9. Oxidation-Reduction

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9. Oxidation-Reduction
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Oxidation-Reduction Redox Reactions

• Reactions in which some elements change their
  oxidation number

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Oxidation numbers

• An oxidation number is the charge an atom would
  have if electrons in its bonds belonged
  completely to the more electronegative atom.
• in CO2 O has a higher EN
• the oxidation numbers are O -2, C 4

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• There are a few rules to remember
• The oxidation number of an element is zero.
• The oxidation number of an ion is equal to the
  charge of the ion.
• Hydrogen has an oxidation number of 1 (this
  doesn't apply to hydrides when hydrogen is -1).
• Oxygen has an oxidation number of -2 (except in
  peroxides when it is -1).
• For polyatomic ions, consider the charge of the
  ion.

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N2
N2O5
Cl-
HNO3
CO3-2
KMnO4
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Practice Assigning Oxidation Numbers
Fe(OH)3
K2Cr2O7
CO32-
CN-
K3Fe(CN)6

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• Oxidation
• Gain of oxygen
• Loss of electrons

• Reduction
• Loss of oxygen
• Gain of electrons

Increase in oxidation number
Decrease in oxidation number
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• Zn CuSO4 ? ZnSO4 Cu

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Using Oxidation Numbers

• An increase in the oxidation number indicates
  that an atom has lost electrons and therefore
  oxidized.
• A decrease in the oxidation number indicates that
  an atom has gained electrons and therefore
  reduced
• Zn CuSO4 ? ZnSO4 Cu
• 0 2 6-2 26-2 0
• Zn 0 ? 2 ? Oxidized
• Cu 2 ? 0 ? Reduced

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LEO
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LEO says
GER!
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GER!
LEO says
Loss of Electrons Oxidation Gain of Electrons
Reduction
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Reducing and Oxidizing Agents

• http//chemed.chem.purdue.edu/genchem/topicreview/
  bp/ch9/redox.phpagents
• Oxidizing Agent ( ozone)
• A substance that brings about the oxidation of
  substances by accepting electrons from the
  substance they oxidize.
• 0
  -2
• 2KI O3 H2O KOH O2 I2

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• Reducing Agent
• A substance that brings about the reduction of a
  substance by donating electrons to the substance
  it reduces.
• 2
  4
  
  
• Fe2O3 3CO gt 2Fe 3CO2
• CuSO4 Zn gt ZnSO4 Cu

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Substances that can act as both

• H2O2
• H2O2 2H 2I- gt 2 H2O I2
• 5H2O2 2MnO4- 6H gt 5 O2 2Mn2
  8H2O

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Some examples of Oxidizing Agents

• MnO4- (aq) H gt Mn2 (aq) H2O
• 14 H Cr2O7-2 6e gt 2Cr3 H2O
• I2 (aq) 2e gt 2 I-(aq)

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Oxidation-Reduction Reactions

• All oxidation reduction reactions have one
  element oxidized and one element reduced
• 2
  0 2
  0
• CuSO4 Zn gt ZnSO4 Cu
• Do Hodder page 253 Q4 , Q2

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Auto Redox Reaction

• A chemical reaction in which a substance oxidizes
  and reduces simultaneously is called auto
  oxidation reduction reaction.
• 0
  -1
  1
• Cl2 H20 gt HCl HOCl

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Writing Half Reactions

• Mg O2 gt MgO
• Reduction and Oxidation must occur
  simultaneously,
• e lost e gained
• Oxidation
• Reduction

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Writing Half Reactions

• Mg O2 gt MgO
• Reduction and Oxidation must occur
  simultaneously,
• e lost e gained
• Oxidation Mg gt Mg2 2e
• Reduction O2 4e -gt 2O2-

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• The two half reactions should be added together

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Balancing Redox Reactions in Acidic or Alkaline
Solution

• Acidic
• Balance O by adding water
• Balance H by adding H

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http//www.kentchemistry.com/aplinks/chapters/4che
mrxns/BalancingRedox.htm
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Reactivity Series

• http//group.chem.iastate.edu/Greenbowe/sections/p
  rojectfolder/flashfiles/redox/home.html
• The more readily the metal loses its outer
  electrons the more reactive it is.
• It is possible to organize a group of similar
  chemicals that undergo either oxidation or
  reduction according to their relative reactivity.
  
• The zinc metal is more reactive than copper metal
  and so it can force the copper metal ions to
  accept electrons and become metal atoms.
• Zn CuSO4 gt Cu
  ZnSO4
• Zn(s) gt
  Zn2(aq) 2e
• Cu2(aq) 2e gt
  Cu(s)

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http//group.chem.iastate.edu/Greenbowe/sections/p
rojectfolder/flashfiles/redox/home.html
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Important

• Although a reaction may be predicted as
  feasible it does not mean that it will happen
  spontaneously.
• If the activation energy is high then it may
  need an extra "push" to get it going. - for
  example the reaction between chlorine and
  hydrogen needs a spark or ultraviolet light and
  then it is explosively fast.

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• Hodder page 254 Paper 2 Q1

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Reactivity series involving non-metals

• Non-metals react by gaining electrons - they are
  oxidising agents.
• The non-metals can be ordered in terms of their
  oxidising strength.
• The halogens are a typical example of a non-metal
  reactivity series.
• Reactivity of the halogens http//ibchem.com/IB/i
  bnotes/full/red_htm/10.2.htm

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• Chlorine will displace bromine from solutions
  containing bromide ions
• Cl2 2Br- gt Br2 2Cl-
• 2. Bromine will displace iodine from solutions
  containing iodide ions
• Br2 2I- gt I2
  2Br-

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Corrosion of Iron

• Corrosion is when a metal undergoes a reaction
  and oxidizes due to the interaction of water, air
  and /or salt solutions.

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The figure below illustrates the oxidation
process of iron where what we know as rust is
formed (Fe2O3 )Iron can only rust if water is
present and does not oxidize directly between
itself and air but by a series of reactions and
it tends to rust faster at a low pH and high salt
concentration.
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Corrosion Prevention