Redox Reactions

Chapter 18

+ O2

Oxidation-Reduction (Redox) Reactions

“redox” reactions: rxns in which electrons are transferred from one species to another

oxidation & reduction always occur simultaneously

we use OXIDATION NUMBERS to keep track of electron transfers

Rules for Assigning Oxidation Numbers:

1) the ox. state of any free (uncombined) element is zero. Ex: Na, S, O2, H2, Cl2, O3

Rules for Assigning Oxidation Numbers:

2) The ox. state of an element in a simple ion is the charge of the ion.

Mg2+ oxidation of Mg is +2

Rules for Assigning Oxidation Numbers:

3) the ox. # for hydrogen is +1

(unless combined with a metal, then it has an ox. # of –1)

Ex: NaOH (H bonded to O) v. NaH (H bonded to Na)

H = +1 H = -1

Rules for Assigning Oxidation Numbers:

4) the ox. # of fluorine is always –1.

Rules for Assigning Oxidation Numbers:

5) the ox. # of oxygen is usually –2.

Why USUALLY? Not -2 when it’s in a peroxide, such as hydrogen peroxide:

H2O2

Rules for Assigning Oxidation Numbers:

6) in any neutral compound, the sum of the oxidation #’s = zero.

Rules for Assigning Oxidation Numbers:

7) in a polyatomic ion, the sum of the oxidation #’s = the overall charge of the ion.

Rules for Assigning Oxidation Numbers:

**use these rules to assign oxidation #’s; assign known #’s first, then fill in the #’s for the remaining elements:

Examples: Assign oxidation #’s to each element:

a) NaNO3 Na = +1O = -2

Therefore, N = +5

Examples: Assign oxidation #’s to each element:

b) SO32-

O = -2, therefore S must = +4 to balance the charges and have an overall charge of 2-

-2+4

Examples: Assign oxidation #’s to each element:

c) HCO3-

-2+1 +4

Examples: Assign oxidation #’s to each element:

Do on your own:

d) H3PO4

e) Cr2O72-

f) K2Sn(OH)6

Definitions

Oxidation: the process of losing electrons (ox # increases)

Reduction: the process of gaining electrons (ox # decreases)

Oxidizing agents: species that cause oxidation (they are reduced)

Reducing agents: species that cause reduction (they are oxidized)

To help you remember…

OIL RIG Oxidation Is Loss Reduction Is Gain

OR… LEO GER Losing Electrons → Oxidation Gaining Electrons → Reduction

Are all rxns REDOX rxns?You must determine this…

a reaction is “redox” if a change in oxidation # happens; if no change in oxidation # occurs, the reaction is nonredox.

Examples

MgCO3 MgO + CO2

+2 -2

MgCO3 is an ionic compound, so what is Mg’s charge in an ionic compound?

The carbonate ion CO32- is

the other ion, let’s figure out C because we already know O.

+4-2+2 -2+4

Is this a redox or nonredox reaction?

NONREDOX (no change in oxidation numbers)

Examples

Zn + CuSO4 ZnSO4 + Cu

Which oxidation numbers do we already know?

0, free element

0Break down this ionic compound into its ions

Cu and SO42-

So, Cu must be Cu2+

+2

O = -2 in SO42-, so S must

be…?

-2+6-2+2 +6

Is this a redox or nonredox reaction?

REDOX reaction

Examples

NaCl + AgNO3 AgCl + NaNO3

Redox or nonredox?

Examples

CO2 + H2O C6H12O6 + O2

-2+4 -2+1 0-2+10

What happened to Carbon? It went from +4 oxidation # to 0.

Was Carbon oxidized or reduced?

REDUCED

OIL RIG (oxidation is losing electrons so oxidation number increases, where as reduction is gaining electrons so oxidation number decreases)