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻; Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻

What are the half-equations for a hydrogen-oxygen fuel cell in acidic conditions?

Anode: H₂ → 2H⁺ + 2e⁻; Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O

What are the advantages of hydrogen-oxygen fuel cells?

Water is only product (no pollution at point of use), high efficiency, no combustion, and can be used in spacecraft (water for drinking)

What are the disadvantages/risks of hydrogen-oxygen fuel cells?

Hydrogen is flammable and explosive, difficult to store (requires high pressure), production relies on fossil fuels currently, and platinum catalysts are expensive

What is the hydrogen economy?

The concept of using hydrogen as a clean fuel, with fuel cells generating electricity and producing only water

What happens to the E°cell value when a cell is being recharged?

It becomes negative, indicating energy is being put into the system to force the reverse reaction

Why do non-rechargeable cells eventually stop working?

The reactants are consumed and the irreversible reactions reach completion, so the potential difference drops to zero

Why do rechargeable cells degrade over time?

Repeated charging and discharging causes physical changes to electrodes and loss of active material

What environmental concerns are associated with batteries?

Toxic chemicals and heavy metals can leak into environment; mining for lithium and cobalt causes habitat destruction; disposal requires careful recycling