Topic 3 Redox I
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Redox reaction
Reaction that involves both reduction and oxidation.
Oxidation
Loss of electrons.
Reduction
Gain of electrons.
Oxidising agent
A species that oxidised another species by removing one or more electrons. When an oxidising reagent reacts, it gains at least 1 electron, so is reduced.
Reducing agent
A species that reduces another species by adding at least 1 electron. When a reducing agent reacts, it loses one or more electrons, so is oxidised.
Oxidation number
The charge that an ion has, or that it would have, if it were fully ionic.
Oxidation number of an uncombined element
0
The sum of Oxidation numbers of all the elements in a neutral compound
= 0
The sum of Oxidation numbers of all the elements in an ion=
The charge on the ion.
The more electronegative element in a substance
Given a negative oxidation number.
Oxidation number of fluorine
-1 always.
Oxidation number of hydrogen
+1, except when combined with a less electronegative element, then it becomes -1.
The oxidation number of oxygen
-2 except in peroxides where it is -1 and when combined with fluorine, when it is +ve.
Disproportionation
The simultaneous oxidation and reduction of an element in a single reaction.
What happens to Oxidation number of an element when it is reduced?
It decreases.
What happens to oxidation number of an element when it is oxidised?
It increases.
Disproportionation reaction when NaOH reacts with chlorine:
2NaOH + Cl2 —> NaCl + NaClO + H2O (all aq except water, which is l).
SO2
Sulfur(IV) oxide
SO3
Sulfur (VI) oxide
When are systematic names (Roman numerals in brackets) used?
When an element can have more than 1 Oxidation State.
How can we use oxidation numbers to identify the stoichiometric ratio?
Identify the elements whose oxidation numbers have changed. E.g., S changes from +4 to +6, so this is a 2 electron change and Ag changes from +1 to 0– a 1 electron change. This means SO2 to Ag+ is 1:2. Then Balance the electrons, als and Hs.
First step when balancing redox equations
Check you have the existing atoms balanced.
When balancing equations, what are you allowed to add?
Electrons. Water. H+ (or OH- if in alkaline conditions).
Typical sequence for balancing equations in acidic or neutral conditions:
Balance the atoms except O and H. Balance the Os by adding water molecules. Balances the Hs by adding H+ ions. Balance the charges by adding electrons.