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Oxidation - Reduction
RedOx
Oxidation States
. . . of an element are determined from the number of electrons that are ______ other atoms Gained from Lost toAND Shared with
Rules of assigning oxidation states
Atoms have a negative oxidation state if they have the higher e-neg in the bond NH3
N=-3 H=+1 Oxidation states on
all the atoms of a molecule & compoundmust add upto equal zero
Uncombined elementsfree state
Elements that are not bonded to a different element have oxidation states of zero.
Al(s) Cu(s) Ag(s) Cl2 O2 O3 Ne
H2S sulfur has a -2 oxidation state
H2SO3
sulfur has a +4 oxidation state
H2SO4
sulfur has a +6 oxidation state
NOW More complicated . . .
Assigning oxidation states to atoms in a polyatomic ion
Oxidation states on individual atoms must equal the charge on the ion
(SO4)(S+6 O4-2
)
-8+6 = -2
(S O4) = -2
(S O4-2
)
-8 = -2
Try these . . .
hydroxide
dichromate
ammonium
Assign oxidation states to each atom
NaOH
MnCrO4
(NH4)2SO4
K2O2
Balancing RedOx reactionsusing oxidation states
Zn + HNO3 Zn(NO3)2 + NO2 + H2O
Go through the steps on paper…
1. Assign oxidation states to all atoms determine the
number of e- lost and gained
+4
+3
+2
+1
0
-1
-2
-3
-4L
osin
g e- o
xida
tion
Gain e
- Reduction
2. Select product coefficients that balance the e- HINT: switch #’s balance major elements with reactants
3. Balance leftovers with inventory
*coefficients must be lowest possible whole #s
Balance - use oxidation states
Cu + HNO3 Cu(NO3)2 + H2O + NO
3Cu + 8HNO3 3Cu(NO3)2 + 4H2O + 2NO
K2Cr2O7 + H2O + S SO2 + KOH + Cr2O3
2K2Cr2O7 + 2H2O + 3S 3SO2 + 4KOH + 2Cr2O3
Balancing net ionic rxns
Mg + Al3+ Mg2+ + Al
Al3+ and Mg+2 are ionsif they give you the charge use it! Its part of a net ionic rxn
they have removed the spectator ions.
Balance for mass and charge
Balance . . . for mass and charge
Cu2+ + K K+ + Cu
RedOx reactions
Chemical reactions involving an
electron transfer between
reactants
Recognizing RedOx
assign oxidation states to the individual elements in reactants and the products
if the oxidation state changes for some of the particles it is considered a RedOx reaction
Single Replacement Reactions are ALWAYS RedOXDouble Replacement Reactions are NEVER RedOX
In a RedOx reaction
If electrons are lost by one species in a reaction they
all MUST be gained by another!
If one atom is being oxidized another must be reduced.
In other words . . . oxidation and reduction always occur
together
Oxidation
Oxidation results in an increase in oxidation state
Atoms that Lose e-lectrons undergo
Oxidation
Metals tend to undergo oxidation
+3 +2 +1 0 -1 -2 -3
ReductionReduction results in a decrease
in oxidation state
Atoms which Gain e-lectrons
undergo Reduction
Non-metals tend to undergo reduction
+3 +2 +1 0 -1 -2 -3
GeR
RedOx: Yes or NO?
HCl + NaOH HOH + NaCl
Mg + 2HCl MgCl2 + H2
MnO2 + 4HBr MnBr2 + Br2 + 2H2O
Al + CuCl2 ? Write a balanced equation based
on these reactants. Is this a RedOx reaction?What species is oxidized?
Reduced?
Predicting Products of Single Replacement Reactions
Roaring Animals Often Attack
Elements that undergo oxidation cause reduction Reducing agent
Elements that undergo reduction cause oxidation Oxidizing agent
LEO GER
RA OA
2Al + 3CuCl2 2AlCl3 + 3Cu
What is the Oxidizing Agent? Cu+2 because it gets reduced
What is the Reducing Agent? Al because it gets oxidized
These answers always come from the reactant
side
MnO2 + 4HCl MnCl2 + Cl2 + 2H2O
Assign oxidation states to all atomsWhat species is oxidized?
How do you know?What species is reduced?
How do you know?
What species is the oxidizing agent? Mn+4 because it gets reduced
What species is the reducing agent? Cl- because it gets oxidized
RedOx Practice
Remember . . .+4
+3
+2
+1
0
-1
-2
-3
-4L
osin
g e- o
xida
tion
Gain e
- Reduction
Writing half-reactions
A half reaction shows either oxidation or reduction of a RedOx reaction.
The electrons being lost (oxidation) or gained (reduction) are also shown.
Oxidation half-reaction
Fe(s) Fe+3(aq) + 3e-
oxidation number increaseselectrons are lost
productsconservation of mass AND CHARGE
charges on both sides equal each other
Reduction half-reaction
Sn+4 + 2e- Sn+2
oxidation number decreaseselectrons are gained
reactantsconservation of mass AND CHARGE
charges on both sides equal each other
Balancing RedOx using 1/2 reactions
1. ASSIGN OXIDATION STATES . . .2. Write the oxidation 1/2 reaction3. Write the reduction 1/2 reaction4. Balance the two half reactions so that
the number of electrons transferred is equal
5. Use these coefficients to balance the RedOx atoms
6. Balance leftover atoms by inventory
. . . use 1/2 reactions
KMnO4 + HCl MnCl2 + KCl + Cl2 + H2O
ZnS + O2 SO2 + ZnO
The End
I came . . . I saw . . . I RedOxed