Redox – Part 2
a. demonstrate an understanding of:
iv. how oxidation number is a useful concept in terms of the classification of
reactions as redox and as disproportionation
b. write ionic half-equations and use them to construct full ionic equations.
Task: Match up the formulas with their
correct names
Naming Compounds
• Some elements can have more than one oxidation
number.
• So to avoid confusion when naming the compounds they
form, its helpful to use these numbers in their names.
Naming Compounds
What are the
missing
oxidation
numbers here
for Manganese
and Chromium,
respectively?
?
?
Task Answer – sometimes, instead of using the
systematic name, scientists still use the traditional
names for some substances.
Redox and Oxidation Numbers
• An increase in oxidation number means oxidation has
occurred.
• While a decrease in oxidation number means reduction
has occurred.
What would be
• E.g.
0
+1
-2
+1
-1
0
Therefore,
chlorine has been reduced, and
sulphur has been oxidised.
the oxidation
numbers for
each of the
species in this
reaction?
Redox and Oxidation Numbers
• Another example is the Thermite Reaction (click here to
see it in action!!)
+3
0
-2
0
+3
• But what is oxidised and what is reduced?
Therefore,
iron has been reduced, and
aluminium has been oxidised.
-2
Disproportionation
• Is a type of reaction in which a substances both oxidised
and reduced in the same reaction. E.g. the breakdown of
hydrogen peroxide:
Disproportionation – another
example
• The reaction of copper(I) oxide with dilute sulphuric acid:
• Study the equation carefully – what disproportionates?
• The answer is copper (Cu) which gets oxidised from +1 to
+2 and also reduced from +1 to zero, in the same reaction.
• Other examples of such reactions include reactions of
chlorine with water, and chlorine with hot sodium hydroxide
solution.
Questions
Answers
Oxidation States
• Some elements show a range of oxidation states.
• One colourful example is the reduction of vanadium (V)
to vanadium (II) through sucessive oxidation number.
Vanadium – its different oxidation states
Ammonium Vanadate (V) (white solid) can be added to dilute
HCl to give the orange Dioxovanadium (V) ion:
VO3- + 2H+
VO2+ + H2O
+5
+5
If granulated zinc is now added it reduces the vanadium over
a period of several minutes and gives several colour
changes:
Blue [VO(H2O)5]2+ Vanadium (IV)
Green
[VCl2(H2O)4] + Vanadium (III)
Violet [V(H2O)6]2+ Vanadium (II)
Common Oxidising Agents
• What is an oxidising agent?
– A species that reacts by oxidising something else and getting
reduced itself.
• So, as we will see, electrons in the half equations are
always on the left-hand side of the equation, as oxidising
agents gain electrons.
More Oxidising Agents
More Oxidising Agents
Common Reducing Agents
• What is an reducing agent?
– A species that reacts by reducing something else and getting
oxidised itself.
• So, this time, electrons in the half equations are always
on the right-hand side of the equation, as reducing
agents loose electrons.
More Reducing Agents
More Reducing Agents
Writing balanced equations from ionic
half equations - Worked Example…
Firstly, write
down the
half equations
you
need.
Worked Example…..cont….
X5
Worked Example…..cont….
Notice, that in adding the two equations together
The electrons are cancelled out and not
Included in the final equation
Worked Example 2…
Again, start
with the half
equations
Have a go
before you turn to
the next slide for
the answer
Worked Example 3…
Again, start
with the half
equations
?
Or
X
Ionic to Molecular Equation
• To turn an ionic equation to a molecular equation, you would
simply add the spectator ions.
• Eg. Reaction of potassium manganate (VII) solution with iron
(II) sulphate solution in the presence of dilute sulphuric acid:
Ionic equation – which you work out
Add in spectator ions by using clues
In question
molecular equation
Questions
Answers