20 Chemistry
Chapter 13 – States of Matter
The Kinetic-Molecular Theory
All matter is made up of tiny particles.
The particles are moving (and thus have kinetic energy given by 𝐾. 𝐸. =
1
2
𝑚𝑣2 )
The particles are attracted to each other (and to other particles).
In the Case of a Gas
The particles are much much smaller than the space separating the particles.
The motion is random, translational (rotational and vibrational).
The attraction between particles is not significant.
Temperature – the measure of the average kinetic energy of the particles of a substance
Gas Properties
Low density, compressible and expandable
They diffuse – diffusion is the movement of one substance through another. Because the
particles of a gas typically have a lot of “space” around them and because they are not
significantly attracted to each other, gases diffuse readily. Consider mixing two gases at the
same temperature. Their particles have the same kinetic energy. The particles of two
different gases have different masses but have the same kinetic energy. The lighter particles
must therefore move faster. This is why lighter gases will diffuse quicker and diffusion
explains why we smell things.
Pressure
Pressure is defined as the force per unit area. Specifically, gases have pressure because their
particles move and collide with things in their surroundings. Thus forces are exerted on the
surfaces that the particles come in contact with. You can increase the pressure by
i) increasing the force by
a) increasing the speed of the particles that collide
b) increasing the number of particles that collide
ii) decreasing the amount of surface area that the particles collide with
Gravity’s pull on the air in the atmosphere gives rise to atmospheric pressure. At the surface,
atmospheric pressure is 1 atmosphere which is equal to the weight of a 1 kg mass (10
Newtons) pushing down on a square cm surface OR
101,300 N/m2
or
101,300 Pascals
760 mm of Hg
1 atmosphere
or
101.3 kPa
14 lbs/in2
An interesting thing about the pressure of a gas is this. It depends on the number of moles of
gas, the temperature of the gas and the size of the container that the gas is in. Notice that it
doesn’t depend on the type of gas. This means that 1 mol of Hydrogen in a 1 litre container
at 20 degrees Celcius has the same pressure as 1 mol of oxygen in a 1 litre container at 20 oC.
Dalton’s Law (Optional) – the total pressure of a mixture of gases is equal to the sum of the
partial pressures of the gases that make up the mixture.
PT = P1 + P2 + …+ Pn
Sample p. 391
Practice p. 392 # 4 – 6
Inter-Molecular Forces – why are some substances gases and others liquids?
Dispersion Forces – are the weakest. They are so much weaker than other intermolecular forces that if others are present, dispersion forces are negligible. So discuss
dispersion forces for anything but non-polar substances (i.e. diatomic substances such as
O2, F2, Cl2, Br2 and I2 and symmetric substances). Dispersion forces are the result of
temporary shifts in the density of electron clouds. This temporary shift is subtle. (p. 394)
Dipole – Dipole Forces – are the result of more pronounced and permanent dipoles that
exist in polar substances. The negative end of one molecule is attracted to the positive
end of another. The polar molecules “line up” optimizing this effect.
Hydrogen Bonds – this is a dipole-dipole attraction but it is yet stronger. These bonds
(attractive forces) exist only between hydrogen atoms and small highly electronegative
atoms such as nitrogen, oxygen and fluorine. Furthermore there must be at least one
unshared pair of electrons in the molecule to constitute a hydrogen bond.
So what makes hydrogen bonds so strong?
a) the great difference in electronegativites creates a pronounced partial positive
side (the hydrogen side of the molecule) and a partial negative side.
b) the non-hydrogen atom (the negative side is small) – this allow the negative
end of one dipole to get close to the positive end of another dipole – the closer
these oppositely charged dipoles can get, the stronger their attractive force
(Coulombs law)
This is why for instance why water is a liquid when its similar (but non-hydrogen
bonded) counterparts are not.
Coumpound
Molar Mass
I.M. Force
Boiling Point
State
(g)
(oC)
Water (H2O)
18.0
Hydrogen Bond
100
Liquid
Methane (CH4)
16.0
Dispersion Force
-164
Gas
Ammonia (NH3)
17.0
Dipole-Dipole
-33.4
Gas
So why do the atoms of ammonia not form hydrogen bonds? No unshared pairs of electrons.
Draw dot structures of each to see this.
Mon D Video “ Bonding Between Molecules” Uses cookies as a metaphor for inter molecular
forces.