Oxidation Reduction Reactions

Oxidation Reduction Reactions…

are chemical changes that occur when electrons are transferred between reactants

What is oxidation?• The terms oxidation and reduction have been known to

chemists for centuries. One of the earliest definitions tells us that…

oxidation means combining an element with oxygen

such as…

Mg + O2 2MgOand…

What is reduction?

reduction means losing oxygen from a compound

such as…

2KClO3 2KCl + 3O2

Today…

• chemists define oxidation-reduction in terms of the transfer or shift of electrons.

• Oxidation is the complete or partial loss of electrons.

• Reduction is complete or partial gain of electrons.

• Oxidation and reduction occur simultaneously.

• Electrons are transferred from one reactant to the other.

“LEO the lion goes GER!”

• LEO – Loss of Electrons is Oxidation

• GER – Gain of Electrons is Reduction

“OIL RIG”

Oxidation Is a Loss and Reduction Is a Gain of electrons

Mg + S Mg2+ + S2-

In the formation of magnesium sulfide…

• Mg atoms become Mg2+ ions in the compound.

Oxidation: Mg Mg2+ + 2 e- (loss of electrons)

• S atoms become S2- ions in the compound.

Reduction: S + 2 e- S2- (gain of electrons)

Mg + S Mg2+ + S2-

• The substance in a redox reaction that donates electrons is a reducing agent. In the above example, magnesium reduces sulfur. Magnesium is the reducing agent.

• The substance in a redox reaction that accepts electrons is an oxidizing agent. By accepting electrons, sulfur oxidizes magnesium. Sulfur is the oxidizing agent.

Identify what is oxidized and reduced:

2Na + S Na2S

Na is oxidized, S is reduced2K + Cl2 2KCl

K is oxidized, Cl2 is reduced

3O2 + 4Al 2Al2O3

Al is oxidized, O2 is reduced

With compounds of carbon:

• The addition of oxygen OR the removal of hydrogen is always oxidation.

• The removal of oxygen OR addition of hydrogen is reduction.

Assigning oxidation numbers

• A helpful tool in the study of electron transfer is assigning oxidation numbers.

• To identify oxidation-reductions, look for a change in oxidation states.

• The oxidation number of an element in a compound is defined as the charge an atom has, or appears to have.

• Oxidation numbers help us keep track of electrons transferred in redox reactions.

Guidelines for determining oxidation numbers:

1. Free, uncombined elements have an oxidation number of zero (as in Fe, H2, O3).

0 0

Na Br2

2. For a single, monatomic ion, the oxidation number is equal to the charge on the ion.

+1 -3

K+ N3-

Guidelines…

3. The oxidation number of H is +1 in most compounds (except in hydrides, -1) and the oxidation number of O is -2 in most compounds (except in peroxides, -1).

Guidelines…

4. The sum of the oxidation numbers in a neutral compound must be zero.

? -2

N2O4

Totals ? + -8 = 0

N must carry a total of +8, so each N = +4 +4 -2

N2O4

Guidelines…

• The sum of the oxidation numbers in a polyatomic ion must be equal to the charge on the ion.

? -2

PO43-

Totals ? + -8 = -3

P must carry a total of +5 +5 -2

PO43-

Extra details:

• In compounds and polyatomic ions, the more electronegative atom will have an oxidation number equal to the charge, as if it was an ion.

• Oxidation numbers are written with the sign of the charge first (such as +3), while ion charges are written with the number first (such as 3+).

Determine the oxidation number of sulfur in the following ions or compounds:

• SO2

S = +4

• Al2(SO4)3

S = +6

• Na2S2O3

S = +2

• K2SO3

S = +4

• S8

S = 0

Using oxidation number changes to identify oxidation and reduction

• The oxidation number of an element

increases when it is oxidized, and

decreases when it is reduced.

Identify what is oxidized and reduced, then identify the oxidizing and reducing agents:

Cl2 + 2HBr 2HCl + Br2

• Answer: the element chlorine is reduced (0 -1) and the element bromine is oxidized (-1 0)

Identifying Redox Reactions

• All oxidation-reduction reactions involve a change in oxidation numbers.

• If there is no change in oxidation state, it is not a redox reaction.

Identifying Redox ReactionsUse the change in oxidation number to identify which of these reactions are redox reactions:

A) N2O4 2NO2

B) Cl2 + 2NaBr 2NaCl + Br2

• Answer: A is not redox, while B is redox. Bromine is oxidized and chlorine is reduced. Bromine is the reducing agent and chlorine is the oxidizing agent