Vapor pressure is determined by temperature, not by quantity of the liquid.
Boiling occurs when the vapor pressure of a liquid equals the atmospheric
pressure above that liquid.
So, depending on the atmospheric pressure, a liquid can have many boiling
points.
Water for example, boils at 100oC...at 101.3kPa.
(Common misconception is water always boils at 100oC)
If we lower the atmospheric pressure, water does not need as much
energy (temperature) to overcome the atmospheric pressure above the
liquid.
Therefore, the boiling point of water will be lower.
Example- Lowering the atmospheric pressure to 50kPa, will change
waters boiling point to about 78oC.
Intermolecular Forces (IMF's) and Boiling
Point- When comparing IMF's, the higher the boiling point the
stronger the IMF's. It requires more energy to break the attractive
forces between molecules, therefore a higher boiling point.
On Table H, ethanoic acid has the strongest IMF's. It has the
highest boiling point ( due to it having the lowest vapor pressure).
Demo #1- Hot water is sealed in a flask and allowed to
cool. If the pressure above the liquid is lowered by lowering the
temperature of the gas above the liquid (using ice) the water will boil.
Demo #2- Warm water is placed into a vacuum chamber.
The pressure is lowered and the water starts to boil.
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explanations
Regents Questions
Answers
Explanations
June 2004-41 Using your knowledge of chemistry and the information
in Reference Table H, which statement concerning propanone and water at
50°C is true?
(1) Propanone has a higher vapor pressure and stronger
intermolecular forces than water.
(2) Propanone has a higher vapor pressure and weaker intermolecular
forces than water.
(3) Propanone has a lower vapor pressure and stronger
intermolecular forces than water.
(4) Propanone has a lower vapor pressure and weaker intermolecular
forces than water.
2
more vapor weaker intermolecular
forces
Aug 2002-28 As the pressure on the surface of a liquid
decreases, the temperature at which the liquid will boil
(1) decreases
(2) increases
(3) remains the same
1
less pressure lower BP,
table H
Aug 2007-16 Which liquid has the highest vapor pressure at
75°C?
(1) ethanoic acid (3) propanone
(2) ethanol (4) water
3
Table H
June 2008-14 Which liquid has the lowest vapor pressure at
65°C?
(1) ethanoic acid (3) propanone
(2) ethanol (4) water
1
Table H
June 2010-39 Which compound has the lowest vapor pressure at
50°C?
(1) ethanoic acid (3) propanone
(2) ethanol (4) water
