Transcona Community Learning Centre
CHEM 30S
LESSON 6: VAPOR PRESSURE
We have learned about the production of vapour during evaporation and boiling. In this
lesson, we will study the relationship between the pressure produced by this vapour and
the boiling point of a substance.
Lesson Outcomes
When you have completed this lesson, you will be able to:
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Operationally define vapour pressure in terms of observable and measurable
properties.
Construct and interpret a graph of vapour pressure versus temperature.
Interpolate and extrapolate vapour pressures of substances at various temperatures
from vapour versus temperature graphs.
Describe vaporization at the molecular level using diagrams.
Explain why vapour pressure varies with temperature, citing examples (e.g.,
explain why Canadian homes are ‘drier’ in the winter than in the summer).
Describe the practical value of knowing the vapour pressures of various
substances.
Operationally define normal boiling temperature and boiling temperature in terms
of vapour pressure.
Pressure
According to the Kinetic Molecular Theory, the particles of a gas are constantly moving
in random straight-line motion. If the gas particles are in a container, the particles must
eventually collide with the sides of the container or other gas particles. When the
particles collide with the sides of the container, they exert a force upon the container’s
walls. We call this force gas pressure. Pressure is defined as force per unit area.
The Earth's atmosphere is about 100 km thick. This means each object on or
above the Earth has a column of air (the atmosphere) pushing down on it. The
force created by this column of air is called air pressure or atmospheric pressure.
Units of Pressure
Units of pressure are often related to units of force. The kiloPascal (kPa) is a Newton of
pressure per square metre of area. The kPa is the SI unit of pressure.
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Another unit of pressure is the millibar. The millibar is a meteorological unit of
atmospheric pressure. One bar is equal to standard atmospheric pressure or 1
atmosphere.
The unit “atmosphere (atm)” was derived from standard atmospheric pressure at
sea level. 1 atmosphere is equal to 760 mmHg, or 101.325 kPa. Two
atmospheres is twice standard atmospheric pressure, and so on.
Mm of mercury is not a common unit used today outside the laboratory, however,
many aneroid barometers found in the home use both mm of mercury as well as
another unit like kilopascals.
We will discuss the historical development of these units further in the next
module.
Measuring Pressure
Pressure can be measured using several devices or instruments. A manometer usually
has a bulb or glass container on one end and can be open or closed on the other. A liquid,
often mercury, is placed in a U-shaped tube. The pressure is measured by finding the
difference in height on both sides of the tube. An example of a manometer is shown
below.
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The greater the pressure of the gas in the bulb, the greater the force on the
mercury and the higher the level of the mercury on the right side of the U-shaped
tube.
We will discuss measuring gas pressure to much greater extent in the next
module.
Behaviour of a Liquid in a Closed Container
Liquid left in an open container will eventually evaporate to dryness. The particles on the
surface of the liquid vaporize and leave the container. If the container is sealed, the
vapour is trapped and cannot leave the container. Eventually, the space above the liquid
becomes saturated with vapour and for every molecule that evaporates, another
condenses.
Liquid in a closed container, at a constant temperature, reaches equilibrium with
its vapour. At equilibrium,
rate of evaporation = rate of condensation
The pressure created by the vapour at equilibrium is
known as the vapour pressure, abbreviated Pvap. Vapour
pressure is a characteristic physical property because
many substances have different vapour pressures.
Vapour Pressure and Intermolecular Forces
Intermolecular forces determine the rate of evaporation. Try putting a few drops of
rubbing alcohol and water on the countertop. Which evaporates faster? The alcohol
should disappear more quickly. The temperature of both liquids is the same (if both at
room temperature), so their intermolecular forces must determine which evaporates
faster. The alcohol has lower intermolecular forces than the water, so it evaporates faster.
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CHEM 30S
If the intermolecular forces are lower, the amount of vapour that accumulates
increases because it is easier for the particles to escape the forces of attraction
in the liquid and become gaseous.
The manometers above show the difference in the height of the mercury to be
much greater with alcohol in the vessel, as compared to water in the vessel.
Generally, the lower the forces of attraction, the higher the vapour pressure.
Substances that evaporate easily, such as alcohol and acetone, are said to be
volatile. The lower the forces of attraction, the greater the volatility. Often, volatile
substances are also very flammable.
Vapour Pressure and Temperature
There also exists a relationship between temperature and vapour pressure for a substance.
As temperature increases, the amount of vapour also increases. The rising temperature
causes more particles to have enough energy to overcome the forces of attraction of the
liquid. This increases the rate of evaporation, increasing the number of vapour particles..
If the number of vapour particles increases, the pressure increases.
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Vapour Pressure and Boiling Point
In an earlier lesson we described boiling in terms of when vaporization occurs throughout
the liquid and the temperature of the liquid remains constant. This is known as an
operational definition of boiling. The boiling point of a substance is defined as the
temperature at which the vapour pressure of a substance is equal to the atmospheric
pressure.
Let's examine how this works.
As the temperature of a liquid increases, the vapour
pressure also increases. Bubbles of vapour begin to form
in the liquid and rise towards the surface. Once the
pressure inside the bubble is great enough to overcome
the atmospheric pressure, the bubble is able to release
the vapour it contains. If the atmospheric pressure is
greater than the vapour pressure, the bubble will be kept
below the surface until the pressure increases.
Atmospheric Pressure and Boiling Point
We have already determined that intermolecular forces affect boiling point. The greater
the intermolecular forces, the lower the vapour pressure and the greater amount of energy
needed to produce enough vapour, so that vapour pressure equals atmospheric pressure.
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Since boiling occurs when atmospheric pressure and vapour pressure are equal,
any change in pressure will produce a change in the boiling point. The normal
boiling point of a substance is defined as the temperature when vapour pressure
equals standard pressure (1 atmosphere, 101.3 kPa, 760 mmHg, 760 torr).
The lower the atmospheric pressure, the lower the boiling point. Since less
vapour pressure is required, less heat is needed. This explains why it requires
longer cooking time to prepare foods at higher altitudes. At higher altitudes, the
atmospheric pressure is lower because the column of air is shorter. The lower
the air pressure, the lower the boiling point. If we were to try to hard boil an egg
at a high altitude, the water would boil at a much lower temperature. The egg
would take much longer to cook. See the table below.
Altitude Boiling Point of Water
Sea Level
100°C
2000 feet
98°C
5000 feet
95°C
7500 feet
92°C
10 000 feet
90°C
Pressure cookers are often used to speed up cooking. A pressure cooker
operates by increasing the pressure inside the pot. As pressure increases, the
boiling point of the water also increases because more energy is needed to
create more vapour. The higher the temperature at which water boils, the shorter
the cooking time.
Vapour Pressure Curves
We can plot the vapour pressure as a function of temperature and get a graph like the one
that follows.
You will notice that regardless of the substance there is a similar trend in the
vapour pressure as temperature increases.
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We can use this graph to determine the normal boiling point of each substance
and the temperature each will boil at various pressures.
Example 1
What is the normal boiling point of ethanol?
Solution
From the graph, we look for the temperature at which the vapour pressure
of ethanol is 101.3 kPa (standard pressure). According to the graph, the
normal boiling point is about 77°C.
Example 2
If the pressure in a city is 90 kPa, at what temperature will water boil?
Solution
When the atmospheric pressure is 90 kPa, water will boil when its vapour
pressure is 90kPa. We start at 90 kPa on the y-axis and move to where it
intersects with the curve for water, then see to what temperature this
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corresponds. According to the graph, water boils at about 97°C at 90 kPa
of pressure.
Example 3
Which substance has the largest forces of attraction at 60°C?
Solution
The substance with the largest forces of attraction, will have the lowest
vapour pressure. The substance with lowest vapour pressure at 60°C is
acetic acid.
Lesson Summary
In this lesson, you have learned:
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A barometer is used to measure atmospheric or air pressure and a manometer is
used to measure gas pressure.
Gas pressure is the force of gas particles on the sides of their container.
Gas pressure is directly proportional to the temperature and the number of
particles in the container.
Vapour pressure is the pressure produced by a vapour in a closed container in
equilibrium with its liquid phase.
Vapour pressure increases with increased temperature.
The lower the intermolecular forces, the higher the vapour pressure.
The boiling point of a substance is the temperature at which the vapour pressure
equals the air pressure.
The normal boiling point is the temperature at which the vapour pressure of the
liquid is equal to standard pressure.
Assignment
Use the vapor pressure curves on Page 38 to answer the questions below:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Aug 2006
What is the vapour pressure of ethanol at 60°C ?
What is the vapour pressure of acetic acid at 80°C ?
What is the approximate vapour pressure of chloroform at 0°C?
What is the temperature at which the vapour pressure of ethanol is 50 kPa?
Which of these substances has the highest vapour pressure?
Which of the three substances would evaporate fastest at room temperature?
Which of the three substances would evaporate slowest at room temperature?
Which substance has the weakest intermolecular forces?
From the graph, what are the normal boiling points of the four substances?
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10. What would be the boiling point of water on a day when the atmospheric
pressure is 95 kPa?
11. Alcohol is heated in a container in which there is a partial vacuum. The air
pressure in the container is 25 kPa. At what temperature will the alcohol boil?
12. If substance “X” had a normal boiling point of 30°C, where would you
expect to find the vapour pressure curve of “X”? Explain your answer.
i) to the left of the chloroform curve?
ii) between the chloroform and ethanol curve?
iii) between the ethanol and water curve?
iv) between the water and acetic acid curve?
v) to the right of the acetic acid curve?
13. In the definition of vapour pressure, there are the words “in a closed
container”. Why is it necessary to have a closed container?
14. What would the atmospheric pressure have to be in order to have ethanol boil
at 20.0°C ?
15. What would the atmospheric pressure be to have acetic acid boil at 80.0°C ?
16. If the temperature was 50.0°C and the atmospheric pressure was 20 kPa,
which substances if any would boil?
17. If the temperature was 80.0°C and the atmospheric pressure was 100 kPa,
which substances if any would boil?
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