Early Definitions of Redox Reactions Any substance that gains oxygen is

oxidized, and any substance that loses oxygen is reduced

Modern Concepts

Oxidation is the the complete or partial loss of electrons or the gain of oxygen. The substance that loses electrons is the

reducing agent Reduction is the complete or partial gain

of electrons or loss of oxygen. The substance that accepts the electrons is

the oxidizing agent.

Oxidation and Reduction

Practice

Determine what is oxidized and what is reduced in each reaction. Identify the oxidizing agent and the reducing agent in each case.

2Na(s) + S(s) Na2S(s)

4Al(s) + 3O2 2Al2O3(s)

Redox in Covalent Compounds

Oxidation Numbers

An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

What is the oxidation number of each kind of atom in the following compounds :

a. SO2 b. CO3- c. Na2SO4 d. (NH4)2S

Identifying Oxidized and Reduced Atoms

An increase in oxidation number of an atom or ion indicates oxidation.

A decrease in oxidation number of an atom or ion indicates reduction.

Identifying Oxidized and Reduced Atoms

Identify which atoms are oxidized and which are reduced in the following reaction also identify the oxidizing agent and reducing agent:

Cl2(g) + 2HBr(aq) 2HCl(aq) + Br2(l)

2KNO3(s) 2KNO2(s) + O2(g)

Identifying Redox Reactions

All reactions can be classified as reactions that transfer electrons (redox reactions) or reactions that do not transfer electrons. Redox reactions include: single

displacement, combination, decomposition, and combustion reactions

Double displacement and most acid-base reactions do not transfer electrons

Oxidation Number-Change Method (agents appear once on each side)

1. Assign oxidation numbers

2. Identify which atoms are oxidized and which are reduced

3. Use a bracket to connect the atoms that undergo oxidation and reduction and write the oxidation number change on the line

4. Make the total increase in oxidation number equal the total decrease by using appropriate coefficients

5. Check to see if balanced for both atoms and charge

Fe2O3(s) + CO(g) Fe(s) + CO2(g)

Oxidation Number-Change Method Balance the following equation:

K2Cr2O7(aq) +H2O(l) + S(s) KOH(aq) + Cr2O3(s) +SO2(g)

HNO2(aq) +HI(aq) NO(g) +I2(s) +H2O(l)

Half-Reaction Method (reaction in acidic or basic solutions)

S(s) + HNO3(aq) SO2(g) + NO(g) + H2O(l)

Write unbalanced equation in reaction in ionic form.

Write separate half-reactions for the oxidation and reduction.

Half-Reaction Method

Balance the atoms in the half-reactions. In an acidic solution use H2O to balance the

O, and H+ to balance the H In a basic solution use OH- to balance the

O, and H2O to balance the H

Half-Reaction Method

Add electrons to balance the charge

Half-Reaction Method

Multiply each half-reaction by an appropriate number to make the number of electrons equal on both sides.

Then add the half-reactions to show the overall net ionic equation.

Half-Reaction Method

Write the net ionic equation and Identify any spectator ions:

KMnO4(aq) + HCl(aq) MnCl2 (aq) + Cl2(g) + H2O(l) + KCl(aq)

Half-Reaction Method

Write the net ionic equation and Identify any spectator ions:

Zn(s) + NO3-(aq) NH3 + Zn(OH)4

2-(aq)

Redox Reactions and Electrochemistry During Redox Reactions electrons are transferred

from a more reactive atom to a less reactive ion. If the movement of electrons is made to pass

through an external circuit usefully electrical energy can be produced.

Redox Reactions and Electrochemistry

Voltaic Cell

Electric Potential

The electric potential is the cells ability to produce electrons.

Standard Cell Potential

E°cell is the potential when the concentration of the half-cell is 1M, gases are at 1 atm, and the temperature is 25°C.

The half-cell potential cannot be measured directly, so a standard hydrogen electrode (E°H+ = 0.00V) is used to find the values for half-cell potentials. Negative values mean that

the substance was reduced Positive values mean that

the substance was oxidized

Standard Reduction Potentials

Reduction Potentials (Easily Oxidized)

Reduction Potentials (Easily Reduced)

Determining Reaction Spontaneity If the cell potential is positive the reaction is

spontaneous and will occur, however if the cell potential is negative the reaction is non-spontaneous.

Is the redox reaction between zinc metal and silver ions spontaneous?

Writing Cell Reactions and Calculating Standard Cell Potentials Determine the cell reaction for a voltaic

cell composed of the following half-cells and its standard cell potential.

Fe3+(aq) + e- Fe2+(aq) E°Fe3+ = +0.77 V

Ni2+(aq) + 2e- Ni(s) E°Ni2+ = -0.25 V