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9. Oxidation-Reduction. Oxidation-Reduction Redox Reactions. Reactions in which some elements change their oxidation number. Oxidation numbers. An oxidation number is the charge an atom would have if electrons in its bonds belonged completely to the more electronegative atom.

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9. Oxidation-Reduction

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9. Oxidation-Reduction


Oxidation-Reduction Redox Reactions

Reactions in which some elements change their oxidation number


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


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.


Practice Assigning Oxidation Numbers


  • Oxidation:

  • Gain of oxygen

  • Loss of electrons

  • Reduction:

  • Loss of oxygen

  • Gain of electrons

Increase in

oxidation

number

Decrease in

oxidation

number


Zn + CuSO4 ZnSO4 + Cu


An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized.

A decreasein the oxidation number indicates that an atom has gained electrons and therefore reduced

Zn + CuSO4 ZnSO4 + Cu

0+2 +6-2 +2+6-2 0

Zn: 0+ 2Oxidized

Cu: +20Reduced

Using Oxidation Numbers


LEO


LEO says

GER!


GER!

LEO

says

Loss of Electrons = Oxidation

Gain of Electrons =Reduction


Reducing and Oxidizing Agents

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch9/redox.php#agents

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


Reducing Agent

A substance that brings about the reduction of a substance by donating electrons to the substance it reduces.

+2 +4

Fe2O3 + 3CO => 2Fe + 3CO2

CuSO4 + Zn => ZnSO4 + Cu


Substances that can act as both:

H2O2

H2O2 + 2H+ + 2I- => 2 H2O + I2

5H2O2 + 2MnO4- + 6H+ => 5 O2 + 2Mn+2 + 8H2O


Some examples of Oxidizing Agents

  • MnO4- (aq) + H+ => Mn2+(aq) + H2O

  • 14 H+ + Cr2O7-2 + 6e => 2Cr+3 + H2O

  • I2 (aq) + 2e => 2 I-(aq)


Oxidation-Reduction Reactions

  • All oxidation reduction reactions have one element oxidized and one element reduced

    +2 0 +2 0

    CuSO4 + Zn => ZnSO4 + Cu

    Do Hodder page 253 : Q4 , Q2


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 => HCl + HOCl


Writing Half Reactions

Mg + O2 => MgO

Reduction and Oxidation must occur simultaneously,

# e lost = # e gained

Oxidation:

Reduction:


Writing Half Reactions

Mg + O2 => MgO

Reduction and Oxidation must occur simultaneously,

# e lost = # e gained

Oxidation: Mg => Mg2+ + 2e

Reduction: O2 + 4e -=> 2O2-


  • The two half reactions should be added together:


Balancing Redox Reactions in Acidic or Alkaline Solution

Acidic:

Balance O by adding water

Balance H by adding H+


http://www.kentchemistry.com/aplinks/chapters/4chemrxns/BalancingRedox.htm


Reactivity Series

  • http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/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 => Cu + ZnSO4

    Zn(s) => Zn2+(aq) + 2e

    Cu2+(aq) + 2e => Cu(s)


http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/redox/home.html


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.


  • Hodder page 254 : Paper 2 Q1


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/ibnotes/full/red_htm/10.2.htm


  • Chlorine will displace bromine from solutions containing bromide ions

    Cl2+ 2Br- => Br2+ 2Cl-

    2. Bromine will displace iodine from solutions containing iodide ions

    Br2+ 2I- => I2+ 2Br-


Corrosion of Iron

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


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.


Corrosion Prevention


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