Redox Reactions: Oxidation and Reduction

I. ELECTRON TRANSFER AND REDOX REACTIONS

A. Reactions have the ability to gain or lose electrons when reacting with other elements B. Transfer of electrons from one atom to another – 1. 2Na + Cl 2  2Na + + 2Cl - – 2. electron is transferred from the sodium atoms to the Cl 2 molecule

C. Magnesium Oxide 1. 2Mg + O 2  2MgO 2. Each magnesium transfers 2 e- To each oxygen atom D. Oxidation – Reduction reaction- a reaction in which electrons are transferred from one atom to another aka REDOX REACTIONS

Consider this: Single-Replacement 2KBr + Cl 2  2KCl + Br 2 – Re-write as net ionic equation – 2Br - + Cl 2  Br 2 + 2Cl - Chlorine gets electrons from bromine When bromine loses electrons, the two form a covalent bond to make Br 2 Formation of covalent bond by sharing electrons is also a redox reaction

How do oxidation and reduction differ? Oxidation- loss of electrons from atoms of a substance – Na  Na + + e - – Sodium is oxidized because it loses an electron – For oxidation to take place the electrons lost must be accepted by another substance Reduction- the gain of electrons by atoms of a substance – Cl 2 + 2e -  2Cl - – Chlorine is reduced because it gains electrons

LEO the lion says GER – LEO GER – Loss of Electrons is Oxidation, Gain of Electrons is Reduction

Changes in Oxidation Number Oxidation number- the number of electrons lost or gained by an atom when it forms ions – Complete: 2K + Br 2  2KBr – Net: 2K + Br 2  2K + + 2Br - – Each Potassium loses 1 electron so its oxidation number is +1. It changes from 0 to +1. – Each Bromine gains 1 electron so its oxidation number is -1. It changes from 0 to -1. When an atom or ion is reduced its numerical value or oxidation is reduced, which is why its called reduction.

II. OXIDIZING AND REDUCING AGENTS

Oxidizing agents- the substance that oxidizes another substance by accepting its electrons Reducing agent- the substance that reduces another substance by losing its electrons 2K + Br 2  2KBr Bromine is the oxidizing agent, potassium is the reducing agent oxidized reduced

III. REDOX AND ELECTRONEGATIVITY

Some redox reactions involve changes in molecular substances or polyatomic ions, in which atoms are covalently bonded to other atoms – Ammonia N 2 + 3H 2  2NH 3 – Does not evolve ions or obvious electron transfers – Nitrogen is the oxidizing agent and hydrogen is the reducing agent – The more electronegative atom is reduced by gaining electrons

IV. DETERMINING OXIDATION NUMBERS

Rules for determining oxidation numbers 1. the oxidation number of an uncombined atom is 0. – O 2, Cl 2, etc 2. the oxidation number of a monoatomic ion is equal to the charge on the ion – Ca 2+ is the oxidation number of the more electronegative atom in a molecule or a complex ion is the same as the charge it would have if it were an ion. – NH 3 – N is more electronegative and has a charge of -3

4. the most electronegative element, fluorine, always has an oxidation number of -1 when it is bonded to another element 5. The oxidation number of oxygen in a compound is always -2, except in peroxides, such as hydrogen peroxide (H 2 O 2 ) where it is - 1. When it is bonded to fluorine, the only element more electronegative than oxygen, the oxidation number is The oxidation number of hydrogen in most of its compounds is +1. – Exceptions: LiH, NaH, CaH and AlH 3, it is -1

7. The metals of groups 1A and 2A and aluminum in group 3A form compounds in which the metal atom always has a positive oxidation number equal to the number of its valence electrons (+1, +2, +3 respectively) 8. The sum of oxidation number in a neutral compound is The sum of oxidation number of the atoms in polyatomic ions is equal to the charge on the ion. – NH 4 +, N -3 H +1(4) = +1

Use the rules to determine the oxidation numbers of each element KClO 3 SO 3 2-

V. OXIDATION NUMBER IN REDOX REACTIONS

2KBr + Cl 2  2KCl + Br 2 – Assign numbers to all elements in the balanced equation – Determine the change in numbers – Determine either oxidation, reduction, or none