Chemical bonding and strucure 2
Objectives of this lesson:
 Recall what you have learned in the previous lesson
specifically and the current topic in general;
 Bring new ideas based on what you have learned
so far;
 Understand the forces holding particles (atoms,
molecules etc.,);
 How these forces affect the physical properties
such as boiling point and melting point of a
substance;
 To understand the energy requirement to break
holding forces (bonds) is not the same;
 To learn and compare electrostatic attraction
between particles.
What you think about the following
statements?
 When we boil water, energy is supplied and the
energy is used to break the bonds between atoms
and hence water boils.
 The attractive force between solid particles is much
higher than liquid particles and the particles in
gaseous state do not attract.
 Ice floats in water due to the air trapped between
water molecules in ice structure.
 Water and ethanol; both are liquids and they both
boil at the same temperature.
 Molecules have polarity when they are formed and
cannot be induced.
Some questions to answer by the
end of the lesson.
When we boil water, do the bonds between
hydrogen and oxygen break?
Do inter molecular attraction exists among non
polar substances?
Why all liquids do not have equal boiling point?
Ice floats in water. Why?
Water,
Ethanol
and Ethanoic acid
which one has highest boiling point? Why?
Forces holding particles
 Atoms are attracted within a molecule by
intramolecular attractive forces; ionic, covalent.
 Between molecules there is attractive force which is
responsible for different states of matter.
 All molecules have intermolecular attractive forces
and hence all substances (solids, liquids and
gases)have boiling and melting points. Including
inert gases.
 The inter molecular attractive forces include:
 Van der Waals force (London dispersion force)
(Induced dipole)
 Dipole-dipole attraction (permanent or induced)
 Hydrogen boning
Forces holding particles…..
 Dipole attraction can be between polar molecules-dipoledipole or
 polar and non-polar molecules-induced dipole-dipole
attraction
 Van der Waals force of attraction is a kind of induced
temporary dipole attraction.
 For Hydrogen bonding, Hydrogen atom should be joined to a
highly electronegative atom such as Oxygen, Nitrogen or
Fluorine. The lone pair of electrons in oxygen, Nitrogen or
Fluorine are used to attract Hydrogen.
 The strength of inter-molecular attractive forces is represented
below:
 Van der Waals force<Dipole-dipole<Hydrogen bonding
London Dispersion Forces
(Van der Waals Force)
 http://my.hrw.com/sh/hw5ny/0030363292/student/ch1
1/sec03/qc10/hw5ny11_03_q10fs.htm
Dipole to Dipole
Forces
 http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIP
OLED.html
Hydrogen Bonding in Water
 http://www.northland.cc.mn.us/biology/Biology1111/
animations/hydrogenbonds.html
Hydrogen bonding in water
Hydrogen bonding in HF and CO
Hydrogen bonding in ammonia
BP 19.0 C
BP -33.0C
BP 100.0C
Lone pair of electrons in
oxygen, fluorine or
nitrogen is used to attract
hydrogen
H2O; HF and NH3 which one has highest bp? Explain
BP 78.1C
BP 118.0C
BP 100.0C
Which one has higher bp? Explain
ICE and hydrogen bonding
 When temperature goes below 4 degree C in water,
there is a strong tendency to form a network of hydrogen
bonds, where each hydrogen atom is in a line between
two oxygen atoms.
 This hydrogen bonding tendency gets stronger as the
temperature gets lower (because there is less thermal
energy to shake the hydrogen bonds out of position).
 The ice structure is completely
hydrogen bonded, and these
bonds force the crystalline
structure to be very "open“
 This makes the molecules kept
apart with strong hydrogen
bonds. So the volume
increases and density
decreases.
 In liquid water, the hydrogen
bonds are more flexible and
molecules can come closer at
times give relatively less
volume and higher density.
 So ice floats in water.
How aquatic animals survive when the
temperature is very low?
Ice is an insulator to heat.
Ice prevents heat loss from water below.
This helps aquatic animals to stay alive in winter
when the temperature is too low.
Question time
 1. Helium is an inert gas. Why it has melting and boiling
points?
 2. Which intermolecular force is present in all substances?
 3. Propane C3H8 and Hexane C6H14 both are alkanes.
Which one has a higher bp? Explain.
 4. Hydrogen bonding is the strongest among
intermolecular forces. On boiling water, why the bond
between oxygen and hydrogen does not break instead
of the hydrogen bonds?
 5. On cooling, substances will have higher density. But
when we change water to ice, density decreases.
Explain.
 6. Which compound contains hydrogen bonding?
COCl2; PH3; H2CO; CH3OH
Question time cont….
 7. Which of the following has the strongest attraction
between molecules?
SiH4; CH2=O; CH3-CH3; O2
 8. Write reasons for:
 (a) Sulphur and Oxygen both are in group 6. Why
water is a liquid while SO2 is a gas at room
temperature?
 (b) Ethanol (CH3-CH2-OH) has a higher boiling point
than dimethyl ether(CH3-O-CH3)
 (c) The boiling point of sulphur dioxide is 24 C higher
than that of Chlorine gas.
Answers…………..
 1. Helium is an inert gas. Why it has melting and
boiling points?
 The force of attraction among helium atoms is Van
der Waals force (London dispersion).
 2. Which intermolecular force is present in all
substances?
 Van der Waals force of attraction.
 3. Propane C3H8 and Hexane C6H14 both are
alkanes. Which one has a higher bp? Explain.
 Hexane has higher boiling point. It has larger
molecular mass and hence bigger number of
electrons to form larger Van der Waals force of
attraction.
 4. Hydrogen bonding is the strongest among intermolecular
forces. On boiling water, why the bond between oxygen
and hydrogen does not break instead of the hydrogen
bonds?
 Covalent bond between Hydrogen and Oxygen has much
higher bond enthalpy (energy) than Hydrogen bonding
between molecules.
 5. On cooling, substances will have higher density. But
when we change water to ice, density decreases. Explain.
 When we change water to ice, the Hydrogen bond
between water molecules become more rigid (less
movement of bond due to low energy) making the ice with
larger volume. Larger the volume, smaller the density when
the mass is the same.
 6. Which compound contains hydrogen bonding?
COCl2; PH3; H2CO; CH3OH
 CH3OH
 7. Which of the following has the strongest
attraction between molecules?
SiH4; CH2=O; CH3-CH3; O2
 CH2=O ; because of dipole-dipole attraction
together with London force. All others have weak
Van der Walls force only.
 8. Write reasons for:
 (a) Sulphur and Oxygen both are in group 6. Why
water is a liquid while SO2 is a gas at room
temperature?
 Water has strong Hydrogen bonding together with
London force and Dipole attraction. SO2 has no
Hydrogen bonding.
 (b) Ethanol (CH3-CH2-OH) has a higher boiling
point than dimethyl ether(CH3-O-CH3)
 Ethanol has strong Hydrogen bonding which is
not in dimethyl ether.
 (c) The boiling point of sulphur dioxide is 24 C
higher than that of Chlorine gas.
 Chlorine molecules have only Van der Waals
force of attraction. Sulphur dioxide has Van
der Waals force and diploe-dipole attraction.