Reduction Oxidation

Reduction Oxidation reactions are often referred to as REDOX reactions.

Definition

A redox reaction occurs when 1 or 2 elements change their oxidation number during a chemical reaction.

The next two frames contain 4 common examples fo redox reactions. Which ones are you familiar with?

Examples

Metals rust when exposed to water or oxygen.

Batteries produce electricity to run a flashlight or to start a car.

More examples

An apple turns brown after a bite is taken.

A piece of jewelry is plated with a thin layer of gold or silver.

How about in Chemistry class?

One of the most interesting aspects of redox chemistry is the different colors.

Many transition metals lose different numbers of electrons. Each different oxidation number usually has a different color.

Click to the next frame to see some examples

Colors of Transition Metals

Assigning Oxidation Numbers

The oxidation number of a free element is 0. The charge of an ion is it’s oxidation number. The sum of the oxidation numbers in a

compound is equal to 0. H in a compound has an oxidation number of

+1, except in metallic hydrides. Oxygen in a compound has an oxidation

number of -2, except in peroxides. The sum of the oxidation numbers in a

polyatomic ion is equal to it’s charge.

The oxidation number of a free element is 0. Free elements are elements that are not

bonded to another element.

Any element has the same number of (+) protons and (-) electrons, so they do not have a charge.

Diatomic elements are also considered free elements. We all love ClIF H. BrON

The charge of an ion is it’s oxidation number. The elements in column I like to lose 1

electron. They have an oxidation number of +1

The elements in column II like to lose 2 electrons. They have an oxidation number of +2

The elements in column III like to lose 3 electrons. They have an oxidation number of +3

The charge of an ion is it’s oxidation number. The elements in column VII like to gain 1

electron. They have an oxidation number of -1

The elements in column VI like to gain 2 electrons. They have an oxidation number of -2

The elements in column V like to gain 3 electrons. They have an oxidation number of -3

Column IV

The elements in column IV often share electrons.

Sometimes they lose 4 electrons.

Sometimes they gain 4 electrons.

Therefore, their oxidation number can be either +4 or -4.

The sum of the oxidation numbers in a compound is equal to 0. Transition Metals can lose different numbers

of electrons. Therefore, they have different oxidation numbers. To determine their oxidation number, add up the oxidation numbers in the compound and set the sum equal to zero.

For instance, what is the oxidation number of the elements in Nickel(II) chloride?

Nickel(II) chloride is NiCl2

To start this problem, we know that chlorine is in column VII, therefore it has an oxidation number of -1.

Since there are two chlorine atoms, that is a total charge of -2.

For the sum of the oxidation numbers to equal 0, what is the ox # of the nickel?

The ox # of Ni is +2

+2 from the nickel and 2 chlorines, each -1 adds up to a total of zero. In other words, the number of electrons lost by the nickel is equal to the number of electrons gained by the chlorines.

What about Rust?

Rust has the formula Fe2O3. What is the oxidation number of each element in rust?

First, we know that oxygen has an oxidation number of -2 in a compound. There are 3 oxygen atoms for a total charge of -6.

The oxidation numbers of a compound must add up to zero. So what is iron’s ox #?

Iron has an ox # of +3

Each iron atom loses three electrons for an oxidation number of +3.

2(+3) + 3(-2) =0

Fe2O3

Practice

What is the oxidation of each transition metal in the following compounds? FeCl3

MnO

Cu2O

CoBr2

Answers

Fe in FeCl3 is +3

Mn in MnO is +2

Cu in Cu2O is +1

Co in CoBr2 is +2

The sum of the oxidation numbers in a polyatomic ion is equal to it’s charge.

To help understand this rule, let’s look at the polyatomic ion sulfate, SO4

-2.

Oxygen atoms have an oxidation number of -2

? + 4(-2) = -2

SO4

It must add up to -2

6 + 4(-2) = -2

SO4

Therefore the oxidation number of the sulfur in sulfate is +6

What about CrSO4?

Since this is a compound, the sum must be 0

+2 +6 4(-2) = 0

CrSO4

The Cr is +2, S is +6 and the O is -2

One last problem…….

What is the oxidation number of each element in Cu(NO3)2?

NO3-1 has an oxidation number of -1 and there

are two of them.

The compound must add up to zero.

What is copper’s oxidation number?

+2 2(-1) = 0

Cu(NO3)2

Each nitrate is -1, so the copper must be +2

So what is the nitrogen?

+2 ?? 6(-2) = 0

Cu(NO3)2

There are six oxygen atoms, each one with an oxidation number of -2.

The compound must add up to zero.

+2 2(+5) 6(-2) = 0

Cu(NO3)2

There are two nitrogen atoms and the must have a total charge of +10.

Therefore each nitrogen has an oxidation number of +5

Final answer………

Copper has an oxidation number of +2

Nitrogen has an oxidation number of +5

Oxygen has an oxidation number of -2