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Chemistry Unit 3 Summary Notes Part 3
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The third document of summary notes for chemistry unit 3.
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Topic 2.6 RedoxRedox reactions
Oxidation is the loss of electrons or an increase in oxidation number
Reduction is the gain of electrons or a decrease in oxidation number
A redox reaction is one in which oxidation and reduction take place simultaneously. One chemical species
must be oxidised and a second must be reduced.
e.g. When chlorine water is added to a solution of potassium iodide, a redox reaction takes place
- -
Cl2(aq) + 2I (aq) → 2Cl (aq) + I2(aq)
Oxidation numbers: 2x(0) 2x(-1) 2x(-1) 2x(0)
The iodine species is oxidised from -1 to 0
The chlorine species is reduced from 0 to -1
Chlorine molecules are acting as an oxidising agent, themselves being reduced
Iodide ions are acting as a reducing agent, themselves being oxidised
A Half equation is a chemical equation which show the redox change for one species, with electrons added
to balance the equation in terms of charge. The overall redox reaction is written by combining the two half
equations, first checking that the numbers of electrons balance. Add the two equations together, cancel the
electrons, and add state symbols. Thus for the reaction above:
- -
Half equation 1: Cl2 + 2e → 2Cl
- -
Half equation 2: 2I → I2 + 2e
- - - -
Add together: Cl2 + 2e + 2I → 2Cl + I2 + 2e
- -
Cancel electrons: Cl2 + 2I → 2Cl + I2
- -
Add state symbols: Cl2(aq) + 2I (aq) → 2Cl (aq) + I2(aq)
Similarly, for the reaction between chlorate(I) ions and iodide ions in acidified solution to form iodine, water,
and chloride ions.
- + - -
ClO + 2H + 2e → Cl + H2O
- -
2I → I2 + 2e
- + - -
ClO (aq) + 2H (aq) + 2I (aq) → Cl (aq) + H2O(l) + I2(aq)
A disproportionation reaction is one in which the same species is simultaneously oxidised and reduced
- + - -
ClO + 2H + 2e → Cl + H2O
+1 -1
- - + -
ClO + 2H2O → ClO3 + 4H + 4e
+1 +5
Double the first equation so that there are four electrons in each equation
- + - -
2ClO + 4H + 4e → 2Cl + 2H2O
2x(+1) 2x(-1)
Then add the equations,
- + - - - - + -
2ClO + 4H + 4e + ClO + 2H2O → 2Cl + 2H2O + ClO3 + 4H + 4e
and cancel out to obtain the overall equation for the disproportionation
- - -
3ClO → 2Cl + ClO3
02/06/2009
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