redox

• Redox chemistry involves oxidation and reduction reactions
• The term "redox" is a combination of "ox" for oxidation and "red" for reduction
• In chemistry, oxidation and reduction can be defined in four ways:

  • Oxygen gain or loss
  • Hydrogen gain or loss
  • Electron gain or loss
  • Oxidation state change
• Gain of oxygen leads to oxidation, while loss of oxygen leads to reduction
• Loss of hydrogen results in oxidation, while gain of hydrogen leads to reduction
• Loss of electrons is oxidation, while gain of electrons is reduction
• Oxidation state change: Oxidation occurs when the oxidation state of an atom increases, and reduction occurs when the oxidation state decreases
• The mnemonic "SHOE" can help remember the four definitions of oxidation and reduction:

  • S: Oxidation state
  • H: Hydrogen
  • O: Oxygen
  • E: Electron
• Convenient definitions of oxidation and reduction are based on oxygen, hydrogen, and electron gain or loss
• The most valid definition is based on oxidation state changes
• In reactions, if something is oxidized, something else must be reduced; oxidation and reduction occur together
• Half equations show only half of the reaction; combining them gives the overall reaction
• Oxidation state is a number indicating how oxidized or reduced an atom is in a compound
• Rules for assigning oxidation states:

  1. Atoms in free elements have an oxidation state of zero

  2. Monoatomic ions have oxidation states equal to their charges

  3. Some elements have fixed oxidation states in compounds
• In single-celled organisms, substances can easily enter the cell due to a short distance, while in multicellular organisms, the distance is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen due to their higher surface area to volume ratio
• Atoms of an element in compounds have fixed oxidation states:

  • Group one elements have an oxidation state of +1
  • Group two elements have an oxidation state of +2
  • Oxygen has an oxidation state of -2
  • Hydrogen has an oxidation state of +1
• Examples of oxidation states:

  • Sodium in sodium chloride: +1
  • Magnesium in magnesium oxide: +2
  • Hydrogen in hydrogen chloride: +1
  • Oxygen in water: -2
  • Hydrogen in methane: +1
• Exceptions to fixed oxidation states:

  • Oxygen in hydrogen peroxide: -1
  • Hydrogen in metal hydrides: -1
• Rule for sum of oxidation states in a compound:

  • The sum of oxidation states of all atoms in a compound is equal to zero
• Examples applying the sum of oxidation states rule:

  • Magnesium chloride: Cl has an oxidation state of -1
  • Sodium carbonate: C has an oxidation state of +4
  • Potassium permanganate: Mn has an oxidation state of +7
• Exceptions to fixed oxidation states:

  • Chromium in sodium dichromate: +6
• Rule for polyatomic ions:

  • The sum of oxidation states for all atoms in a polyatomic ion is equal to the charge of the ion
• Examples applying the polyatomic ion rule:

  • Sulfur in sulfate: +6
  • Nitrogen in nitrate: +5
  • Nitrogen in ammonium ion: -3
• Oxidation is when the oxidation state of an atom increases in a reaction, while reduction is when the oxidation state decreases
• To identify the substance that is oxidized and reduced in a reaction, assign oxidation states to all atoms in the reactants and products
• For the reaction involving zinc and copper oxide:

  • Zinc has an oxidation state of 0
  • Copper in copper oxide has an oxidation state of 0
  • Oxygen in compounds has an oxidation state of -2
  • Applying the rule that the sum of oxidation states in a compound is 0, the oxidation state of copper in copper oxide is +2 and zinc in zinc oxide is +2
  • Zinc is oxidized as its oxidation state increases from 0 to +2
  • Copper is reduced as its oxidation state decreases from +2 to 0
• Template for identifying oxidized and reduced substances:

  • The substance that is oxidized is [Name of Substance], as the oxidation state of [Atom] increases from [Initial State] in [Reactant] to [Final State] in [Product]
  • The substance that is reduced is [Name of Substance], as the oxidation state of [Atom] decreases from [Initial State] in [Reactant] to [Final State] in [Product]
• In the decomposition of hydrogen peroxide:

  • Oxygen in oxygen gas has an oxidation state of 0
  • Oxygen in hydrogen peroxide has an oxidation state of -1
  • Hydrogen in hydrogen peroxide has an oxidation state of +1
  • Oxygen in water has an oxidation state of -2
  • Hydrogen in water has an oxidation state of +1
  • Hydrogen peroxide is both oxidized and reduced, known as disproportionation
• Common mistakes in assigning oxidation states:

  • Oxidation states are assigned to individual atoms, not compounds
  • Reacting ratios are not considered when assigning oxidation states
  • The oxidation state of hydrogen is +1, even if there are multiple hydrogen atoms
  • The substance that is oxidized or reduced must be a reactant, not a product
• Types of reactions that are not redox:

  • Acid-base reactions
  • Precipitation reactions
  • Decomposition reactions (thermal decomposition of metal carbonates)
• Quick method to identify redox reactions at O level:

  • If any element is present in the reactants or products of a reaction, it is a redox reaction