Electrochemical cells

Standard Hydrogen Electrode (SHE)

A reference electrode with a standard electrode potential of 0.00 V, consisting of hydrogen gas at 1 atm pressure in contact with a 1 mol dm⁻³ solution of H⁺ ions, using a platinum electrode.

Platinum in SHE

Platinum is inert and conducts electricity, providing a surface for electron transfer.

Conditions of SHE

1. 1 mol dm⁻³ H⁺ solution; 2. Hydrogen gas at 100 kPa; 3. 298 K (25°C); 4. Inert platinum electrode.

Standard Electrode Potential (E⁰)

The voltage of a half-cell compared with the standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1 mol dm⁻³ solutions).

More Positive E⁰ Value

Indicates a half-cell with a greater tendency to gain electrons (reduction); the species is a stronger oxidizing agent.

More Negative E⁰ Value

Indicates a half-cell with a greater tendency to lose electrons (oxidation); the species is a stronger reducing agent.

EMF Calculation

EMF = E⁰ (more positive) – E⁰ (more negative).

Positive Terminal in Electrochemical Cell

The electrode with the more positive E⁰ value.

Flow in the External Circuit

Electrons flow from the more negative electrode to the more positive electrode.

Flow in the Salt Bridge

Ions flow to maintain charge balance in each half-cell; the salt bridge allows ion movement but not electron flow.

Inert Ions in Salt Bridge

Necessary to prevent reaction with the ions in the half-cells.

Necessity of Salt Bridge

To complete the circuit and prevent charge build-up.

High-Resistance Voltmeter Purpose

To measure EMF without drawing current, ensuring standard conditions.

Effect of Concentration on EMF

Increasing ion concentration in the reduction half-cell increases EMF; in the oxidation half-cell, it decreases EMF.

Effect of Temperature on EMF

EMF can increase or decrease depending on the enthalpy change and entropy effects of the cell reaction.

Fuel Cell

A device that converts the chemical energy of a fuel directly into electrical energy by a redox reaction.

Products of Hydrogen Fuel Cell (Acidic)

Water.

Half-Equations in Acidic Hydrogen Fuel Cell

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

Half-Equations in Alkaline Hydrogen Fuel Cell

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

Advantages of Fuel Cells Over Combustion Engines

Higher efficiency, less CO₂ produced, continuous operation with fuel supply.

Disadvantages of Hydrogen Fuel Cells

Hydrogen is flammable, storage and transport are difficult, production may involve fossil fuels.

Increasing Electrode Surface Area Effect on EMF

More surface area for redox reactions allows more efficient electron transfer.

Feasible Reaction in Electrochemistry

One where the cell EMF is positive, indicating a thermodynamically favorable redox reaction.

Non-feasibility Despite Positive EMF

If kinetic factors like activation energy or lack of a catalyst prevent the reaction.

Predict Feasibility of Redox Reactions Using E⁰ values

1. Identify the half-equations; 2. Reverse the more negative one (oxidation); 3. Add the half-equations; 4. Ensure EMF is positive.

Need for Platinum or Graphite in Half-Cells

Required when neither redox species is a solid conductor, a solid electrode is needed to transfer electrons.

Fe³⁺/Fe²⁺ Metal Electrode Usage Example

Cannot use a metal electrode as both Fe³⁺ and Fe²⁺ are aqueous ions; platinum is needed for redox reactions.

Non-Metal Species in Electrochemical Cells

Yes, but a solid inert electrode (e.g. Pt) must be used to allow electron transfer.

Conventional Cell Diagram Writing

Most oxidized species on the left; vertical line for a phase boundary; double vertical line for the salt bridge.

Need for Inert Electrode in Cell Diagram

When redox species are in solution only (no metal present). Example: Pt(s) | Fe²⁺(aq), Fe³⁺(aq).

Drawing an Electrochemical Cell Setup

Two beakers with half-cell solutions; metal electrodes connected by wires and voltmeter; salt bridge in both solutions.

Standard Cell Conditions

1 mol dm⁻³ concentrations, 100 kPa pressure, 298 K temperature.