Transcript REDOX

REDOX
Oxidation-Reduction Reactions
Oxidation-Reduction Reactions
• electrons transferred 1 atom to another
• examples of redox reactions:
– ALL SR rxns
– ALL combustion rxns
LEO GOES GER!!!
oxidation is the loss of electrons
LOSS
of
ELECTRONS
is
OXIDATION
LEO
Reduction is the gain of electrons
GAIN
of
ELECTRONS
is
REDUCTION
GER
REDOX
• Oxidation & Reduction:
– complementary reactions
– occur together & simultaneously
or not at all
Oxidation Numbers
• In Ionic Compounds:
– # of electrons lost/gained by atom determined
by ions that form compound
Assigning Oxidation Numbers
Rule #1:
ox # of free, uncombined element = 0
Na He
O2
N2
S8
Cl2
P
Rule # 2:
ox # of monatomic ion = charge of ion
Ca+2 = +2
Cl-1 = -1
Al+3 = +3
Remember: IONS form ionic cmpds:
CaCl2, Al(NO3)3, etc.
Rule # 3:
F always -1
CF4
Rule # 4:
H is nearly always +1
● except if bonded to metal then 1
H2O, HNO3, H2SO4, LiH, CaH2, NaH
Rule # 5:
O is nearly always -2 except when:
- bonded to F: O is +2
OF2
-peroxide ion: O is -1
O22-
(when bonded to group
1 & 2 elements only)
Rule # 6
sum ox # in neutral compound = 0
H2O
CO2
NO
SO3
Rule # 7
sum ox # in polyatomic ion =
charge of ion
sum in SO4-2 = -2
sum in NH4+1 = +1
Rule # 8
covalent cmpds:
ox # more electroneg atom is negative
SiCl4: Si = +4, Cl = -1
*NH3: N = -3, H = +1
Assign Oxidation Nos
• KCl
K = +1, Cl = -1
• CaBr2 Ca = +2, Br = -1
• CO
C = +2, O = -2
• CO2
C = +4, O = -2
• 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
4
3
2
1
0
-1
-2
-3
-4
2) if you’re
lucky you strike
oil & it shoots
up
1) you dig down
with an oil rig
Electrons are Negative!
• why use the word “reduced” when
electrons are gained?
look at how the oxidation number changes
ex: Cl2 gains an electron → Cl-1
• oxidation # ↓ from 0 to -1 so the # was reduced
Writing Equations
• even though oxidation & reduction
reactions occur together we write separate
equations for each process
– known as Half-Reactions
Reduction Half-Reactions
• I2 + 2e-  2I-1
• O2 + 4e-  2O-2
electrons are gained so put
on reactant side!
• ALL half-reactions must demonstrate:
– conservation of mass & conservation of charge
– # atoms on left must = # atoms on right
– total charge on left must = total charge on right
Oxidation Half-reactions
• K  K1+ + 1e• Fe2+  Fe3+ + 1e• Cu  Cu2+ + 2e-
electrons are lost so they are
put on product side!
• total charge on left = total charge on right
• # atoms on left = # atoms on right