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Redox, Electrochemistry, and
Radioactivity
--Whew.
Redox
• A reduction is a gain of electrons, an
oxidation is a loss of electrons
• A reduction is always conjoined with an
oxidation (electrons are neither created
nor destroyed, charges must balance)
• Remember “OILRIG”
OILRIG
Oxidation is loss of electrons
Reduction is gain of electrons
Or…
LEO says GER
Loss of electrons is oxidation
Gain of electrons is reduction
For example:
+2
+2
Cu (aq)+Fe(s)Fe (aq)+Cu(s)
(right?)
For example:
+2
+2
Cu (aq)+Fe(s)Fe (aq)+Cu(s)
(right?)
• The iron loses custody of its electrons to
form the +2 ion.
• The iron is oxidized to an iron (II) ion, the
copper ion is reduced to copper atom
For example:
2H2 +O22H2O
(right?)
For example:
2H2 +O22H2O
(right?)
• The hydrogen loses (partial) custody of its
electrons when it makes the polar covalent
bond.
For example:
2H2 +O22H2O
(right?)
• The hydrogen loses custody of its
electrons when it makes the polar covalent
bond.
• The hydrogen is oxidized, the oxygen is
reduced
In general…
• If an atom forms a (-) ion, it has been
reduced—happens to nonmetals
• If an atom forms a (+) ion, it has been
oxidized—happens to metals
• Combining with O2 (or F2 or Cl2) is an
oxidation—the O2 (or F2 or Cl2) is reduced
In general…
• If an atom forms a (-) ion, it has been
reduced—happens to nonmetals
A (-) ion forming an element is an oxidation
• If an atom forms a (+) ion, it has been
oxidized—happens to metals
A (+) ion forming an element is a reduction
• Combining with O2 (or F2 or Cl2) is an
oxidation—the O2 (or F2 or Cl2) is reduced
Mark the oxidation and reduction
2H2 +O22H2O
Mark the oxidation and reduction
reduction
2H2 +O22H2O
oxidation
Mark the oxidation and reduction
reduction
2H2 +O22H2O
Hydrogen
is the
reducing
agent
oxidation
Oxygen
is the
oxidizing
agent
Mark the oxidation and reduction
reduction
2H2 +O22H2O
Hydrogen
is the
reducing
agent
(it gets oxidized)
oxidation
Oxygen
is the
oxidizing
agent
(it gets reduced)
Mark the oxidation and reduction
(include the agents and what
happens to each)
Cu+2(aq)+Fe(s)Fe+2(aq)+Cu(s)
Which gets oxidized?
Which gets reduced?
6Li + N2 2 Li3N
O2 + N2  2 NO
16K + S8  8 K2S
S8 + 16F2  8 SF4
Br2 + 2NaI  I2 + 2 NaBr
2NaCl  2 Na + Cl2
H2 + Cl2  2 HCl
Which gets oxidized?
Which gets reduced?
6Li + N2 2 Li3N
O2 + N2  2 NO
16K + S8  8 K2S
S8 + 16F2  8 SF4
Br2 + 2NaI  I2 + 2 NaBr
2NaCl  2 Na + Cl2
H2 + Cl2  2 HCl
Which gets oxidized?
Which gets reduced?
6Li + N2 2 Li3N
O2 + N2  2 NO
16K + S8  8 K2S
S8 + 16F2  8 SF4
Br2 + 2NaI  I2 + 2 NaBr
2NaCl  2 Na + Cl2
H2 + Cl2  2 HCl
What if it’s not so easy?
2NH3+NaOClN2H4+NaCl+H2O
Oxidation Numbers
Summary of Oxidation State Rules
1) Free element Oxidation State = 0
2) Compound total Oxidation State = 0
3) Ion Oxidation State = its charge
4) Oxygen Oxidation State = -2
(except peroxides =-1 each)
5) Hydrogen Oxidation State = +1
(except hydrides =-1)
What is the oxidation number?
of each atom in sodium hypochlorite
NaOCl
What is the oxidation number?
Sodium
hypochlorite is a
compound, all
together
oxidation
numbers = 0
NaOCl
What is the oxidation number?
The Na+ ion has
+1 charge
+1
NaOCl
.
Oxidation
number = +1
What is the oxidation number?
+1
NaOCl
.
The OCl- ion has -1
charge
Oxidation numbers
add up to -1
What is the oxidation number?
+1 -2 .
NaOCl
The O atom: -2
charge
Oxidation numbers
add up to -1
What is the oxidation number?
+1 -2 +1
NaOCl
.
So the Cl must be
+1!
What is the oxidation number?
NaCl
KMnO4
diamond
CO2
CO
KCN
Na4Fe(CN) 6
Fe2O3
Fe3O4
ClO4-
ClO3ClO2ClOClCl2
P2O5
P4O6
H3PO4
Mg3N2
MgH2
NH3
N 2H 4
NH4+
Au
NO3NO2NO2
NO
N 2O
Na2O2
Now, it’s easy
2NH3+NaOClN2H4+NaCl+H2O
Now, it’s easy
-3 +1
+1 -2 +1
-2 +1
+1
-1
+1 -2
2NH3+NaOClN2H4+NaCl+H2O
Now, it’s easy
reduction
-3 +1
+1 -2 +1
-2 +1
+1
-1
+1 -2
2NH3+NaOClN2H4+NaCl+H2O
oxidation
Does a redox reaction occur?
• Look for an oxidizing agent and a reducing
agent.
• If there is one of each, then ask, “Can this
oxidizing agent oxidize this reducing
agent” Answer by comparing reduction
potentials. (page 688)
• (Don’t memorize a rule, compare the
values to a reaction you know will occur)
Does a redox reaction occur?
If you combine…
• Na+ and Fe+3?
• Cl- and Ag?
• Cu and K+ ?
• Pb+2 and I- ?
• Fe+2 and Mg?
Does a redox reaction occur?
If you combine…
• Na+ and Fe+3?
• Cl- and Ag?
• Cu and K+ ?
• Pb+2 and I- ?
• Fe+2 and Mg?
Can be reduced, but
not oxidized further
Does a redox reaction occur?
If you combine…
• Na+ and Fe+3?—No. There is no reducer.
• Cl- and Ag?
• Cu and K+ ?
• Pb+2 and I- ?
• Fe+2 and Mg?
Does a redox reaction occur?
If you combine…
• Na+ and Fe+3?—No. There is no reducer.
• Cl- and Ag?—No. There is no oxidizer.
• Cu and K+ ?—No. This oxidizer can’t do it
• Pb+2 and I- ?—No, but it will precipitate.
• Fe+2 and Mg?—Yes.
Fe+2 + Mg  Fe and Mg+2
Redox—half reactions
• Balance the atoms
• Add H2O and H+ to balance oxygen and
hydrogen
• Rectify the electrons, add and cancel
• Check that charges are balanced
• (Add OH- if the reaction is specified as in a
basic solution)
Try it.
• S2O3-2 + NO3- 
• H2O2 + Fe+2 
• Cr2O7-2 + I- 
• MnO4- + C2H5OH 
Try it.
• S2O3-2 + NO3-  SO4-2 + NO
• H2O2 + Fe+2  H2O + Fe+3
• Cr2O7-2 + I-  Cr+3 + I2
• MnO4- + C2H5OH  Mn+2 + CO2 + H2O
Try it.
3x(S2O3-2 + 5H2O 2 SO4-2 +10H + + 8 e-)
8x(NO3- +4H + + 3 e-  NO + 2 H2O)
3S2O3-2 + 15H2O 6 SO4-2 +30H + + 24 e-)
8NO3- +32H + + 24 e-  8NO + 16H2O)
3S2O3-2+8NO3-+2H+6SO4-2 +8NO+H2O
Reduction potentials
• -- Measured as compared to the reduction
of 2H+ + 2e-  H2. (0.00V)
• Half reactions that can accomplish this
have (-) reduction potentials
• Half reactions that force the reverse have
(+) reduction potentials
Reduction potentials
• Specifically: Magnesium reduces H+
• 2H+ + Mg  Mg+2 + H2 which implies that
Mg+2 + 2 e-  Mg
has a Eo<0
• While bromine oxidizes hydrogen gas
• H2 + Br2  2H+ +2Br - which implies that;
Br2 + 2 e-  2Br –
has a Eo>0
Electrochemical Cells
• When half reactions are separated, and
the electrons are connected in a circuit.
• A salt bridge is needed to allow charges
to migrate to offset the motion of electrons
• Cathode=reduction
• An electrode carries electrons to or from a
half reaction
Shorthand notation
• The Danielle Cell, using copper and zinc,
Zn|Zn+2||Cu+2|Cu
…makes 1.1 V
Zn|Zn+2||Cu+2|Cu
(or, in general)
product reactant
Anode of
of
cathode
oxidation reduction
If non-metals are used…
Pt|H2|H2O||O2|H2O|Pt
• The (non-reactive) metal electrode is
noted outside the bars
Standard cell potentials
• Eo=Ered-Eox
Be able to:
• Sketch a cell (include salt bridge and
circuit)
• Label anode and cathode
• Write the half reactions, complete reaction
• Calculate Eo, show direction of electron
flow
• Describe the oxidation and reduction—
with mass changes, observations.
• Read and write the shorthand notation
Electrolytic Cells
• Applying an external voltage will allow a
non-spontaneous reaction to occur.
2H2O2H2+O2 is not spontaneous (Right?)
If you apply a voltage to water (with some
electrolyte added to carry a charge), it will
decompose (or electrolyse)
Nuclear Chemistry
• --breaks the rules that one atom cannot be
converted to another.
Chemistry is the dance of the
electrons—nuclear reactions
change the nuclei of atoms
• --charge and mass are still conserved.
Nuclide Notation
• A nuclide is a nucleus or atom of a specific
isotope of an element
39
19
K
• This is potassium-39. It has 19 protons
(atomic number = 19), making it
potassium, and 20 neutrons, making a
mass number of 39
How many p, n, e- in each?
What is the mass number and
atomic number?
3
1
59
26
H
+3
Fe
36
17
Cl
90
38
+2
Sr
131
53
228
90
I
Th
Natural decays
•
a—the loss of a particle from a nuclide
--The a particle is composed of 2p and 2n,
the 4He nucleus
--decreases the mass by 4 and the atomic
number by 2
• b—emission of an electron (b particle) from the
nucleus by the conversion of a n  p + e--the electron is the b particle
--increases the atomic number by 1, does
not affect mass
Write the reaction
•
•
•
•
•
•
Argon-39 undergoes a b decay
Thorium-228 undergoes an a decay
An a decay forms lead-204
A b decay forms nitrogen-14
A natural decay forms Sc-45 from Ca-45
A natural decay forms Ac-227 from Pa-231
Notice what they do
• A b decay lowers the n/p ratio in small
nuclei, or when the ratio is too large.
• An a decay lowers the total size, and
raises the n/p in large nuclides, or when
the ratio is too small.
What is “too large” or “too small”?
What is “too large” or “too small”?
What is “too large” or “too small”?
b decay
a decay
What is “too large” or “too small”?
What is “too large” or “too small”?
n:p = 2:1
n:p = 1:1
This just in.
• Researchers report the first creation of the
long-lived nucleus hassium-270, a "doubly
magic" combination of 108 protons and
162 neutrons. Its long lifetime of 22
seconds supports the theory of an "island
of stability" for the heaviest elements. (J
Dvorak et al. 2006 Phys. Rev. Lett. 97,
242501)
Nuclear reactions
• Many nuclear reactions involve colliding
nuclei or smaller particles at some
significant fraction of the speed of light,
• --find the missing particle by balancing
mass and charge.
Particles might include…
•
•
•
•
•
•
p
n
e- (AKA b)
d
a
g (OK, it’s not a particle, but it’s often
written in)
In comparison
• Physical changes:
joules/mole range
• Chemical changes:
kilojoules/mole range
• Nuclear changes:
megajoules/mole range
Fission vs Fusion
• Fission=breaking up large nuclei—
--natural radioactive decay of large atoms
--used for nuclear power
• Fusion=combining small nuclei
--occurs naturally in stars
--prospects for nuclear energy—no
radioactive byproducts
Both are transmutations—one nuclide is
converted into another
Consider the relationships
•
•
•
•
Half life
Original amount
Final amount
Time elapsed
Consider the relationships
t/t1/2
A=A0(1/2)
•
•
•
•
A is the amount of the sample remaining
A0 is the original amount in the sample
t is the time that has passed, and
t 1/2 is the half-life of the nuclide
Please notice
t/t1/2
A=A0(1/2)
• A / A0 is the fraction remaining
• t / t 1/2 is the number of half-lifes that have
passed
Try it.
• Hydrogen-3 has a half life of 12.3 years.
If you start with a 20 g sample of H-3
--how much is left after 12.3 years?
--how much is left after 24.6 years?
--how much is left after 30.2 years?
Try it.
• Br-82 has a half life of 35.3 hours. If
you start with a 6.5 mg sample of Br-82
--how much is left after 4 days?
--how long will it take to reach .75 mg?
Try it.
• Br-82 has a half life of 35.3 hours. If
you start with a 6.5 mg sample of Br-82
--how much is left after 4 days?
--how long will it take to reach .75 mg?
How do you solve
for an exponent?
Use a log function
t/t1/2
log (A/A0)= log(1/2)
log (A/A0)=t/t1/2log(1/2)
t/t1/2 = log (A/A0)
log(1/2)
Try it.
• Br-82 has a half life of 35.3 hours. If
you start with a 6.5 mg sample of Br-82
--how much is left after 4 days?
--how long will it take to reach .75 mg?
Try it.
• If you start with 1.38 mg of U-234 and
t1/2=2.44 x 105 yrs for its decay:
--how much is left after 20,000 years?
--how long will it take to reach 0.40 mg?
Try it.
• A .350 mg sample of K-42 decays to
only .066 mg in 29.7 hours.
--what is the half life?
--how much was left after 20.0 hours?
--how long will it take to reach .010 mg?
Th-th-that’s all, folks.
Atomic theory
• All matter is composed of atoms.
--atoms of one element are identical
--atoms of different elements are
different
--reactions form different combinations
of atoms, not different atoms
• Atoms are composed of protons,
neutrons, and electrons.
(Are all of the little kids in bed?—Now we
can tell you the real story…)
Modern Theory
In this Universe,
you will find:
Fermions
and
Bosons
(force
mediating
particles)
Modern Theory
Bosons (force mediating particles)
include:
g, (electromagnetic force)
Zo, (weak nuclear force)
Gluons (Strong nuclear force)
Graviton (gravity)
W±, (weak nuclear force)
Higgs (mediates mass)
Modern Theory
Fermions are the
fundamental
particles, including:
Quarks
and
Leptons
Modern Theory
up
down
Types of
quarks
include:
top
(formerly
known as
truth)
charm
bottom
(formerly
known as
beauty)
strange
(and their antiparticles)
Modern Theory
Leptons include:
Electrons, e-,
Muons, m,
Tauons, t,
and three types
of neutrinos
e, m, t
(and their
antiparticles)
Modern Theory
Combinations
of quarks make
hadrons, either:
Mesons
(2 quarks each)
Including:
p+, K-, r+, B0, c
Baryons
(3 quarks each)
including
p, n, p-, L, -
Modern Theory
In this Universe, you will find
fermions which
include the
fundamental
particles
Quarks
u,d,c,s,t,b
(and antiparticles)
Quarks make
up the
hadrons,
either
and
Bosons (force mediating particles)
g, (electromagnetic force
Zo, (weak nuclear force)
Gluons (Str. nuclear force)
Graviton (gravity)
W±,(weak nuclear force)
Higgs (mediates mass)
Leptons e-,m,t,e, m,
t (and antiparticles)
Mesons (2
quarks each)
p+,K-,r,+B0,c
baryons (3 quarks each)
including p,n,p-,L,-