Redox reactions

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What is a redox reaction?

RedOx - Reduction, oxidation

Reduction is a gain of electrons

Oxidation is a loss of electrons


OIL RIG ---> Oxidation is loss, reduction is gain


Redox reactions always involve the movement of electrons so are also called electron transfer reactions

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Half equations

Example

Copper and oxygen ---> copper oxide

Cu + 1/2O2 ---> Cu(2+) + O(2-)

Copper - lost 2 electrons - has been oxidised

Cu ---> Cu(2+) +2e-

Oxygen - gained 2 electrons - has been reduced

1/2O2 + 2e- ---> O(2-)

Add the two half equations together

Cu (s) + 1/2O2 (g) ---> [Cu(2+) + O(2-)] (s)

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Oxidising and reducing agents

Oxidising agents help other things to be oxidised

Oxidation is loss of electrons

If something else is losing electrons, and oxidising agents help this to happen, oxidising agents must gain electrons

Reducing agents help other things to be reduced

Reduction is gain of electrons

If something else is gaining electrons, and reducing agents help this to happen, reducing agents must lose electrons

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Oxidation states

Each element in a compound is given an oxidation state.

In an ionic compound ---> tells us how many electrons it has lost or gained

In a molecule ---> tells us the distribution of electrons between elements of differet elecronegativities

General rules

  • an element in its uncombined state has an oxidation state of 0
  • a positive number shows the element has lost electrons (has been oxidised)
  • a negative number shows the element has gained electrons (has been reduced)
  • numbers always have a + or - sign, unless they are 0
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Finding oxidation states

Hydrogen

always +1, except in metals hydrides (e.g. NaH) where it is -1

Group 1 elementsalways +1

Group 2 elementsalways +2

Aluminiumalways +3

Oxygen

always -2, except in peroxides where it is -1, and compounds with OF2 where it is +2

Fluorinealways -1

Chlorine

always -2, except in compounds with F and O where it has a positive value

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Balancing redox reactions

For an equation to be balanced:

  • the numbers of atoms of each element must be the same on either side of the equation
  • the total charge on each side of the equation must be the same

To balance an equation

  • if there are more hydrogens on one side, add H+ to the other side
  • if there are more oxygens on one side, add H20 to the other side
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