Transcript Oxidation and Reduction

Oxidation and Reduction
Historically....
• Oxidation was defined as the addition
of oxygen to a substance Eg. when coal
was burned
C + O2
CO2
or the rusting of iron
4Fe + 3O2
2Fe2 O3
• Chemists discovered it was possible to
remove oxygen from some substances
• Eg. When hydrogen gas is passed over
heated copper oxide, copper metal is
obtained
CuO + H2
Cu + H2O
• This process was called reduction as it
was often used to extract metals from
their ores thus getting a reduced
amount of metals out of a larger amount
• Reduction became known as the removal
of oxygen from a substance or since
hydrogen was often used the addition
of hydrogen to a substance
Oxidation and Reduction in terms
of Electron transfer
• Many chemical reactions involve the
transfer of electrons
• In the oxidation of Magnesium to
magnesium oxide
2Mg + O2
2Mg2+O2-
Mg loses 2 electrons
oxidation
• In this reaction Magnesium is losing two
electrons and oxygen is gaining the two
electrons
• Oxidation of an element takes place
when it loses electrons
• In the equation we have just studied
what is oxidised ?
• Consider the reaction of copper oxide
to copper metal
Cu2+O2- + H2
Cu
2+
gains 2 electrons
reduction
Cu + H2O
• We can see Cu2+ gains 2 electrons to
become a copper atom
• Reduction of an element takes place
when it gains electrons
• Thinks of OIL RIG to help you
remember
• (oxidation is loss of electrons,
reduction is gain of electrons)
Redox Reactions
• Oxidation and Reduction must always occur
together, if one substance loses electrons
there must be another substance there to gain
those electrons
• Think of the reaction between sodium and
chlorine to form sodium chloride
2Na + Cl2
2Na+ Cl-
Can you identify what is reduced and what is
oxidised?
• These types of reactions are called
oxidation-reduction reactions or Redox
reactions for short
• It is clear from the equation between
sodium and chlorine that neither
hydrogen or oxygen are present showing
that oxidation and reduction is much
more broadly defined in terms of
electron transfer
Reaction Between Zinc Metal and
Copper Ions
• When zinc metal is left to stand in
copper sulfate solution it is found that
the zinc becomes covered in copper
deposits
• As this reaction takes place the blue
colour of the solution fades indicating
the Cu2+ ions are being used up
• On analysis of the liquid Zinc ions are
found
• It appears the following reaction has
taken place
Zn + Cu2+
Zn2+ + Cu
• The Zn is oxidised and the Cu is
reduced
• The Zn could not lose its electrons
unless there was something there to
accept them (in this case the Cu2+ ions)
• We say that the Cu2+ ion is an oxidising
agent
Oxidising Agents
• An oxidising agent is a substance that
brings about oxidation in other
substances
• In oxidising the Zn the Cu2+ ion gains
electrons and is itself reduced
• The oxidising agent is always reduced
Reducing Agents
• Since Zn is the substance that causes
the Cu2+ ion to be reduced we call it the
reducing agent
• A reducing agent brings about
reduction in other substances
• In giving electrons to the Cu2+ ion the
zinc loses electrons and thus is oxidised
• The reducing agent is always oxidised
Identifying Oxidising and
Reducing agents in Equations
• One of the most common oxidising agents
is Hydrogen Peroxide (H2O2)
used to bleach hair
• It converts coloured hair pigment to
colourless hair pigment which appears
blonde
Coloured Hair + H O
Colourless Hair + H O
2 2
2
pigment
pigment
• Iodine used to treat cuts and chlorine
used to disinfect swimming pool water
work by oxidising chemicals in the cells
of germs
Live Germ + I2
2I- + Dead Germ
Live Germ + Cl2
2Cl- + Dead Germ
• Carbon monoxide is a very useful
reducing agent in industry
• It is used to remove the oxygen from
iron ore in order to convert it to pure
iron
Fe2O3 + CO
2Fe + 3CO2
• By using the group number of the elements
involved in the reactions it can be
determined whether the atoms will want to
lose or gain electrons
• For example group 1 elements will tend to
want to lose one electron hence will be
oxidised and are reducing agents
• We will study more detailed rules for this
later on
• Try p190 Q14.2
Oxidation Numbers
• In order to keep track of electrons
during chemical reactions involving
covalent compounds chemists introduced
the idea of oxidation numbers/states.
• The Oxidation number of an atom is the
charge that an atom appears to have
when the electrons are distributed
according to certain rules
Rules for Assigning oxidation
Numbers
1.The Oxidation No of any uncombined
element is zero Eg. In Zn the Zn has an
ON of 0, in O2 each O has an ON of 0
2.The Oxidation No of an ion of an
element is the same as its charge
Eg. The ON of Br in the Br- ion is -1 the
ON of Na in Na+Cl- is +1 and of Mg in
Mg2+O2- is +2 etc
3.The sum of all the elements in a compound
must add up to zero
Eg. in H2O each H is +1 and O is -2 therefore
2(+1) + 1 (+2) = 0
4.Oxygen has an Oxidation No of – 2 except in
a peroxide or if joined with fluorine
In Mg2+O2- the ON of oxygen is -2, however in
H2O2 (hydrogen peroxide) it has an ON of -1 so
that the sum of all the ON in the compound will
be 0 2(+1) + 2(-1) = 0. In Oxygen difluoride OF2
Fluorine is more electronegative than oxygen
and attracts electrons to itself each F has a
charge of -1 so oxygen has a charge of +2 to
ensure the ON’s add up to 0 (2(-1) + 1(+2) = 0
5.Hydrogen is assigned + 1 except if
joined with metals when it will have an
ON of -1
In NH3 each H atom is assigned the
number of +1 , this would mean N must
be -3 as 3(+1) + 1(-3) = 0 However in
Sodium Hydride NaH because sodium is
a metal H has an ON of -1
6.Halogens are – 1 unless bonded to a more
electronegative atom
Fluorine will always have an ON of -1 as it is the
most electronegative atom in the periodic table,
Chlorine usually has an ON of -1 except when it
is bonded to oxygen or fluorine (as these are
more electronegative than chlorine) in Cl2O each
Chlorine has an ON of +1 in Cl2O7 each chlorine
has an ON of +7
7. The sum of all the oxidation numbers in a
complex ion must add up to the charge on the
ion
Examples of complex ions are NO3- , SO42-,
CO32- etc. (ions like Cl- and Mg2+ are called
simple ions)
Oxidation and Reduction in terms
of oxidation numbers
• When an element is oxidised its
oxidation number increases, ie.
oxidation is an increase in oxidation
number
• When an element is reduced its
oxidation number decreases, ie.
Reduction is a decrease in oxidation
number
Balancing Redox Equations
• We will refer to examples p187