Unit 7
How can we analyze a
complex chemical system?
Chemistry XXI
The central goal of this unit is to apply and extend
central concepts and ideas discussed in this course
to the analysis of a complex chemical system.
M1. Tracking Electron Transfer
Detect electron transfer among
reacting species in a system.
M2. Detecting Electron Sharing
Analyze electron. sharing among
reacting species in a system.
M3. Analyzing Coupled Processes
Analyze processes occurring
simultaneously in a system.
Unit 7
How can we analyze a
complex chemical system?
Chemistry XXI
Module 2: Detecting Electron Sharing
Central goal:
To analyze electron
sharing between reacting
species in a system.
The Challenge
Transformation
How do I change it?
Chemistry XXI
We live in a complex environment made of
hundreds of different substances in constant
chemical interaction. Some of these
interactions are crucial for the survival of life on
Earth; others threaten several ecosystems.
How can we analyze the
types of chemical
processes in which
these substances are
involved?
The Reactions
As we discussed in the last module, many REDOX
reactions involving O2 in the atmosphere produce
non metal oxides:
C6H12O6(g) + 6O2(g)  6H2O(l) + 6CO2(g)
N2(g) + O2(g)  2NO(g)
Chemistry XXI
2NO(g) + O2(g)  2NO2(g)
2SO2(g) + O2(g)  2SO3(g)
The amounts of these types of compounds
that go into the environment are pretty large.
CO2 Production
CO2 is naturally produced by animals and plants, and
consumed by the latter. Additionally:
1.75%
Fuel + O2

0.19%
20.65%
Chemistry XXI
43.06%
98.25%
Fuel consumption
Others
US ~ 6000 million tons/year
36.09%
Petroleum
Natural Gas
Coal
Renewables
SO2 and NOx Production
SO2
1.77%
3.53%
0.93%
5.54%
NO, NO2
11.58%
8.46%
38.92%
13.91%
21.62%
Chemistry XXI
71.40%
Electricity Generation
Fossil Fuel Combustion
Industrial Processes
Non Road Equipment
On Road Vehicles
Miscellaneous
US ~ 12.9 million tons/year
22.34%
On Road Vehicles
Electricity Generation
Non Road Equipment
Fossil Fuel Combustion
Industrial Processes
US ~ 17.0 million tons/year
Forming Acids
The environmental problem with non metal
oxides is that they react with water to produce
new compounds with acidic properties.
+4 -2
+1 -2
CO2(g) + H2O(l)
H2CO3(aq)
+5 -2
+1 +5 -2
+1 -2
N2O5(g) + H2O(l)
+6 -2
+1 -2
SO3(g) + H2O(l)
Chemistry XXI
+1 +4 -2
Let’s Think
2HNO3(aq)
Carbonic Acid
Nitric Acid
+1 +6 -2
H2SO4(aq)
Sulfuric Acid
Are these processes redox reactions?
These types of processes are not redox reactions.
There is no change in the ONs.
What are they then?
Lewis Acids and Bases
Non metallic oxides
generate acids when
reacting with water:
CO2(g) + H2O(l) H2CO3(aq)
Carbonic Acid
What drives these chemical processes?
Chemistry XXI
The reactions are driven by the attraction between
electron-rich (-) and electron-poor (+) centers.
d+
d-
d+
Lewis Base
Electron-pair Donor
(Nucleophile)
d-
d+ d-
Lewis Acid
Electron-pair Acceptor
(Electrophile)
Acid-Base Reactions
CO2(g) + H2O(l) H2CO3(aq)
These types of processes correspond to a general
class of chemical reactions that involve electron
sharing between electron-pair donors (Lewis bases)
and electron-pair acceptors (Lewis acids).
Chemistry XXI
+
In these processes, the formation of a new bond
triggers charge redistributions that determine the
reaction mechanism and the nature of the products.
Tracking Charge
Given that the oxidation number of each atom
does not change in these types of processes,
chemists have developed other tools to quickly
evaluate and track charge redistribution.
How do we know the value of these charges?
Chemistry XXI
+
The calculation of the “formal charge” of each
atom is particularly useful.
Formal Charge
Chemistry XXI
The formal charge is defined as the
partial charge that an atom in a
molecule would have if all of the
bonding electrons were shared
equally between atoms, regardless
of their electronegativity
(molecule seen as fully covalent).
Consider this distribution of
electrons in the molecule of CO2.
If the bonding electrons are
equally divided between atoms:
C has 4 valence e-
O has 6 valence e-
Formal Charge
To calculate the formal charge (FC) we compare the
number of valence electron each atom has with those
that it would have in its elemental form:
FC = # of valence e- of elemental atom –
# valence e- in fully covalent molecule.
FC(O) = 6 – 6 = 0
Chemistry XXI
FC(C) = 4 – 4 = 0
What would
happen if the
structure
was? Resonance Structure
Notice that SFC =
charge of molecule
FC(O-Left) = 6 – 5 = +1
FC(C) = 4 – 4 = 0
FC(O-Right) = 6 – 7 = -1
Stability
The most stable structures tend to be those in which:
Chemistry XXI
 The formal charge on each atom is zero or as low
as possible (absolute value);
 The most negative formal charges are on the most
electronegative atoms;
 Opposite charges are as close as possible; equal
charges are as far as possible.
0
0
0
More stable structure
(Predominant)
+1
0
-1
Let’s Think
Consider these two substances of central
importance in our atmosphere: O2 and O3.
Analyze the formal charge in the atoms of
O2 and O3. Use your results to analyze the relative
stability of these two allotropes of oxygen.
Chemistry XXI
0
0
0
+1
-1
-1
+1
0
Having a +1 formal charge on an oxygen atom
makes O3 more unstable.
Stability
6-7 = -1
During Lewis Acid-Base
reactions is common that
some atoms acquire formal
charges in intermediates or
transition states, or even in the
final product.
6-5 = +1
Chemistry XXI
6-6 = -0
Redistribution of electrons or
atoms from one region of the
molecule to another helps to
stabilize the final product
(minimize formal charge).
6-6 = 0
Acid Properties
H2CO3 has acidic properties.
H2CO3(aq) + H2O(l)
HCO3-(aq) + H3O+(aq)
Why is it an acid?
H2O H
O
Lewis Base
H
H+ is actually
considered the
Lewis Acid.
Chemistry XXI
6 - 7 = -1
H2CO3
Brønsted-Lowry acids and
bases are a particular type of
Lewis acids and bases.
H3O+
H
HCO3-
O
H
+
H
6 - 5 = +1
Let’s Think
SO2, an important pollutant, reacts with H2O to
product sulfurous acid H2SO3.
a) Draw the Lewis structures for the reactants and
analyze their partial and formal charges.
Chemistry XXI
-1
+1
H 0
0
d+
Acid
d-
O0 0
H
Base
b) Identify the Lewis acid and the Lewis base in
this pair.
Let’s Think
SO2, an important pollutant, reacts with H2O to
product sulfurous acid H2SO3.
c) Propose a mechanism for the formation of H2SO3
and analyze the process using FC ideas.
-1
+1
-1
0
0
0
0
0
0
0
Chemistry XXI
H
d+
d-
H 0
0
O
H
0
0
+1
H
H0
0
H0
Chemistry XXI
Let′s apply!
Assess what you know
Explain
Let′s apply!
SO2 reacts with O2 to produce SO3, one of the key
substances leading to “ACID RAIN” in our planet.
Chemistry XXI
The most predominant resonance structure
for SO3 has an expanded octet
(more than 8 valence e- on the central atom).
How would you explain that using FC ideas?
0
0
0
-1
+2
-1
0
0
Predict
Let′s apply!
Consider the formation of sulfuric acid, H2SO4,
in our atmosphere:
SO3(g) + H2O(l)  H2SO4(aq)
Chemistry XXI
Propose a mechanism for this reaction. Predict
the structure of the relevant species and analyze
their stability in terms of formal charge.
6-5 = +1
H
H
O
O
H
O
S
O
O
H
+
O
O
S
O
O
-
HO
S
OH
6-7 = -1
O
Explain
Let′s apply!
Although SO3 is not as abundant as CO2 in the
atmosphere, it generates a stronger acid when
reacting in water:
CO2  H2CO3
SO3  H2SO4
Carbonic Acid
pKa = 3.6
Sulfuric Acid
pKa ~ -3
Chemistry XXI
How do you explain it?
Hint: What factors affect acid strength?
O
HO
HCO3-
S
HSO4- OH
O
Resonance makes
the conjugate base
HSO4- more stable.
Identify
Let′s apply!
H2SO4 (sulfuric acid) is an strong acid that
dissociates completely when dissolved in water:
H2SO4(aq) + H2O(l)  HSO4-(aq) + H3O+(aq)
O
Chemistry XXI
HO
S
H
O
OH
H+ is the Acid
O
H
Base
Identify the Lewis acid and base in this
reaction. Justify your reasoning.
Let′s apply!
Explain
Chemistry XXI
The dissolution of CO2, SOx, and NOx in water in the
atmosphere makes rains acidic.
Rain pH is constantly monitored across the US.
What do you
observe?
How would
you explain
the
differences in
pH (location,
time)?
Clean Air Act
35000
30000
25000
20000
15000
10000
NOx
5000
Emissions (Million tons)
Emissions (Million tons)
Chemistry XXI
In 1995, Phase one of the Clean Air Act Amendments
of 1990 took effect for 110 electrical utilities.
Most of these power plants were in the Eastern US.
30000
25000
20000
15000
10000
Larger
impact
SO2
5000
0
0
1940 1950 1960 1970 1980 1990 2000
1940 1950 1960 1970 1980 1990 2000
Year
Year
Chemistry XXI
Working in pairs, summarize two
central ideas discussed in these
module.
Detecting Electron Sharing
Summary
Chemistry XXI
Lewis acid-base reactions involve electron sharing
between electron-pair donors (Lewis bases) and
electron-pair acceptors (Lewis acids).
In these processes, the
formation of a new bond
triggers charge
redistributions that can be
analyzed by determining the
formal charge of the atoms
in reactants and products.
6-5 = +1 O
H
O
H
+
S
O
O
-
6-7 = -1
O
HO
S
OH
O
Formal Charge
The formal charge is defined as the partial charge
that an atom in a molecule would have if all of the
bonding electrons were shared equally between
atoms, regardless of their electronegativity
(molecule seen as fully covalent).
Chemistry XXI
The most stable structures tend to be those in which:
 The formal charge on each atom is zero or as low
as possible (absolute value);
 The most negative formal charges are on the most
electronegative atoms;
 Opposite charges are as close as possible; equal
charges are as far as possible.
Chemistry XXI
For next class,
Investigate how to calculate the overall
equilibrium constant for a set of coupled
chemical processes.
In which way different chemical processes can
interact with each other?