Evaporation and Boiling Point

1. Evaporation and Boiling Point

2. Contents • What is Evaporation? • What is Boiling point? • Comparison between Evaporation and Boiling Point

3. 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 )

4. Evaporation • Occurs at: • Any temperature • Only on the surface • Rate changes when atmosphere pressure changes

5. 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.

6. 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

7. Evaporation • Thus, evaporation only occurs on the surface of the liquid.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. Evaporation • The formula for calculating heat consumed by evaporation • g = Θ A (xs - x)         (1) where g = 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).

15. Boiling

16. Boiling • Factors that affect temperature of boiling point • Atmospheric pressure • Vapor pressure

17. 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.

18. 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

19. Boiling • Formula to calculate boiling point where: TB = the normal boiling point, K R = the ideal gas constant, 8.314 J · K-1 · mol-1 P0 = is the vapor pressure at a given temperature, atm ΔHvap = the heat of vaporization of the liquid, J/mol T0 = the given temperature, K ln = the natural logarithm to the base e

20. Boiling • How boiling occurs -when vapor pressure is greater than atmosphere pressure

21. 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

22. 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

23. Boiling • Different liquids boils at different temperature as they have different vapor pressure which is a factor that determines the boiling point. • Example

24. Boiling Evaporation

25. 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