Oxidation and Reduction

1
Oxidation and Reduction

• Section 9.1

2
Electron Transfer Theory

• According to modern theory
• the gain of electrons is called reduction.
• the loss of electrons is called oxidation.

LEO the lion says GER
3
Redox

• Theory suggests
• the total number of e- gained in a reaction must
  equal the total number of e- lost
• Reduction and oxidation are separate processes
  that occur simultaneously
• The chemical equations of the oxidation,
  reduction and the overall equation must be
  balanced.

4
Writing and Balancing Half-Reactions

• Write a balanced net equation for the reaction of
  copper with aqueous silver nitrate.
• Step 1 Write each half reaction (the metal
  reacting with the solution will undergo an
  oxidation forms a ion thus a loss of
  electrons, while the one in solution will undergo
  a reduction)

5
Example Continued

• Step 2 Remember the number of electrons lost
  must equal the number of electrons gained. So we
  need to multiply the second reaction by two so
  that we will have 2e- in each equation

6
Example Cont

• Step 3 Now add the two half-reaction equations
• Step 4 Cancel out like terms
• Step 5 Write net equation

7
Oxidation States

• An oxidation number is a positive or negative
  number corresponding to the oxidation state of an
  atom
• Using the following rules you can determine the
  oxidation number of other atoms in compounds (pg.
  658)

8
Examples

• What is the oxidation number of sodium in sodium
  hydride (NaH)?
• - hydrogen in a hydride has an oxidation of
  -1
• - b/c there is only 1 Na, it must be 1
• What is the oxidation number of sulphur in
  dihydrogen sulphide (H2S)?
• - hydrogen is always 1
• - b/c there are 2 of them the overall is 2(1)
  2
• - b/c only 1 S, it must be -2

9
Examples Cont

• What is the oxidation number of carbon in the
  carbonate ion (CO22-)?
• - O has an oxidation number of -2
• - there is an overall charge of 2- on the ion
  so we can set up a mini equation
• - solve for x

10
Examples Cont

• What is the oxidation number of sulphur in
  calcium sulphate ( )
• We can assumed that Ca is in ionic form ? its
  oxidation number is 2
• So we can set up our equation and solve

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Oxidation Numbers and Redox Reactions

• An oxidation is an increase in oxidation number
• A reduction is a decrease in oxidation number
• In redox reactions, oxidation numbers change.

12
Example

• When methane burns, carbon dioxide and water
  form. Identify the oxidation and reduction in
  this reaction
• First write the reaction
• Now insert the oxidation numbers and arrows

13
Seatwork

• Pg. 653 2
• Pg. 656 8-9
• Pg. 659 12, 13
• Pg. 662 18, 19, 20