Chemical Bonding and Molecular Structure (Chapter 9)

1
Chemical Bonding and Molecular Structure
(Chapter 9)

• Ionic vs. covalent bonding
• Molecular orbitals and the covalent bond (Ch.
  10)
• Valence electron Lewis dot structures
• octet vs. non-octet
• resonance structures
• formal charges
• VSEPR - predicting shapes of molecules
• Bond properties
• bond order, bond strength
• polarity, electronegativity

2
Bond Polarity

• HCl is POLAR because it has a positive end and a
  negative end (partly ionic).
• Polarity arises because Cl has a greater share of
  the bonding electrons than H.

Calculated charge by CAChe H (red) is ve
(0.20 e-) Cl (yellow) is -ve (-0.20 e-).
(See PARTCHRG folder in MODELS.)
3
Bond Polarity (2)

• Due to the bond polarity, the HCl bond energy is
  GREATER than expected for a pure covalent bond.

BOND ENERGY pure bond 339 kJ/mol
calculated real bond 432 kJ/mol measured
Difference 92 kJ/mol. This difference is the
contribution of IONIC bonding It is proportional
to the difference in
ELECTRONEGATIVITY, c.
4
Electronegativity, c

• c is a measure of the ability of an atom in a
  molecule to attract electrons to itself.

Concept proposed by Linus Pauling (1901-94) Nobel
prizes Chemistry (54), Peace (63) See p. 425
008vd3.mov (CD)
5
Electronegativity, c
Figure 9.7

• F has maximum c.
• Atom with lowest c is the center atom in most
  molecules.
• Relative values of c determines BOND POLARITY
  (and point of attack on a molecule).

6
Bond Polarity
Which bond is more polar ? (has larger bond
DIPOLE) OH OF
c H 2.1 O F 3.5 4.0

• c(A) - c(B) 3.5 - 2.1
• Dc 1.4

3.5 - 4.0 0.5
??(O-H) gt ??(O-F) Therefore OH is more polar
than OF
Also note that polarity is reversed.
7
Molecular Polarity

• Molecules such as HCl and H2O are POLAR
• They have a DIPOLE MOMENT.
• Polar molecules turn to align their dipole with
  an electric field.

• A molecule will be polar
• ONLY if
• a) it contains polar bonds AND
• b) the molecule is NOT symmetric

8
Molecular Polarity H2O
Water is polar because a) O-H bond is polar b)
water is non-symmetric
The dipole associated with polar H2O is the
basis for absorption of microwaves used in
cooking with a microwave oven
9
Molecular Polarity in NON-symmetric molecules
BF bonds are polar molecule is NOT
symmetric

• BF bonds are polar
• molecule is symmetric

Atom Chg. ? B ve 2.0 H ve
2.1 F -ve 4.0
B ve F -ve
10
Fluorine-substituted Ethylene C2H2F2
CF bonds are MUCH more polar than CH bonds.
??(C-F) 1.5, ??(C-H) 0.4

• CIS isomer
• both CF bonds on same side
• ? molecule is POLAR.

• TRANS isomer
• both CF bonds on opposite side
• ? molecule is NOT POLAR.

11
CHEMICAL EQUILIBRIUMChapter 16

• equilibrium vs. completed reactions
• equilibrium constant expressions
• Reaction quotient
• computing positions of equilibria examples
• Le Chateliers principle - effect on equilibria
  of
• addition of reactant or product
• pressure
• temperature

YOU ARE NOT RESPONSIBLE for section 16.7
(relation to kinetics)
12
Properties of an Equilibrium

• Equilibrium systems are
• DYNAMIC (in constant motion)
• REVERSIBLE
• can be approached from either direction

16_CoCl2.mov (16z01vd1.mov)
Pink to blue Co(H2O)6Cl2 ---gt Co(H2O)4Cl2 2
H2O
Blue to pink Co(H2O)4Cl2 2 H2O ---gt Co(H2O)6Cl2
13
Chemical Equilibrium

• After a period of time, the concentrations of
  reactants and products are constant.
• The forward and reverse reactions continue after
  equilibrium is attained.

16_FeSCN.mov 16m03an1.mov
14
Chemical Equilibria

• CaCO3(s) H2O(l) CO2(g)

Ca2(aq) 2 HCO3-(aq)
At a given T and pressure of CO2, Ca2 and
HCO3- can be found from the EQUILIBRIUM
CONSTANT.
15
THE EQUILIBRIUM CONSTANT

• For any type of chemical equilibrium of the type

the following is a CONSTANT (at a given T)
If K is known, then we can predict
concentrations of products or reactants.
16
Determining K

• Place 2.00 mol of NOCl is a 1.00 L flask. At
  equilibrium you find 0.66 mol/L of NO. Calculate
  K.
• Solution
• 1. Set up a table of concentrations
• NOCl NO Cl2

Before 2.00 0 0 Change -0.66 0.66 0.33 Equ
ilibrium 1.34 0.66 0.33
17
Calculate K from equil.

• 2 NOCl(g) 2 NO(g) Cl2(g)
• NOCl NO Cl2
• Before 2.00 0 0
• Change -0.66 0.66 0.33
• Equilibrium 1.34 0.66 0.33

18
Writing and ManipulatingEquilibrium Expressions

• Solids and liquids NEVER appear in equilibrium
  expressions.

19
Manipulating K adding reactions

• Adding equations for reactions

K1 SO2 / O2
NET EQUATION
Ktot
Ktot K1 x K2
ADD REACTIONS ? MULTIPLY K
20
Manipulating K Reverse reactions

• Changing direction

21
Chemistry of Sulfur
Elemental S stable form is S8 (s)
sources desulfurizing natural gas roasting
metal sulfides
Oxides of S SO2 (g) and SO3 (g) - significant
in atmospheric pollution Industrially Oxides
generated as needed stored as the hydrate
SO3 (g) H2O (l) ? H2SO4 (aq)
Sulfuric acid is HIGHEST VOLUME chemical
(fertilizers, refining, manufacturing)
22
Manipulating K Kp for gas rxns

• Concentration Units
• We have been writing K in terms of mol/L.
• These are designated by Kc
• But with gases, P (n/V)RT conc RT
• P is proportional to concentration, so we can
  write K in terms of PARTIAL PRESSURES.
• These constants are called Kp.
• Kc and Kp have DIFFERENT VALUES
• (unless same number of species on both sides of
  equation)

23
The Meaning of K

• 1. Can tell if a reaction is
• product-favored or reactant-favored.

1.5 x 1080
K gtgt 1

Concentration of products is much greater than
that of reactants at equilibrium.
The reaction is strongly product-favored.
24
Meaning of K AgCl rxn
Kc
K ltlt 1
Ag Cl-
1.8 x 10-5
Conc. of products is much less than that of
reactants at equilibrium.
This reaction is strongly reactant-favored.
What about the reverse reaction ?
Krev Kc-1 5.6x104. It is strongly
product-favored.
25
Meaning of K butane isomerization

• 2. Can tell if a reaction is at equilibrium.
• If not, which way it moves to approach
  equilibrium.

If iso 0.35 M and n 0.25 M, is the system
at equilibrium? If not, which way does the rxn
shift to approach equilibrium?
26
Q - the reaction quotient

• All reacting chemical systems can be
  characterized by their REACTION QUOTIENT, Q.

Q has the same form as K, . . . but uses
existing concentrations
If Q K, then system is at equilibrium.




Q 1.4 which is LESS THAN K 2.5
Reaction is NOT at equilibrium.
To reach EQUILIBRIUM
Iso must INCREASE and n must DECREASE.
27
Typical EQUILIBRIUM Calculations
2 general types a. Given set of
concentrations, is system at
equilibrium ?
Calculate Q compare to K
IF Q gt K or Q/K gt 1 ?
REACTANTS Q lt K or Q/K lt 1 ?
PRODUCTS
QK at EQUILIBRIUM
28
Examples of equilibrium questions
b. From an initial non-equilibrium condition,
what are the concentrations at equilibrium?
Place 1.00 mol each of H2 and I2 in a 1.00 L
flask. Calculate equilibrium concentrations.
29
H2(g) I2(g) 2 HI(g) Kc 55.3
Step 1. Set up table to define EQUILIBRIUM
concentrations in terms of initial concentrations
and a change variable
H2 I2 HI

• Initial 1.00 1.00 0
• DEFINE x H2 consumed to get to equilibrium.
• Change -x -x 2x
• At equilibrium 1.00-x 1.00-x 2x

30
H2(g) I2(g) 2 HI(g) Kc 55.3
Step 1 Define equilibrium condition in terms of
initial condition and a change variable H2 I
2 HI At equilibrium 1.00-x 1.00-x 2x

• Step 2
• Put equilibrium concentrations into Kc expression.

31
H2(g) I2(g) 2 HI(g) Kc 55.3

• Step 3. Solve for x. 55.3 (2x)2/(1-x)2
• In this case, take square root of both sides.

Solution gives x 0.79 Therefore, at
equilibrium
H2 I2 1.00 - x 0.21 M HI 2x
1.58 M
32
EQUILIBRIUM AND EXTERNAL EFFECTS

• The position of equilibrium is changed when there
  is a change in
• pressure
• changes in concentration
• temperature
• The outcome is governed by
• LE CHATELIERS PRINCIPLE

Henri Le Chatelier 1850-1936 - Studied mining
engineering - specialized in glass and ceramics.
...if a system at equilibrium is disturbed, the
system tends to shift its equilibrium position to
counter the effect of the disturbance.
33
Shifts in EQUILIBRIUM Concentration

• If concentration of one species changes,
• concentrations of other species CHANGES
• to keep the value of K the same (at constant T)
• no change in K - only position of equilibrium
  changes.

ADDING PRODUCTS - equilibrium shifts to REACTANTS
ADDING REACTANTS - equilibrium shifts to PRODUCTS
- GAS-FORMING PRECIPITATION
REMOVING PRODUCTS - often used to DRIVE
REACTION TO COMPLETION
34
Effect of changed on an equilibrium
INITIALLY n 0.50 M iso 1.25
M CHANGE ADD 1.50 M n-butane What happens ?

• Solution
• A. Calculate Q with extra 1.50 M n-butane.

16_butane.mov (16m13an1.mov)
Q iso / n 1.25 / (0.50 1.50) 0.63
Q lt K . Therefore, reaction shifts to PRODUCT
35
Butane/Isobutane

• Solution

B. Solve for NEW EQUILIBRIUM - set up
concentration table n-butane isobutane Ini
tial 0.50 1.50 1.25 Change - x
x Equilibrium 2.00 - x 1.25 x
x 1.07 M. At new equilibrium position,
n-butane 0.93 M isobutane 2.32 M.
Equilibrium has shifted toward isobutane.
36
Effect of Pressure (gas equilibrium)

• Increase P in the system by reducing the volume.

Increasing P shifts equilibrium to side with
fewer molecules (to try to reduce P). Here,
reaction shifts LEFT PN2O4 increases
16_NO2.mov (16m14an1.mov)
PNO2 decreases
See Ass2 - question 6
37
EQUILIBRIUM AND EXTERNAL EFFECTS

• Temperature change ? change in K
• Consider the fizz in a soft drink

Kc CO2(aq)/CO2(g)

• Change T New equilib. position? New value of
  K?

• Increase T
• Equilibrium shifts left CO2(g) ? CO2
  (aq) ?
• K decreases as T goes up.
• Decrease T
• CO2 (aq) increases and CO2(g) decreases.
• K increases as T goes down

38
Temperature Effects on Chemical Equilibrium

• Kc 0.00077 at 273 K
• Kc 0.00590 at 298 K

?Horxn 57.2 kJ
Increasing T changes K so as to shift equilibrium
in ENDOTHERMIC direction
16_NO2RX.mov (16m14an1.mov)
39
EQUILIBRIUM AND EXTERNAL EFFECTS
Catalytic exhaust system

• Add catalyst ---gt no change in K
• A catalyst only affects the RATE of approach to
  equilibrium.

40
CHEMICAL EQUILIBRIUMChapter 16

• equilibrium vs. completed reactions
• equilibrium constant expressions
• Reaction quotient
• computing positions of equilibria examples
• Le Chateliers principle - effect on equilibria
  of
• addition of reactant or product
• pressure
• temperature

YOU ARE NOT RESPONSIBLE for section 16.7
(relation to kinetics)