Oxidation-Reduction Reactions

Oxidation-Reduction Reactions

Oxidation-reduction (redox) reactions involve transfer of electrons. Oxidation – loss of electrons.Reduction – gain of electrons. Both half-reactions must happen at the same time.

Can be identified through understanding of oxidation numbers. Chromium gives great example of different oxidation numbers. Different oxidation states of chromium have different colors. Chromium (II) chloride = blue

Chromium (III) chloride = green

Potassium chromate = yellow

Potassium dichromate = orange

Oxidation à reactions in which the atoms or ions of an element experience an increase in oxidation state

Ex. combustion of metallic sodium in atmosphere of chlorine gas. Sodium ions and chloride ions made during exothermic reaction form cubic crystal lattice. Sodium cations are ionically bonded to chloride anions.

2Na(s) + Cl2 (g) «2NaCl (s)

Formation of sodium ions shows oxidation b/c each sodium atom loses an electron to become sodium ion. Oxidation state represented by putting oxidation number above symbol of atom and ion.

Na⁰«Na⁺¹+ e^-

Oxidation state of sodium changed from 0 (elemental state) to +1 (state of the ion). A species whose oxidation number increases is oxidized. Sodium atom oxidized to sodium ion

Reduction à reactions in which the oxidation state of an element decreases. Ex. chlorine in reaction with sodium. Each chlorine atom accepts e- and becomes chloride ion. Oxidation state decreases from 0 to -1.

Cl⁰₂ + 2e^- «2Cl^-

A species that undergoes a decrease in oxidation state is reduced

The chlorine atom is reduced to the chloride ion

L ose E lectrons = O xidation. Sodium is oxidized

Gain Electrons = Reduction. Chlorine is reduced

Active metals: Lose electrons easily, Are easily oxidized Are strong reducing agents.

Active nonmetals: Gain electrons easily, Are easily reduced, Are strong oxidizing agents.

Oxidizers

Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers. That is, the oxidant (oxidizing agent) removes electrons from another substance; i.e., it oxidizes other substances, and is thus itself reduced. And, because it "accepts" electrons, it is also called an electron acceptor.

Oxidants are usually chemical substances with elements in high oxidation states (e.g., H₂O₂, MnO⁻₄, CrO₃, Cr₂O²⁻₇, OsO₄), or else highly electronegativeelements (O₂, F₂, Cl₂, Br₂) that can gain extra electrons by oxidizing another substance.

Reducers

Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as reducing agents, reductants, or reducers. The reductant (reducing agent) transfers electrons to another substance; i.e., it reduces others, and is thus itself oxidized. And, because it "donates" electrons, it is also called an electron donor. Electron donors can also form charge transfer complexes with electron acceptors.

Reductants in chemistry are very diverse. Electropositive elemental metals, such as lithium, sodium, magnesium, iron, zinc, and aluminium, are good reducing agents. These metals donate or give away electrons readily. Hydride transfer reagents, such as NaBH₄ and LiAlH₄, are widely used in organic chemistry, primarily in the reduction of carbonyl compounds to alcohols. Another method of reduction involves the use of hydrogen gas (H₂) with apalladium, platinum, or nickel catalyst. These catalytic reductions are used primarily in the reduction of carbon-carbon double or triple bonds.

Part of the reaction involving oxidation or reduction alone can be written as a half-reaction. Overall equation is sum of two half-reactions. Number of e- same of oxidation and reduction, they cancel and don’t appear in overall equation.

Electrons lost in oxidation appear on product side of oxidation half-reaction. Electrons gained in reduction appear as reactants in reduction half-reaction. When copper reacts in nitric acid 3 copper atoms are oxidized to Cu⁺² ions as two nitrogen atoms are reduced from a +5 oxidation state to a +2 oxidation state.

If no atoms in reaction change oxidation state, it is NOT a redox reaction. Ex. Sulfur dioxide gas dissolves in water to form acidic solution of sulfurous acid .

Equations for simple redox reactions can be balanced by looking at them. Most redox equations require more systematic methods. Equation-balancing process needs use of oxidation numbers. Both charge and mass are conserved. Half-reactions balanced separately then combined.

Half-Reaction Method. Also called ion-electron method. Made of seven steps. Oxidation numbers assigned to all atoms and polyatomic ions to determine which species are part of redox process. Half-reactions balanced separately for mass and charge.

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