Transcript Regents Unit 13

REDOX
Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
• Electrons are transferred from 1 atom to
another.
• All single-replacement & combustion rxns
are redox rxns.
Oxidation
= loss of electrons.
LOSS of ELECTRONS = OXIDATION
LEO
Reduction
= gain of electrons.
GAIN of ELECTRONS = REDUCTION
GER
REDOX
• Oxidation & Reduction are
complementary.
• They occur together & simultaneously or
not at all.
LEO GOES GER!!!
Oxidation of Cu
Oxidation Numbers
• In Ionic Compounds: the number of
electrons lost or gained by an atom when
it forms ions.
Oxidation states of Vanadium
Assigning Oxidation Numbers
8 Rules for Oxidation Numbers
1. of a free, uncombined element = 0.
Na He
O2
N2
S8
Cl2
2. of a monatomic ion = charge on
ion.
+2
-1
+3
Ca
= +2. Cl = -1. Al
P
= +3.
Remember: Ions occur in ionic
compounds: CaCl2, Al(NO3)3, etc.
8 Rules for Oxidation Numbers
3.
Fluorine is always -1.
CF4
4. Hydrogen is nearly always +1,
except when it’s bonded to a
metal. Then it’s -1.
H2O, HNO3, H2SO4
LiH CaH2 NaH
8 Rules for Oxidation Numbers
5.
Oxygen is nearly always -2
except when its
-Bonded to fluorine, where O is +2 OF2
-In the peroxide ion, where O is -1.
O22-
8 Rules for Oxidation Numbers
6. The sum of oxidation numbers in a
neutral compound is 0.
H2O CO2 NO SO3
7. The sum of oxidation numbers
in a polyatomic ion = charge of
the ion.
Sum in SO42- = -2. Sum in NO3- = -1.
8 Rules for Oxidation Numbers
8. In covalent compounds, the
oxidation number of the more
electronegative atom is the
negative charge it would have if
it was an ion.
*NH3: N = -3, H = +1.
SiCl4: Si = +4, Cl = -1.
Assign Oxidation Nos
•
•
•
•
•
•
•
•
•
KCl
K = +1, Cl = -1
CaBr2 Ca = +2, Br = -1
CO
C = +2, O = -2
C = +4, O = -2
CO2
Al(NO3)3
Al = +3, O = -2, N = +5
Na3PO4
Na = +1, O = -2, P = +5
H2S
H = +1, S = -2
NH4+1
N = -3, H = +1
SO3-2
S = +4, O = -2
Electrons are Negative!
• Why do we use the word “reduced” when
electrons are gained?
Look at how the oxidation number
changes.
For example, if Cl gains an electron it
becomes Cl-1. The oxidation number
decreased from 0 to -1. The oxidation
number was reduced.
Writing Equations
• Even though oxidation & reduction occur
together, we can write separate equations
for each process.
• Called Half-Reactions.
• In order to balance a redox equation, we
have to split the full equaton into halfreactions.
Conservation of Mass
• # of atoms of each type is the same on
both sides of the equation.
• Still holds for half-reactions.
• Do this step first.
Conservation of Charge
• Total charge on LHS must equal total
charge on RHS.
• In the past, we usually had both sides
neutral. (0 = 0.)
• Note: Total charge can be nonzero. Just
has to be equal on the 2 sides.
• If not balanced, add electrons to
whichever side is too positive.
Reduction Half-Reactions
Electrons are gained so
• I2 + 2e-  2Ithey
are
like
a
reactant!
• O2 + 4e-  2O-2
• Half-reactions must demonstrate
conservation of mass & conservation of
charge.
• # of atoms of each element on LHS equals “
“ “
“ “
“
“ RHS.
• Total charge on LHS = Total charge on RHS
Oxidation Half-reactions
• K  K1+ + 1e• Fe2+  Fe3+ + 1e• Cu  Cu2+ + 2e-
Electrons are lost so
they appear on the
product side!
• Total Charge on LHS = Total Charge on RHS
• # atoms LHS = # atoms RHS
Identifying Half-Reactions
• Reduction: electron term is on reactant
side.
• Oxidation: electron term is on product
side.
Vocabulary Interlude
• Oxidizing Agent: Is itself reduced.
Accepts electrons from something else –
aids oxidation for another species.
• Reducing Agent: Is itself oxidized.
• Loses electrons to something else – aids
reduction for another species.
Figuring out what is what!
• Given an unbalanced equation.
• Goal: Balance it.
• Procedure:
– Assign oxidation numbers to everything
– Split into half-reactions
– Balance them separately
– “Match” the electrons
– Add them together
What’s oxidized & what’s reduced?
USE OIL RIG
4
3
2
1
0
-1
-2
-3
-4
2) And if
you’re lucky
you strike oil &
it shoots up
1) You dig down
with an oil rig
Oxidizing & Reducing Agents
• They are both ALWAYS on the reactant
side.
• Identify them by seeing how the oxidation
4
numbers change.
3
0
+2
+2
0
Mg + Cu2+  Mg2+ + Cu
Mg is oxidized, so
Mg is the reducing agent!
2
1
0
-1
-2
-3
-4
Oxidizing & Reducing Agents
• What’s oxidized & what’s reduced:
0
+2, -1
+2, -1
0
Ca + FeCl2  CaCl2 + Fe
• Assign oxidation numbers
• Figure out what increases & what
decreases.
Ca is oxidized; Fe2+ is reduced.
Ca = reducing agent; FeCl2 = oxidizing agnt.
4
3
2
1
0
-1
-2
-3
-4
Oxidizing & Reducing Agents
3, -1
2, -1
2, -1
4, -1
2 FeBr3 + SnBr2  2 FeBr2 + SnBr4
Fe3+ is reduced to Fe2+
Sn2+ is oxidized to Sn4+
FeBr3 is the oxidizing agent.
SnBr2 is the reducing agent.