Unit #3: Electrochemistry

Lesson 1: Oxidation and Reduction

Oxidation Reduction/Redox Chemistry

Oxidation and Reduction reactions always take place simultaneously.

Loss of electrons – oxidation (Increase in Oxidation Number)

Ex: Na ------> Na+1 + e-1

Gain of electrons - reduction ( Decrease in Oxidation Number)

Cl2 + 2 e-1 ------> 2 Cl-1

Oxidation occurs when a molecule does any of the following:

   Loses electrons    Loses hydrogen    Gains oxygen

If a molecule undergoes oxidation, it has

been oxidized and it is the reducing agent

(aka reductant).

Reduction occurs when a molecule does any of the following:

Gains electrons Gains hydrogen

Loses oxygen

If a molecule undergoes reduction, it has been reduced and it is the oxidizing agent (aka oxidant).

Zn(s) + Cu2+(aq) Zn2+

(aq) + Cu(s)

• The Cu2+(aq) comes from the dissociation of copper (ll) sulfate

in aqueous solution. • The Zn2+

(aq) and sulfate ion; which is still in solution after the reaction, become a zinc sulfate solution.

• The above redox reaction can be broken down into an oxidation reaction and a reduction reaction.

Oxidation: Zn(s) Zn2+(aq) + 2e- (Zinc loses 2 electrons)

Reduction: Cu2+(aq) + 2 e- Cu(s) (Copper gains the 2 electrons lost by zinc)

Redox reactions involve electron transfer:

Diagram pg 374 of text

Identifying Redox Reactions Not all chemical reactions are redox reactions To determine if a reaction is a redox reaction,

you must find the oxidation numbers of each element before and after the reaction

The oxidation number is usually equal to the charge the atom would have if it were an ion

If the oxidation number changes, the reaction is a redox reaction

If the oxidation number does not change, the reaction is not a redox reaction

Assigning Oxidation NumbersThe following is a list of rules for assigning oxidation

numbers: The oxidation number of an atom in an uncombined

element is always zero. For example, elemental copper would have an oxidation number of zero.

The oxidation number of hydrogen is +1 The oxidation number of oxygen is -2 The oxidation numbers of group 1 and 2 elements are +1

and +2 respectively The sum of the oxidation numbers in a compound must

equal zero The sum of oxidation numbers in a polyatomic ion must

equal the charge of the ion

Examples:

Determine the oxidation numbers for the following examples:

KNO3 K=N=O=

H2SO4 H=S=O=

You try!

K2O CaO Fe2O3

Cu(NO3)

SO42-

OH-

Classwork

pp. 377 # 3 a - h

Redox Reactions of MetalsCu + 2 Ag(NO3) 2 Ag + Cu(NO3)2

1. Write out the ionic equation

2. Cancel the ions common on both sides

3. Label the charges

4. Identify the loss and gain of electrons

Practice

1. Ca(s) + Sn2+(aq) Sn(s) + Ca2+

(aq)

2. CuSO4(aq) + Mg(s) Cu(s) + MgSO4(aq)

3. 3CuCl2(aq) + 2Al(s) 3Cu(s) + 2AlCl3(aq)

4. Zn(s) + 2AgNO3(aq) 2Ag(s) + Zn(NO3)2(aq)

Redox Reactions of Non-metals2 KClO3 2 KCl + O2

1. Write out the oxidation numbers for each element

2. Identify the loss and gain of electrons

Practice

1. H2(g) + Cl2(g) HCl(g)

2. 2K(s) + I2(g) 2KI (s)

3. CaCO3(s) CaO(s) + CO2(g)

4. Ca(s) + 2HCl(aq) H2(g) + CaCl2(aq)

Classwork

pp. 384 # 3, 5, 6