Transcript Reactions in which some elements change their

9. Oxidation-Reduction
Oxidation-Reduction
Redox Reactions
Reactions in which some
elements change their oxidation
number
Oxidation numbers
• An oxidation number is the charge an atom
would have if electrons in its bonds belonged
completely to the more electronegative atom.
in
CO2
O has a higher EN
the oxidation numbers are O = -2,
C = +4
There are a few rules to remember:
• The oxidation number of an element is zero.
• The oxidation number of an ion is equal to the
charge of the ion.
• Hydrogen has an oxidation number of +1 (this
doesn't apply to hydrides when hydrogen is -1).
• Oxygen has an oxidation number of -2 (except in
peroxides when it is -1).
• For polyatomic ions, consider the charge of the
ion.
N2
N2O5
ClHNO3
CO3-2
KMnO4
Practice Assigning
Oxidation Numbers
Fe(OH)3
K2Cr2O7
CO32CNK3Fe(CN)6
Oxidation:
Reduction:
•Gain of oxygen
•Loss of oxygen
•Loss of electrons
•Gain of electrons
Increase in
oxidation
number
Decrease in
oxidation
number
Zn
+
CuSO4  ZnSO4 + Cu
Using Oxidation Numbers
• An increase in the oxidation number indicates that
an atom has lost electrons and therefore oxidized.
• A decrease in the oxidation number indicates that
an atom has gained electrons and therefore reduced
Zn
0
+
CuSO4  ZnSO4 + Cu
+2 +6-2
+2+6-2
0
Zn: 0  + 2  Oxidized
Cu: +2  0  Reduced
LEO
LEO says
GER!
LEO
says
GER!
Loss of Electrons = Oxidation
Gain of Electrons = Reduction
Reducing and Oxidizing Agents
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch9/redox.php#agents
Oxidizing Agent ( ozone)
A substance that brings about the oxidation of substances by accepting
electrons from the substance they oxidize.
0
-2
2KI + O3 + H2O =+ KOH + O2 + I2
Reducing Agent
A substance that brings about the reduction of a substance by
donating electrons to the substance it reduces.
+2
+4
Fe2O3 + 3CO => 2Fe + 3CO2
CuSO4 + Zn => ZnSO4 + Cu
Substances that can act as both:
H 2O 2
H2O2 + 2H+ + 2I- => 2 H2O + I2
5H2O2 + 2MnO4- + 6H+ => 5 O2 + 2Mn+2 + 8H2O
Some examples of Oxidizing Agents
1. MnO4- (aq) + H+ => Mn2+ (aq) + H2O
2. 14 H+ + Cr2O7-2 + 6e => 2Cr+3 + H2O
3.
I2 (aq) + 2e => 2 I-(aq)
Oxidation-Reduction Reactions
• All oxidation reduction reactions have
one element oxidized and one element
reduced
+2
0
+2
0
CuSO4 + Zn => ZnSO4 + Cu
Do Hodder page 253 : Q4 , Q2
Auto Redox Reaction
• A chemical reaction in which a substance
oxidizes and reduces simultaneously is
called auto oxidation reduction reaction.
0
-1
+1
Cl2 + H20 => HCl + HOCl
Writing Half Reactions
Mg + O2 => MgO
Reduction and Oxidation must occur simultaneously,
# e lost = # e gained
Oxidation:
Reduction:
Writing Half Reactions
Mg + O2 => MgO
Reduction and Oxidation must occur simultaneously,
# e lost = # e gained
Oxidation:
Mg => Mg2+ + 2e
Reduction:
O2 + 4e -=> 2O2-
• The two half reactions should be added
together:
Balancing Redox Reactions in Acidic or
Alkaline Solution
Acidic:
Balance O by adding water
Balance H by adding H+
http://www.kentchemistry.com/aplinks/chapters/4chemrxns/BalancingRed
ox.htm
Reactivity Series
•
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/redox/home.html
• The more readily the metal loses its outer electrons the more
reactive it is.
• It is possible to organize a group of similar chemicals that undergo
either oxidation or reduction according to their relative reactivity.
• The zinc metal is more reactive than copper metal and so it can
force the copper metal ions to accept electrons and become metal
atoms.
Zn + CuSO4 => Cu + ZnSO4
Zn(s) => Zn2+(aq) + 2e
Cu2+(aq) + 2e => Cu(s)
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/redox/home.html
Important
Although a reaction may be predicted as
feasible it does not mean that it will happen
spontaneously.
If the activation energy is high then it may
need an extra "push" to get it going. - for
example the reaction between chlorine and
hydrogen needs a spark or ultraviolet light
and then it is explosively fast.
• Hodder page 254 : Paper 2 Q1
Reactivity series involving non-metals
• Non-metals react by gaining electrons - they are oxidising agents.
• The non-metals can be ordered in terms of their oxidising strength.
• The halogens are a typical example of a non-metal reactivity series.
Reactivity of the halogens: http://ibchem.com/IB/ibnotes/full/red_htm/10.2.htm
1. Chlorine will displace bromine from solutions containing
bromide ions
Cl2 + 2Br- => Br2 + 2Cl-
2. Bromine will displace iodine from solutions containing iodide ions
Br2 + 2I- => I2 + 2Br-
Corrosion of Iron
• Corrosion is when a metal undergoes a
reaction and oxidizes due to the interaction
of water, air and /or salt solutions.
The figure below illustrates the oxidation process of iron where what we
know as rust is formed (Fe2O3 )Iron can only rust if water is present and
does not oxidize directly between itself and air but by a series of reactions
and it tends to rust faster at a low pH and high salt concentration.
Corrosion Prevention