Kinetic Theory of Liquids: V.P. and B.P. Day 4 - Notes Unit: States of Matter Kinetic Theory of Liquids: V.P. and B.P.

After today you will be able to… Describe vapor pressure and how it is affected by temperature and nature of the liquid Explain the relationship between boiling point and vapor pressure Predict a way to boil a liquid without changing the temperature of the liquid

Vapor Pressure (V.P.) Vapor pressure (V.P.): The pressure caused by the evaporated particles in a closed container. With the lid, the vapor will be steady so the pressure can be measured. Without the lid, the particles will diffuse.

Vapor Pressure and Temperature As temperature increases, vapor pressure will increase. This occurs because more particles have the energy needed to evaporate. There is a direct relationship.

Vapor Pressure As the IM forces in a liquid increases, the vapor pressure decreases. As the forces of attraction are stronger, it is more difficult for particles to break free. There is an indirect relationship.

Vapor Pressure IM forces restrict the movement of the molecules. = Stronger attractive forces (IM forces) Less evaporation of particles (less V.P.)

Boiling Point (B.P.) The temperature at which a substance will boil. Boiling is a conversion from liquid to gas throughout the entire substance. Recall evaporation is a conversion from liquid to gas at the surface.

In order to boil: = Vapor Pressure Atm Pressure

Normal Boiling Point (n.B.P.) The boiling point at standard pressure (1 atm) If not at sea level, will not see the n.b.p. (because atm pressure is different) The n.B.P. for water is 100°C

Boiling Point (B.P.) Boiling point is not constant; it depends on atm pressure! Where there is high atm pressure = B.P. increases Where there is low atm pressure = B.P. decreases

Boiling Point (B.P.) Recall, when a substance is boiling, temperature remains constant. (In any phase change, temperature is constant.) At the boiling point, the added energy goes towards the conversion of liquid  gas instead of increasing temperature. Boiling can also occur by lowering atm pressure until it equals the vapor pressure. (Like in a vacuum!)

Boiling Points for Water at Different Atm Pressures Takes more energy to have v.p.=atm pressure, so a higher b.p. is observed here. Above Sea level 0.8 atm b.p = 90°C Takes less energy to have v.p.=atm pressure. Sea level 1atm b.p = 110°C n.b.p = 100°C 1.5 atm Below Sea level

Questions? Complete WS 4