Transcript oxidized - Fort Bend ISD

Electrochemistry
Electron Transfer Reactions
• Electron transfer reactions are oxidationreduction or redox reactions.
• Results in the generation of an electric
current (electricity) or be caused by
imposing an electric current.
• Therefore, this field of chemistry is often
called ELECTROCHEMISTRY.
Redox Reactions
(Electron Transfer reactions)
Reduction
Oxidation
Oxidation: Reduction:
•Gain of oxygen
•Loss of oxygen
•Loss of electrons •Gain of electrons
Increase in
oxidation
number
Decrease in
oxidation
number
You can’t have one… without the other!
• Reduction (gaining electrons) can’t happen
without an oxidation to provide the electrons.
• You can’t have 2 oxidations or 2 reductions in the
same equation. Reduction has to occur at the
cost of oxidation
LEO the lion says GER!
o l x
s e i
e c d
t a
r t
o i
n o
s n
GER!
a l e
i e d
n c u
t c
r t
o i
n o
s n
Another way to remember
•OIL RIG
x s o
i
s
d
e
a
t
i
o
n
e s
d
u
c
t
i
o
n
a
i
n
Rules for Assigning Oxidation
Numbers
1) The oxidation number of any
uncombined element is zero.
2) The oxidation number of a
monatomic ion equals its charge.
0
0
1
1
2 Na  Cl 2  2 Na Cl
Rules for Assigning Oxidation
Numbers
1) The oxidation number of oxygen in
compounds is -2, except in
peroxides, such as H2O2 where it is -1.
4) The oxidation number of hydrogen in
compounds is +1, except in metal
hydrides, like NaH, where it is -1.
1
2
H2O
Rules for Assigning Oxidation
Numbers
5) The sum of the oxidation numbers of the
atoms in the compound must equal 0.
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca(O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation
Numbers
6) The sum of the oxidation numbers in
the formula of a polyatomic ion is equal
to its ionic charge.
? 2
N O3

X + 3(-2) = -1
N
O
thus X = +5
? 2
S O4
2
X + 4(-2) = -2
S
O
thus X = +6
Oxidised –
gains oxygen
Burning Magnesium
2Mg(s) + O2(g)  2MgO(s)
Must be a redox!
Oxidised – loss of e-
Put the
e- in.
Reduced – gain of e-
Mg

2+
Mg
O +2e-  O2-
+2e-
Copper in silver nitrate solution
Cu(s) + 2AgNO3(aq)  Cu(NO3 )2(aq) + 2Ag(s)
Complete
the halfOxidised?
equations
Reduced?
Oxidised – loss of e-
Reduced – gain of e-
Cu
Ag+ +e-


Cu2+ +2e-
Ag
Oxidation and Reduction
(Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom loses one electron:
1
0
Na  Na  e

Each chlorine atom gains one electron:
0

1
Cl  e  Cl
Not All Reactions are Redox
Reactions
- Reactions in which there has been no
change in oxidation number are NOT
redox reactions.
Examples:
1 5 2
1
1
1
1
1 5 2
Ag N O3 (aq)  Na Cl (aq)  Ag Cl ( s)  Na N O3 (aq)
1 2 1
1
6 2
1
6 2
1
2
2 Na O H (aq)  H 2 S O 4 (aq)   Na 2 S O 4 (aq)  H 2 O(l )
Terminology for Redox Reactions
• OXIDATION—loss of electron(s) by a species;
increase in oxidation number; increase in oxygen.
• REDUCTION—gain of electron(s); decrease in
oxidation number; decrease in oxygen; increase
in hydrogen.
• OXIDIZING AGENT—electron acceptor; species is
reduced. (an agent facilitates something; ex.
Travel agents don’t travel, they facilitate travel)
• REDUCING AGENT—electron donor; species is
oxidized.
LEO says GER :
Lose Electrons = Oxidation
1
0
Na  Na  e

Sodium is oxidized
the substance that loses the electrons is called the reducing agent.
Gain Electrons = Reduction
0

1
Cl  e  Cl
Chlorine is reduced
the substance that gains the electrons is called the oxidizing
agent.
Reducing Agents and
Oxidizing Agents
• Conceptual Problem 20.2, page 641
• An increase in oxidation number = oxidation
• A decrease in oxidation number = reduction
1
0
Na  Na  e

Sodium is oxidized – it is the reducing agent
0

1
Cl  e  Cl
Chlorine is reduced – it is the oxidizing agent
Balancing Redox Equations
Two systematic methods are available,
and are based on the fact that the total
electrons gained in reduction equals
the total lost in oxidation.
The two methods:
1) Use oxidation-number changes
2) Use half-reactions
Using Oxidation-Number
Changes
you compare the increases and decreases in
oxidation numbers.
•start with the skeleton equation
•Step 1: assign oxidation numbers to all
atoms; write above their symbols
•Step 2: identify which are oxidized/reduced
•Step 3: use bracket lines to connect them
•Step 4: use coefficients to equalize
•Step 5: make sure they are balanced for
both atoms and charge
Using half-reactions
A half-reaction is an equation showing just
the oxidation or just the reduction that takes
place
they are then balanced separately, and
finally combined
Step 1: write unbalanced equation in ionic
form
Step 2: write separate half-reaction
equations for oxidation and reduction
Step 3: balance the atoms in the halfreactions (More steps on the next screen.)
Using half-reactions
continued
•Step 4: add enough electrons to one side
of each half-reaction to balance the charges
•Step 5: multiply each half-reaction by a
number to make the electrons equal in both
•Step 6: add the balanced half-reactions to
show an overall equation
•Step 7: add the spectator ions and balance
the equation
Choosing a Balancing Method
1) The oxidation number change
method works well if the oxidized
and reduced species appear only
once on each side of the equation,
and there are no acids or bases.
2) The half-reaction method works
best for reactions taking place in
acidic or alkaline solution.
Corrosion
•Damage done to metal is costly to
prevent and repair
•Iron, a common construction metal often
used in forming steel alloys, corrodes by
being oxidized to ions of iron by oxygen.
•This corrosion is even faster in the
presence of salts and acids, because
these materials make electrically
conductive solutions that make
electron transfer easy
Corrosion
•Luckily, not all metals corrode easily
•Gold and platinum are called noble
metals because they are resistant to
losing their electrons by corrosion
•Other metals may lose their electrons
easily, but are protected from corrosion by
the oxide coating on their surface, such as
aluminum –
•Iron has an oxide coating, but it is not
tightly packed, so water and air can
penetrate it easily
Corrosion
•Serious problems can result if bridges,
storage tanks, or hulls of ships corrode
•Can be prevented by a coating of oil,
paint, plastic, or another metal
•If this surface is scratched or worn away,
the protection is lost
•Other methods of prevention involve the
“sacrifice” of one metal to save the second
•Magnesium, chromium, or even zinc
(called galvanized) coatings can be applied