Download 19-105 W-01 WEEK 6 Lectures 16-18 Begin ELECTROCHEMISTRY

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19-105
W-01
WEEK 6
Lectures 16-18
Begin ELECTROCHEMISTRY
J&A Chapter 18, 18.1, 18.2
Monday - Lecture 16; Wednesday - Lecture 17
18.1 Half reactions Oxidation States
18.2 Balancing Oxidation-Reduction Equations
Friday: Lecture 18
Review class for those who need it on balancing
REDOX equations.
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OXIDATION
Loss of electrons by an atom,
molecule, ion, etc.
REDUCTION
Gain of electrons by an atom,
molecule, ion, etc.
e.g. 2Na(s) + CR2(g) 6 2NaCR(s)
Here the Na metal is oxidized to Na+
(1 electron lost per sodium atom)
and CR2 is reduced to 2CR
(1 electron gained per chlorine atom)
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Oxidation-Reduction or REDOX reactions
Reactions which involve BOTH GAIN (reduction)
and LOSS (oxidation) of electrons
Convenient to think of the overall REDOX reaction as
two individual HALF-reactions, namely an Oxidation
Half-reaction and a Reduction Half-reaction
2Na(s) 6 2Na+ + 2e- (oxidation 1/2-reaction)
-
-
CR2(g) + 2e 6 2CR (reduction 1/2-reaction)
2Na(s) + CR2(g) 6 2NaCR(s)
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Oxidizing AGENT 6
Causes another species to be
oxidized by accepting/taking
electrons from it.
Reducing AGENT 6
Causes another species to be
reduced by donating
electrons to it.
LEORA 6
Loss of Electrons is Oxidation and that
GEROA 6
species is a Reducing Agent.
Gain of Electrons is Reduction and that
species is an Oxidizing Agent.
Oxidation results in an increase in the Oxidation State
(or Number) by loss of electrons.
Reduction results in an decrease in the Oxidation State
(or Number) by gain of electrons.
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Rules for assigning Oxidation Numbers
(apply in this sequence)
1. Pure element, oxidation no. = 0
2. Monoatomic ion, oxidation no. = charge on ion
3. Fluorine in its compounds is always -1
4. Oxygen in its covalent compounds is -2, except in
peroxides where it is -1
5. Hydrogen in covalent compounds with non-metals
is +1, in metal hydrides it is -1
6. The sum of the oxidation numbers = 0 for a
neutral (no charge) compound; for an ion, the sum
of the oxidation numbers = ion charge
7. Look at the column number in the periodic table
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Examples: Assign Oxidation Numbers (or States) to
all the atoms in the following:CO2
PbS
NH3
CH4
SF6
Cr2O72MnO4NO3C2H6O
H3PO4
H2PO4F2O
ARO2CROCRO2-
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Absolutely FOOL-PROOF method for balancing
ANY redox equation
1. Assign Oxidation Numbers and write separate
equations for each half reaction, then work on each
half-reaction separately.
2. Balance atoms being oxidized or reduced.
3. From change in oxidation numbers, get the
number of electrons involved and add them to the
high oxidation number side of the equation.
4. Now balance the charge, using either H+ (if in
ACID solution) or OH (if in base solution).
5. Now balance the OXYGEN by adding H2O to the
appropriate side of the equation.
6. Check that everything balances - it will!
7. Scale the two half reactions, to equalize the
number of electrons in each.
8. Add the two half-reactions.
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Example: (all aqueous)
MnO4 + Fe2+ 6 Fe3+ + Mn2+ in acid solution.
Oxidation numbers
Mn in MnO4 is +7, in Mn2+ it is +2
Fe2+ is +2, Fe3+ is +3
Oxidation Step: Fe2+ 6 Fe3+ + 1e (steps 1,2,3)
Reduction step: MnO4 6 Mn2+ (steps 1 and 2)
5e + MnO4 6 Mn2+ (step 3)
+
8H + 5e + MnO4 6 Mn2+ (step 4)
+
8H + 5e + MnO4 6 Mn2+ + 4H2O (step 5)
Visual check
- all OK
(step 6)
5Fe2+ 6 5Fe3+ + 5e (step 7)
+ 5e + MnO4 6 Mn2+ + 4H2O (step 7)
8H+
Add
8H+ + MnO4 + 5Fe2+ 6
Mn2+ + 4H2O + 5Fe3+ (step 8)
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-
-
In base, NO2 (aq) + AR(s) 6 NH3(g) + ARO2 (aq)
-
Oxid Nos. N in NO2 is +3, in NH3 is -3
-
AR in AR(s) is 0, in ARO2 (aq) is +3
Oxidation: AR 6 ARO2 + 3e (steps 1,2,3)
4(OH ) + AR 6 ARO2 + 3e (steps 4)
4(OH ) + AR 6 ARO2 + 3e + 2(H2O) (steps 5,6)
Reduction: NO2 6 NH3 (steps 1 and 2)
6e + NO2 6 NH3 (step 3)
6e + NO2 6 NH3 + 7(OH ) (step 4)
5(H2O) + 6e + NO2 6 NH3 + 7(OH ) (steps 5,6)
Scale, add and tidy up (cancel superfluous water)
8(OH ) + 2AR(s) 6 2ARO2 + 6e + 4(H2O)
5(H2O) + 6e + NO2 6 NH3 + 7(OH )
5(H2O) + NO2 + 8(OH ) + 2AR 6
NH3 + 7(OH ) + 2ARO2 + 4(H2O)
H2O + NO2 + 8(OH ) + 2AR 6
NH3 + 7(OH ) + 2ARO2
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Give Balanced Half Reactions for the following in
aqueous solution:1. H+ + Cr2O72- + C2H6O 6 Cr3+ + CO2
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2. As2O3(s) + NO3-(aq) 6 H3AsO4(aq) + NO(g)
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3. MnO4- + S2- 6 MnS(s) + S(s) (in base)
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4. CR2(g) 6 CR- + CRO- (in base)
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5. HO2- + Cr(OH)3- 6 CrO42- (in base)
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6. Ag2S(s) + CN-(aq) + O2(g) 6
S(s) + [Ag(CN)2]-(aq)
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7. More devious fiendishly-cunning examples on
redox balancing will be done in class....