Redox Reactions, Voltaic Cells, and Electrolysis

Posted on Oct 30, 2024 in Chemistry

Redox Reactions

Oxidation and Reduction

Oxidation: The process of electron loss by a reductant. The reductant is the substance that contains the element whose oxidation number increases.

Reduction: The process of electron gain by an oxidant. The oxidant is the substance that contains the element whose oxidation number decreases.

• Oxidation: Process where an element’s oxidation number increases (loses electrons).
• Reduction: Process where an element’s oxidation number decreases (gains electrons).

Redox Reactions: Chemical processes where there is a change in the oxidation number of elements involved.

Rules for Oxidation Numbers

• Free elements: 0
• Monatomic ions: Charge of the ion
• Alkali metals: +1
• Alkaline earth metals: +2
• Hydrogen: +1 (except in metal hydrides like NaH, CaH₂)
• Oxygen: -2 (except in peroxides, -1)

A redox reaction involves a conjugate pair: an oxidant and its reduced form (conjugate reductant), and a reductant and its oxidized form (conjugate oxidant).

Voltaic Cells

A voltaic cell is a device that generates an electric current from a spontaneous redox reaction.

Components

• Anode (Zinc): An electrode (zinc sheet) immersed in an aqueous solution of a soluble salt (ZnSO₄). Oxidation occurs at the anode, decreasing the mass of zinc. (Zn – 2e^– → Zn²⁺)
• Cathode (Copper): An electrode (copper sheet) immersed in a solution of a soluble salt containing Cu²⁺ ions (CuSO₄). Reduction occurs at the cathode, increasing the mass of copper. (Cu²⁺ + 2e^– → Cu)
• External Circuit: A metallic conductor allows electron flow from the anode to the cathode.
• Voltmeter: Measures the cell’s electromotive force (potential difference between electrodes).
• Salt Bridge: Contains an inert electrolyte (e.g., KCl) to close the circuit and maintain charge neutrality.

Shorthand Notation

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

Anode: Oxidation, Loss of Electrons

Cathode: Reduction, Gain of Electrons

Standard Hydrogen Electrode

The standard hydrogen electrode potential is assigned 0V. Other electrode potentials are measured relative to it. This electrode consists of a platinum foil immersed in a 1M HCl solution at 25°C, with H₂ gas bubbled at 1 atm pressure.

• Anode: H₂ – 2e^– → 2H⁺
• Cathode: 2H⁺ + 2e^– → H₂

Standard electrode potential is the potential difference of a cell consisting of the electrode and a standard hydrogen electrode, both under standard conditions.

Half-reactions with negative potentials act as anodes against electrodes with positive potentials (which act as cathodes).

Electrolysis

Electrolysis is a process where an electric current passed through a solution or molten electrolyte causes a non-spontaneous reaction to occur.

Components

• Electrolytic Cell: Container holding the solution or molten electrolyte and the electrodes.
• Electrodes: Surfaces where the redox reactions occur. Connected to a DC source.
• Anode: Electrode where oxidation occurs (+ pole).
• Cathode: Electrode where reduction occurs (- pole).

Differences between Voltaic Cells and Electrolytic Cells

Industrial Applications

• Obtaining active metals (Groups 1 and 2, Al) and non-metals (H₂, Cl₂).
• Producing compounds (e.g., NaOH).
• Electroplating: Depositing a thin metal layer (e.g., gold, silver, zinc, nickel, chromium, copper).
• Metal purification (e.g., copper).

Charge: Current × Time. 96,500 C is equal to 1 mole of electrons.