what is evaporation? what is boiling point? comparison between evaporation and boiling point
TRANSCRIPT
Evaporation and Boiling Point
Contents
What is Evaporation? What is Boiling point? Comparison between Evaporation
and Boiling Point
Evaporation
Occurs faster when:- The surface of exposed area is larger- The temperature is higher- The humidity is lower- There is wind- Surface tension is higher( vapor
pressure )
Evaporation
Occurs at:- Any temperature- Only on the surface- Rate changes when atmosphere
pressure changes
Evaporation
Look at the 2 molecules in the liquid.
Molecule 1 on the surface experiences less attractive cohesive forces than Molecule 2 as half of it is exposed to the air while Molecule 2 is surrounded by water molecule all around it.
Evaporation
Molecules never stop vibrating- When they vibrate, they collide with each
other and kinetic energy is passed on to each other. Therefore some molecules will posses more kinetic energy than others.
- The amount of kinetic needed to break free from the attractive cohesive forces is less on the surface than under the surface as explained on the previous slide
Evaporation
Thus, evaporation only occurs on the surface of the liquid.
Evaporation
A B- B has more molecules exposed to
the air than A, thus, more molecules can escape from the attractive cohesive forces.
- Thus, with a larger area of exposed surface, the rate of evaporation would increase.
Evaporation
-Heat-With a higher temperature, there
would be more heat energy and would result in higher kinetic present in the molecule, this would allow the molecule to escape more quickly.
-Thus, a higher temperature would allow the rate of evaporation to increase.
Evaporation <- alcohol
If the surface tension is lower, the molecule would need to posses less kinetic energy to overcome the attractive cohesive forces.
And thus, a lower surface tension would result in a increased rate of evaporation.
Evaporation
There is a point where is saturated vapor point is reached. ( when no more vapor can evaporate anymore)
When the humidity is higher, the amount of vapor that can evaporate into the surrounding is small.
Thus, the rate of evaporation decreases as the humidity rises.
Evaporation
With wind, more air is brought over the surface of the liquid.
Thus, the air over the surface does not get saturated with water vapor. And so the rate of evaporation rises when wind is present.
Evaporation
Why when your sweat evaporate, you would feel cooler.
When the faster moving molecules leave the liquid, only the slower moving molecules are left in the liquid, making it lose heat.
Evaporation
The formula for calculating heat consumed by evaporation
g = Θ A (xs - x) (1)whereg = amount of evaporated water (kg/h)Θ = (25 + 19 v) = evaporation coefficient (kg/m2h)v = velocity of air above the water surface (m/s)A = water surface area (m2)xs = humidity ratio in saturated air at the same temperature
as the water surface (kg/kg)x = humidity ratio in the air (kg/kg)Note! The units don't match since the equation is empirical
(a result of experiments).
Boiling
Boiling
Factors that affect temperature of boiling point
- Atmospheric pressure- Vapor pressure
Boiling
Properties of boiling point( also known as saturation temperature )
- Occurs when vapor pressure is greater than atmospheric pressure.
- Occurs in the entire volume of the liquid
- Corresponds to the temperature at which vapor pressure is greater than atmospheric pressure.
Boiling
Saturation pressure ( pressure where liquid boils into vapor corresponding the saturation temperature ) increases as saturation temperature rises
The higher the vapor pressure, the lower the boiling point
The boiling point does not increase even when more heat is added
Boiling
Formula to calculate boiling point
where: TB
= the normal boiling point, KR= the ideal gas constant, 8.314 J · K-1 · mol-1
P0
= is the vapor pressure at a given temperature, atmΔHvap
= the heat of vaporization of the liquid, J/molT0
= the given temperature, Kln= the natural logarithm to the base e
Boiling
How boiling occurs-when vapor pressure is greater than
atmosphere pressure
Boiling
As the liquid is heated, its vapor pressure increases until it is greater then the atmospheric pressure.
To form vapor, the molecules must overcome the attractive cohesive forces
Boiling
Is boiling point is lowered when the atmpsoheric pressure is lower.
As less energy (heat) is needed to increase to vapor pressure to meet the atmospheric pressure
In other words, the boiling point is determined by the atmospheric pressure and the vapor pressure
Boiling
Different liquids boils at different temperature as they have different vapor pressure which is a factor that determines the boiling point.
Example
Boiling EvaporationBoiling point Evaporation
Occurs in the entire liquid Occurs only on the surface
Not affected by temperature Rate of evaporation increases as temperature increases
Not affected by humidity Rate of evaporation increase as humidity decreases
Not affected by wind Rate of evaporation increases if wind is present
Not affected by area of exposed surface area
Rate of evaporation increases when area of exposed surface area increase
Boiling point increases as atmospheric pressure decreases
Rate of evaporation increases as atmospheric pressure decreases
Occurs only vapor pressure is greater than atmospheric pressure
Occurs in any temperature
Boiling point decreases as surface tension decreases
Rate of evaporation increases as surface tension decreases
Sources
http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html
htthttp://www.chem.purdue.edu/gchelp/liquids/boil. htmlp://www.sciencedaily.com/articles/e/evaporation.htm
http://en.wikipedia.org/wiki/Boiling_point