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 + O 2  2MgO and …

What is reduction?

reduction means losing oxygen from a compound such as… 2KClO 3  2KCl + 3O 2

Today…

•

chemists define oxidation-reduction in terms

of the transfer or shift of electrons.

• • Oxidation electrons.

is the complete or partial loss of Reduction electrons.

is complete or partial gain of

• 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



Mg

2+

+ S

2 In the formation of magnesium sulfide… • Mg atoms become Mg 2+ ions in the compound. Oxidation: Mg  Mg 2+ + 2 e (loss of electrons) • S atoms become S 2 ions in the compound.

Reduction:

S + 2 e  S 2 (gain of electrons)

Mg + S



Mg

2+

+ S

2 • •

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  Na 2 S Na is oxidized, S is reduced 2K + Cl 2  2KCl K is oxidized, Cl 2 3O 2 + 4Al  2Al 2 O 3 is reduced Al is oxidized, O 2 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, H 2 , O 3 ).

0 Na 0 Br 2 2. For a single, monatomic ion, the oxidation number is equal to the charge on the ion.

+1 K + -3 N 3-

3.

Guidelines…

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.

Totals ? -2 N 2 O 4 ? + -8 = 0 N must carry a total of +8, so each N = +4 +4 -2 N 2 O 4

Guidelines…

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

Totals ? -2 PO 4 3 ? + -8 = -3 P must carry a total of +5 +5 -2 PO 4 3-

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 (such as 3 +).

+ 3), while ion charges are written with the number first

• • • • • Determine the oxidation number of sulfur following ions or compounds: SO 2 S = +4 Al 2 (SO 4 ) 3 S = +6 Na 2 S 2 O 3 S = +2 K 2 SO 3 S = +4 S 8 S = 0 in the

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:

Cl

2

+ 2HBr



2HCl + Br

2 • 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 Reactions Use the change in oxidation number to identify which of these reactions are redox reactions: A) N 2 O 4 B) Cl 2  2NO 2 + 2NaBr  2NaCl + Br 2 • 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