Transcript Redox - Solon City Schools

Redox Reactions
Redox Reactions
Reactants: Zn + I2
Product: Zn I2
Redox Equations:
At the conclusion of our time together,
you should be able to:
1. Define redox
2. Figure out oxidation numbers for any
element
3. Show the change in oxidation numbers in a
reaction
What’s the Point ?
REDOX reactions are important in …
• Purifying metals
(e.g. Al, Na, Li)
C3H8O + CrO3 + H2SO4 
Cr2(SO4)3 + C3H6O + H2O
• Producing gases
(e.g. Cl2, O2, H2)
• Electroplating metals
•
Electrical production (batteries, fuel cells)
• Protecting metals from corrosion
• Balancing complex chemical equations
• Sensors and machines (e.g. pH meter)
What is Redox?
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REDOX stands for REDuction/OXidation
Oxidation is often thought of as a combination of
a substance with oxygen (rusting, burning)
Oxidation refers to a loss of electrons
Reduction refers to a gain of electrons
As a mnemonic remember LEO says GER
• Lose Electrons = Oxidation
• Gain Electrons = Reduction
Let’s See How You’re Doing??
Q- What is oxidation? What is reduction?
Represent each as a chemical equation.
oxidation
reduction
= loss of e– …
= gain of e– …
X 
X+ + e–
X + e–  X–
Q- Why are 2Na + Cl2  2NaCl & 2H2 + O2  H2O
considered redox reactions?
Both involve the transfer of electrons. (Na has no
charge, the atoms in diatomic molecules have no
partial charge. After reaction the atoms have
different shares of the electrons because of
different EN values)
Let’s See How You’re Doing??
Q- Is it possible to oxidize a material without reducing
something else?
No. A lost e– is taken up by something else.
Let’s See How You’re Doing??
Q- Calcium combines with chlorine gas to form
calcium chloride. Show the redox reactions.
CaCl2 is an ionic compound with a positive calcium ion
and negative chlorine ions
Ca + Cl2  CaCl2
Ca  Ca2+ + 2e–
Cl2 + 2e– 2Cl–.
Thus Ca is losing electrons (oxidation) and Cl is
gaining electrons (reduction).
These are called “half reactions”
Determination of Oxidation and Reduction
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If oxidation # decreased;
substance reduced
If oxidation # increased;
substance oxidized
Review of Oxidation Numbers
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We will see that there is a simple way to keep track
of oxidation and reduction
This is done via “oxidation numbers”
An oxidation number is the charge an atom would
have if electrons in its bonds belonged completely to
the more electronegative atom
e.g. in HCl, Cl has a higher EN. Thus, oxidation
numbers are Cl = -1, H = +1
Notice that oxidation numbers are written as +1 vs.
1+ to distinguish them from charges.
Review of Oxidation Numbers
1. Any element, when not combined with atoms of a
different element, has an oxidation # of zero.
(O in O2 is zero, Na by itself is zero)
2. Any simple monatomic ion (one-atom ion) has an
oxidation number equal to its charge
(Na+ is +1, O2– is –2)
3. The sum of the oxidation numbers of all of the
atoms in a formula must equal the charge written
for the formula. (if the oxidation number of O is –2,
then in CO32– the oxidation number of C is +4)
Review of Oxidation Numbers
4. In compounds, the oxidation # of IA metals is +1,
IIA is +2, IIIA is +3, Zn & Cd is +2, Ag is +1.
5. In ionic compounds, the oxidation # of a
nonmetal or polyatomic ion is equal to the charge
of its associated ion.
(MgCl2, Mg is +2, therefore Cl is –1)
6. F is always –1, O is always –2
(unless combined with F),
H is usually +1, except when it is
bonded to metals in binary compounds.
(ex. NaH, H oxidation # is –1 or when it’s in
elemental form H2, oxidation # is 0).
Oxidation numbers
of all the elements
in the following ?
NaIO3
Na = +1 O = -2
3x(-2) + 1 + ? = 0
I = +5
IF7
F = -1
7x(-1) + ? = 0
I = +7
Redox Equations:
Let’s see if you can:
1. Define redox
2. Figure out oxidation numbers for any
element
3. Show the change in oxidation numbers in a
reaction
Let’s Apply the Oxidation Rules
or rule 5
Rule 6 3 6 4 3 6 3 6 6 4 5 6 3 6
Total +1 +5 -6 +2 +12 -14 -4 +6 -2 +1 -1 +2 +5 -8
Ox.# +1 +5 -2 +1 +6 -2 -2 +1 -2 +1 -1 +1 +5 -2
H N O 3 K2Cr2O7
C2H6O
AgI
H2PO4–
Corrosion – Deterioration of Metals by
Electrochemical Process
Let’s consider
what happened to
the iron ribbing of
the Statue of
Liberty
Iron loses
e-,
therefore
is the
reducing
agent
Iron loses e-, therefore is oxidized
Oxygen gaines e-, therefore is reduced
Oxygen
gaines e-,
therefore
is the
oxidizing
agent
Corrosion – Deterioration of Metals by
Electrochemical Process
Determine and balance the
reaction, determine what is
oxidized and what is reduced.
Corrosion of Silver
Ag + O2  Ag2O
4 Ag + O2  2 Ag2O
Each Ag loses 1eEach O gains 2eSilver
Oxygen
Silver
Oxygen
=
=
=
=
oxidized
reduced
reducing agent
oxidizing agent