Oxidation and Reduction Introduction

1
Oxidation and Reduction Introduction

• LEO goes GER
• Examples
• Balancing simple equations
• Why gain/lose electrons?
• Electronegativity

2
Defining Oxidation and Reduction

• Chemistry is all about atoms making or breaking
  bonds with other atoms. Sometimes atoms lose the
  hold over their electrons, or they may gain
  another atoms electrons
• Oxidation when an atom loses its electrons
• Reduction when an atom gains more electrons
  than it had before
• A quick way to remember this is
• LEO goes GER
• Loss of Electrons is Oxidation
• Gain of Electrons is Reduction

3
Examples

• Oxidation
• Al ? Al3 3e-
• Fe2 ? Fe3 e-
• 2F- ? F2 2e-
• Reduction
• Al3 3e- ? Al
• Fe3 e- ? Fe2
• F2 2e- ? 2F-

4
Exercises Oxidation or Reduction?

• Identify these reactions as either oxidation or
  reduction reactions
• O2 4e- ? 2O2-
• H2 ? 2H 2e-
• Zn ? Zn2 2e-
• MnO4- 8H 5e- ? Mn2 4H2O
• Cr2O72- 14H 6e- ? 2Cr3 7H2O

5
Balancing Simple Equations

• We know from previous years that equations need
  to be balanced, e.g.
• H2 Cl2 ? 2HCl
• We also have to balance equations to make all the
  charges balance, e.g.
• H ? H
• The equation above has a total of no charge on
  the left but one on the right.
• We can add electron(s) (i.e. one negative) to any
  side to make both sides charges balance, e.g.
• H ? H e-

6
Exercises Completing and Balancing Equations

• Balancing Equations
• Na Cl2 ? NaCl
• Mg HNO3 ? Mg(NO3)2 H2
• Balancing Ionic Equations
• Na O2- ?
• Al3 OH- ?
• Balancing Redox Half Equations
• Na ? Na
• F2 ? 2F-
• O2- ? O2

7
Why Gain or Lose Electrons?

• Atoms hold their electrons in shells (otherwise
  known as energy levels).
• As we add more electrons we fill up these levels
• The first level can hold 2 electrons
• The second level can hold 8 electrons
• The third level can hold 8 electrons
• Atoms become stable when their levels (shells)
  are full.
• Note Electron configuration is the name we
  give to the arrangement of the electrons within
  atoms or ions, e.g. Sodiums electron
  configuration is 2, 8, 1

8
Exercises Stable Shells
9
Electronegativity

• Some elements are better at bonding to electrons
  than others. This feature is called
  electronegativity and is discussed in more
  detail in the Atoms and Bonding topic.
• Elements with greater electronegativity will hold
  their electrons more strongly they are less
  likely to be oxidised (in fact they are more
  likely to end up gaining electrons)
• Elements with weak electronegativity have a weak
  hold on their electrons, they are more likely to
  lose them and therefore are more likely to be
  oxidised

10
Electronegativity Trends

• The general rule is that electronegativity
  increases as you move towards the right and the
  top of the periodic table
• This means that fluorine is the best holder and
  taker of electrons (notice that weve ignored
  group 18 why?)
• Therefore the best losers of electrons are at the
  bottom, left hand corner of the periodic table

11
Applied Electronegativity

• We can use this trend to predict what will happen
  when two chemicals are put together. Here are
  some examples
• Cl- and F2 are put together Chlorine is a
  better loser than Fluorine so
• 2Cl- F2 ? 2F- Cl2
• (Chlorine loses electrons and Fluorine gains)
• Na and Cl2 are put together
• 2Na Cl2 ? 2Na 2Cl-
• (Sodium is the loser, chlorine is the gainer)