Oxidation, Reduction and Electrochemistry

In this chapter you will be exposed to some basic concepts involved in a special type of chemical reaction.

Time will not permit an in-depth study of this, but the main concepts will be important to learn.

Oxidation-Reduction Reactions

Oxidation-Reduction reactions are also termed Redox reactions.

A redox reaction is a chemical reaction which involves the loss and gain of electrons.

Redox Reactions

In a redox reaction one substance is oxidized (loses electrons) and another substance is reduced (gains electrons)

OIL RIG ( oxidation is loss, reduction is gain of electrons)

LEO the lion goes GER (loss of electrons is oxidation, gain of electrons is reduction.

2 Na(s) + Cl2(g) 2 NaCl

Na – loses 1 electron (oxidized)

Cl – gains 1 electron (reduced)

Practice Problem

Identify the substance oxidized and reduced in the following redox equation.

Ag (s) + NO3 (aq) AgNO3

Oxidation Numbers

To analyze an equation as a redox reaction it’s essential to identify the oxidation numbers of the reactants and products.

Changes in oxidation numbers reveal if a substance is oxidized or reduced.

Oxidation Number Rules

When assigning oxidation numbers use the following rules:

1) The oxidation number for any “free” or uncombined element in atomic or molecular form is 0.

2) The oxidation number of a monatomic ion is equal to the charge on the ion.

3) The oxidation number of an atom of fluorine in a compound is always –1 because it is the most electronegative.

Oxidation Numbers

4) An atom of the more electronegative element in a binary compound is assigned the number equal to its ion charge.

5) In compounds, atoms of group I, II, and aluminum have positive oxidation numbers : +1, +2, +3 respectively.

6) Hydrogen is +1 unless combined with a metal atom, then its –1.

7) The oxidation number of oxygen is usually –2. When combined with fluorine it is +2. In peroxides, such as H2O2 oxygen has the oxidation number –1.

Oxidation Numbers

8) The sum of the oxidation numbers of all atoms in a particle must equal to zero in a compound and to its charge for a polyatomic ion.

Oxidation Number Practice

NH4+ :

Al :

BF3 :

KClO3

CO32-

Oxidation Number Practice

NH4+ : H= +1 N= -3

Al : 0

BF3 : F=-1 B: +3

KClO3 K=+1 O= -2 Cl= +5

CO32- O= -2 C=+4

Identifying Redox Reactions

To identify a redox reaction you must analyze the oxidation numbers of reactants and products.

Redox Reaction

Identify oxidized and reduced substance:

1) Cl2 + 2HBr 2HCl + Br2

Assign oxidation numbers to each atom in the equation.

0 +1 -1 +1 -1 0

Consider only the atoms for which there was a change in oxidation number.

Identify oxidized and reduced substance:

1) Cl2 + 2HBr 2HCl + Br2

0 +1 -1 +1 -1 0

An increase in oxidation number indicates an oxidation.

A decrease in oxidation number indicates a reduction.

The oxidation number of Cl decreased (from 0 to -1): Cl was reduced

The oxidation number of Br increased (from -1 to 0): Br was oxidized

Identify oxidized and reduced substance:

2) 2H2 + O2 2H2O0 +1 -20

An increase in oxidation number indicates an oxidation.

A decrease in oxidation number indicates a reduction.

The oxidation number of O decreased (from 0 to -2): O was reduced

The oxidation number of H increased (from 0 to +1): H was oxidized

Classwork

Oxidizing and reducing handout