Classification of Chemical Reactions

Definition: Oxidation-reduction reactions or redox reactions are reactions in which electrons are transferred from one atom to another.
Learning Objective: Recognize redox reactions and identify the species being oxidized and reduced.

Electron Transfer:
- The number of electrons assigned to individual atoms changes during the reaction.
- Involves covalent compounds where electrons are rearranged as bonds are broken and new bonds are formed.
Types of Reactions Considered as Redox:
- Single replacement reactions
- Combustion reactions (always)
- Combination reactions

Historically:
- Oxidation: The addition of oxygen atoms to an element.
- Reduction: The removal of oxygen from a compound to yield the element.
Today:
- Oxidation: Defined as the loss of one or more electrons (OIL - Oxidation Is Loss).
- Reduction: Defined as the gain of one or more electrons (RIG - Reduction Is Gain).

Oxidation Examples:
- For A²-: A²- → A⁻ + electron
- For A⁻: A⁻ → A + electron
- For A: A → A⁺ + electron
- For A⁺: A⁺ + electron → A²⁺ + electron
Reduction Reactants: Reactant A can be a:
- Neutral atom
- Monatomic ion
- Polyatomic ion
- Molecule

Oxidation and reduction always occur together in a reaction (redox).
When one substance loses an electron (is oxidized), another gains that electron (is reduced).
Reducing Agent:
- Causes reduction to happen.
- May lose hydrogen atoms, undergo oxidation.
- Becomes more positive (or less negative).
Oxidizing Agent:
- Causes oxidation to occur.
- May lose oxygen atoms, undergo reduction.
- Becomes more negative (or less positive).

Electroplating: Copper wire reacts with aqueous Ag⁺ ions, becoming coated with metallic silver. Copper is oxidized, and silver ions are reduced.
Iodide Ion and Bromine Reaction:
- Iodide ion gives an electron to bromine, forming iodine and bromide ions.
- Oxidation: Iodide ions (–1 to 0).
- Reduction: Bromine atoms (0 to –1).

Alkali Metals and Alkaline Earth Metals: Powerful reducing agents.
- React with pure water due to low ionization energy.
- As ionization energy increases, reducing power decreases.
- Low Ionization Energy: Easy to lose electrons.
Reactive Nonmetals: Powerful oxidizing agents.
- High ionization energies and favorable electron affinities.
- Electron affinity relates to the ability to accept electrons.

Reactions Involving Metals and Nonmetals:
- Metals tend to lose electrons, while nonmetals tend to gain electrons.
- Trends can be predicted based on the position in the periodic table.
Reactions Involving Nonmetals:
- The more metallic element (further down/left) tends to lose electrons.
- The less metallic element (up/right) tends to gain electrons.

Corrosion: Deterioration of metal by oxidation, e.g., rusting of iron in moist air.
Combustion: Burning of fuel through rapid oxidation with oxygen.
Respiration: Biological redox reactions provide energy for living organisms by using oxygen.
Bleaching: De-colorization of materials through redox reactions.
Metallurgy: Extraction and purification of metals involves multiple redox processes.

Coenzyme FAD: The biomolecule FAD (flavin adenine dinucleotide) is reduced to FADH₂ through the transfer of two hydrogen atoms (2H⁺ and 2e⁻).
In biochemical reactions, hydrogen transfers are often necessary for energy production.

Indicators of Redox:
- Changes in the charges of ions involved.
- Gain/loss of electrons can be predicted, particularly in metal-nonmetal reactions.
- Gain/loss of oxygen or hydrogen atoms indicates a redox reaction.
- Changes in electron sharing and electronegativity differences provide insight.
- An atom is oxidized when it loses electron share (less electronegative).
- An atom is reduced when it gains electron share (more electronegative).

Example Reaction for Analysis:
- 2Ag^+ + 2Cl^-
  ightarrow 2Ag + Cl_2
- A: Identify which reactant is oxidized.
- B: Identify which reactant is reduced.
Identify as Oxidation or Reduction:
- A. Sn(s)
  ightarrow Sn^{4+}(aq) + 4 e^-
- B. Fe^{3+}(aq) + 1 e^-
  ightarrow Fe^{2+}(aq)
- C. Cl_2(g) + 2 e^-
  ightarrow 2Cl^-(aq)

Identifying Oxidized and Reduced Species:
- a) Si (s) + 2Cl_2 (aq)
  ightarrow SiCl_4(s)
- b) 2Ca (s) + CO_2 (g)
  ightarrow 2CaO (s) + C (s)
Identifying Oxidized and Reduced Species Analysis:
- Oxidation is defined as loss of electrons, increase in charge, or gain of oxygen; reduction entails gain of electrons, decrease in charge, or loss of oxygen.
- Redox detection can occur through charge changes or through an examination of metals and nonmetals behaviors in reactions.

Determine type as single or double replacement, combination, decomposition, or combustion.
Indicate acts as precipitation, acid-base neutralization, or redox reactions.

Example Reactions for Analysis:
- a) Ca(OH)_2(aq) + 2HBr(aq)
  ightarrow 2 H_2O(l) + CaBr_2(aq)
- b) Pb(ClO_4)_2(aq) + 2NaCl(aq)
  ightarrow 2PbCl_2(s) + 2NaClO_4(aq)
- c) 2AgNO_3(aq) + Cu(s)
  ightarrow 2Ag(s) + Cu(NO_3)_2(aq)
- d) MgCO_3(s) + HCl(aq)
  ightarrow MgCl_2(aq) + CO_2 + H_2O(l)
- e) Mg(s) + O_2(g)
  ightarrow MgO(s)
- f) Mn_2(CO_3)_3(s)
  ightarrow Mn_2O_3 + 3CO_2(g)
Analysis:
- Identifying reaction types may involve looking at the products to match them with descriptions of reaction classifications.