6.1 Pure Substances
Condensed phases of matter are solid and liquid.
Gaseous elements (under room conditions) are found at the top right hand side of the
Periodic Table
Examples of element that are found as gases are:
Hydrogen, H2 Helium, He Radon, Rn
Nitrogen, N2 Neon, Ne
Oxygen, O2 Argon, Ar
Fluorine, F2 Krypton, Kr
Chlorine, Cl2 Xenon, Xe
Potential energy diagram
The exothermic reactions are: condensation, freezing (solidification) and deposition.
The endothermic reactions are: vaporization (evaporation), melting and sublimation
Endothermic reaction: a reaction that consumes energy
Exothermic reaction: a reaction that releases energy.
One gram of steam, H2O (g) causes more severe burns than one gram of water, H2O(l) at
100oC. At the same temperature, both have the same average kinetic energy but steam has a
higher potential energy than water.
A volatile liquid is a liquid that evaporates at room temperature. A liquid with a low
boiling point is easy to vaporize
Compounds A, B and C have the following boiling points: -15℃, 60℃ and 38℃
respectively. List the above compounds from the least to the most volatile.
Vapor pressure of a liquid: is the pressure of the gas above the liquid with which it is at
equilibrium (Both liquid and gas exist indefinitely).
Vapor pressure of a liquid in a sealed container depends on temperature of the flask. As
the temperature increases the vapor pressure of a liquid increases.
At the boiling point, the temperature of a pure substance stays constant as the liquid is
being heated until all the liquid changes into gas. The heat given to the liquid causes more liquid
to change into gas.
Molar heat of vaporization is the minimum energy required to change one mole of a
substance from liquid to gas at the same temperature.
General equation for Molar heat of vaporization: X (l) + heat ⇌ X (g)
General equation for Molar heat of condensation: X (g) ⇌ X (l) + heat
Typical exercise: Find the heat released by a given mass from the heat of vaporization
If the molar heat of vaporization of water is 42 kJ/mole. What is the amount of heat needed to
change 36 g of water to steam?
NOTES:
1. Vapor pressure and temperature are proportional NOT directly proportional.
2. At the same temperature, the vapor pressure is the SAME.
3. For the same liquid, the only factor affecting the pressure of the liquid is the
temperature.
Minimum conditions for liquid molecules to vaporize:
1) Molecules are supposed to be on the surface.
2) Molecules are supposed to have an average kinetic energy greater than the energy
keeping the molecules in the liquid state.
Water has a vapor pressure of 17.5 mmHg at 20oC. Which of the following will increase
the vapor pressure of water?
a) Transferring water to a larger container.
b) Cooling water to 10oC
c) Taking the container to the top of the mountain.
d) Heating the water to 32oC
Note: If the question asks “what changes the vapor pressure” then the correct answer is b
and d
Boiling point: is the temperature at which the liquid vaporizes anywhere in the solution.
At the boiling point:
a. Vapor pressure is equal to the surrounding pressure.
b. Bubbles of vapor can form anywhere within the liquid.
c. Molecules escape from the surface of the liquid to enter the gas phase as vapor (this also
happens at room temperature).
d. With increasing altitude, atmospheric pressure decreases and so does boiling point.
Normal boiling point: is the temperature at which the vapor pressure is exactly 1 atm or
760 mmHg.
Molar heat of fusion: is the energy required to change one mole of a substance from
solid to liquid at the same temperature and constant pressure
X (s) + heat ⇌ X (l)
Molar heat of fusion is less than the molar heat of vaporization. Molar heat of
vaporization of water is 7 times molar heat of fusion of water.
In general, a pure substance that has a high melting point is expected to have a high molar
heat of fusion.
Applications:
1. Exercise 6 page 8
How much heat would 180g of water release when they change from water at 0ºCto ice
at 0°C?
2. Exercise 5 (b, c) page 8
Because of its excellent heat conductivity, liquid sodium has been proposed as acooling
liquid for use in nuclear power plants.
b) How much heat would be absorbed per kilogram of sodium to melt the solid when
the cooling system is put in operation?
c) How much heat would be absorbed per kilogram of sodium if the temperaturerose
too high and the sodium vaporized?
Use the data in Tables 6.1 and 6.2.
Comparison between the energy of the molecules on the three states of matter.
Highest PE in gases and lowest PE in solids. The PE of liquid is less than in gases and more
than in solids
Average KE is the same in all the states of matter at the same temperature.
During melting, the energy absorbed is stored as PE. This stored energy is used to change
state form solid to liquid and not to raise the temperature. Therefore, the temperature stays
constant.
Heating curve:
HINT:
a) As temperature increases: KE increases, PE stays the same
b) At constant temperature: KE stays the same, PE increases.
. The average kinetic energy is directly proportional to absolute temperature. At constant
temperature, average kinetic energy remains the same.
. If a molecule on the surface of a liquid is given a sufficient push from below, it may be
able to escape and join the gaseous state.
. The molecule that leaves the surface jumps out with a lot of kinetic energy, energy that is
stored as potential energy.
. The average kinetic energy of the remaining molecules decreases. The liquid cools down.
. When a liquid evaporates its vapor pressure starts to rise. Eventually, molecules come
back and join the liquid at the same rate they are escaping it. The net amount of liquid molecules
and gaseous molecules becomes constant. No more cooling takes place because the rate at which
molecules leave and cool the liquid is equal to the rate at which the molecules come back and
heat the liquid. Equilibrium is reached. It is this “maximum” or “equilibrium” pressure that is
called the vapor pressure of the liquid at the given temperature.
. When we sweat on very humid days our bodies do not cool down.
. The vapor pressure increases with increasing temperature.
. When the vapor pressure equals the atmospheric pressure, the liquid boils.
. Sometime a liquid is heated to a temperature above its boiling point and it does not boil.
The liquid is said to be superheated. This is dangerous because any small agitation will cause the
liquid to evaporate explosively spraying all with very hot liquid.
. To prevent superheating in the laboratory, we add a few boiling chips to a liquid before
boiling it.