Download redox - trcshshw

When magnesium burns in the
presence of oxygen the octet rule
explains why magnesium & oxygen
come toghether.
Mg has 2 valence and O has 6….
When magnesium reacts with
chlorine the octet rule also allows us
to understand why the atoms come
together.
Determine the formula for
Magnesium Chloride.
What did these have in common?
In both of these reactions the
Magnesium gave up electrons and its
partner (the O or the Cl) gained
electrons.
When an atom gains or loses an
electron we have names to describe
it.
Reduction
When something gains an
electron it has been reduced.
This makes sense because electrons
are ________charged.
So gaining a negative makes it
reduced.
Oxidation
When something loses an
electron it is oxidation.
Which one of these gained and which
on lost electrons.
Which one was oxidized and which
one was reduced
They always go together
One atom cannot gain an electron
(reduced) unless another one can
lose an electron (oxidized)
Since they always go together we
call the reaction a Redox reaction
(reduction & oxidation)
Oxidation numbers
For some reactions it is easy to find
the charges and see the oxidation &
reduction.
For other reactions it is not so easy.
In those reactions we can use the
oxidation numbers at the top right of
the reference table.
Oxidation numbers
Each element has different possible
numbers of electrons it could gain or
lose.
If something gains 1 electrons
(reduced) it will be -1, if it is reduced
by 2 electrons it is -2
If something gained 1 or 2 electrons
it will be +1 or + 2
Oxidation number rules
These are 5 main rules of oxidation
numbers.
Oxidation rules
1) Uncombined elements have an
oxidation number of zero (Na, Mg,
Cl2 or O2)
2) For group 1 & 2 elements when
combined they always have a
charge of +1 or +2 (look at the
reference table)
3) Oxygen is almost always -2 (except
when with fluorine then it’s a +2 -the regents wont ask on this--)
Oxidation rules
4) Fluorine is always a -1 (the other
halogens are also almost always -1).
5) Hydrogen is a -1 when with a metal
but +1 when with a non metal
6) The sum of all oxidation numbers in
a compound need to equal zero.
(there will be an exception soon)
Oxidation
These rules may sound complicated
but once you get the hang of them it
becomes very simple.
Examples
What are the oxidation numbers of
the atoms in HNO3
--------------------------
What are the oxidation numbers of
the atoms in HNO3
O is (almost) always -2 (rule 3)
H is with a non metal so it is -1 (rule
5)
The N then must have an oxidation
of ______ to make everything even.
Examples
What is the oxidation number of
platinum in K2PtCl6
What is the oxidation number of
Sulfur in H2SO4
How to spot a redox reaction
Not all reactions are redox reactions
2NaOH + H2SO4 Na2SO4 + 2H2O
– This is not a redox
Mg +Br2  MgBr2
– This is a redox
The way to spot a redox is seeing if
oxidation numbers have changed!
Example
CuO + H2  Cu + H2O
Find the oxidation numbers before
and after.
Spot the redox
CaCO3 + 2HCl  CaCl2 +H2O + CO2
Or
Zn + 2AgNO3  2Ag + Zn(NO3)2
Hint for redox’s
When ever you see a reaction that
has an element by itself on one
side… and bonded on the other…..
That tells you the oxidation number
MUST have changed. Meaning that
reaction is always the redox.
Half reactions
A chemical equation shows the
formulas of the reactents & products,
but doesn’t show how electrons are
exchanged.
A half reaction shows the electrons
being gained or lost.
Reduction
A reduction half reaction shows an
atom gaining electrons….
& the oxidation charge goes ______
Fe3+ + 3e-  Fe
Oxidation
In an Oxidation half reaction we
show an atom losing electrons and
the oxidation charge goes _______
Fe  Fe3+ + 3e-
Notice how charge is conserved!!
Example
Write the half reaction showing the
reduction of Fe(II)
Write the half reaction showing the
oxidation of Mg
Example
Write the oxidation half reaction for
1) K
2) Ca
Write the reduction half reaction for
1) Ag+
2) F
Which was oxidized & reduced
Cu + 2AgNO3  Cu(NO3)2 + 2Ag
Then write the half reactions
Electro chemical cells
In a redox reaction there is an
exchange of electrons…
In electro chemical cells we can
harness that electron exchange for
our own purposes…
Battery
Electro chemical cells
In an electro chemical cell there are
two sides called electrodes
The electrode that oxidation occurs
at (loses electrons) in the ANODE
The electrode that gains electrons is
the CATHODE
Reduction = cathode
Oxidation = Anode
RED CAT
AN OX
Voltaic cells
A voltaic cell is where this electron
exchange happens spontaneously (A
battery)
Voltaic cells
In this example the Zn Anode lost
electrons, which traveled thru the
wire to the copper cathode. There
Cu2+ from the solution left the liquid
and went into the cathode to accept
the e-’s
Salt bridge
There needs to be another
connection called a salt bridge
between the beakers so that the ions
can stabilize
(not as important for the regents)
How can you tell the anode &
cathode?
Look at the overall reaction and
determine which gained or lost
electrons…
The one that was reduced (lost) is
the _______
The one that was oxidized (gained) is
the _______
Describe the direction of electron
flow in the external circuit in this
operating cell. [1]
State the purpose of the salt bridge
in this voltaic cell?
Allow 1 credit. Acceptable responses
include, but are not limited to:
The salt bridge allows for the
migration of ions between the halfcells
A simpler way to find anode &
cathode
Look at table J on your reference.
The metal that is higher on the
activity chart will be oxidised (anode)
The metal is where the reduction
will occuer (cathode)
Electrolytic cells
So far we have learnt about voltaic
cells that are spontaneous, (happen
without us forcing them to)
The reverse can also happen, where
the electrons are pushed in a non
natural way.
When electricity is used to force a
chemical reaction to occur it is called
electrolysis.
In this type of reaction the oxidized
metal is lower down on the chart
Electrolysis is used to plate a metal
The electrons leave the anode, and
travel thru the wire.
The metal ions leave the anode.
And travel to the cathode (the
spoon) to meet up with the
electrons.