VAPOR PRESSURE OF WATER Experiment #2

What are we doing in this experiment? 1.Determine the vapor pressure of water at different temperatures. 2. Determine the heat of vaporization of water.

Review of Gas laws Boyle’s Law: Charle’s Law: Avogadros’s Law: IDEAL GAS LAW

Units of P: atmospheres Units of V: Liters Units of T: kelvin Units of R:

Dalton’s law of partial pressure A + B + C P total = P A + P B + P C

Relation between mole fraction and partial pressure of a gas A + B P total = P A + P B

What is VAPOR PRESSURE OF WATER? It is the pressure (technically, partial pressure) exerted by the water molecules in the vapor phase (gas, water vapor) above the surface of a liquid at equilibrium at that temperature. Heat Vapor phase or Water vapor Liquid phase P vap

Define Pressure It is defined as force applied per unit area Units of pressure:

Vapor phase or Water vapor Liquid phase Why does water not vaporize by itself quickly? Atmosphere Strong inter molecular hydrogen bonds O--H-O-H - - O H H H H

When does a liquid boil? Vapor pressure of the liquid = surrounding atmospheric pressure What is boiling point of a liquid ? It is the temperature at which, Vapor pressure of the liquid = surrounding atmospheric pressure Normal boiling point: If P atm = 1.0 atm

Composition of Dry Air Gas% by volume N 2 O 2 Ar CO 2 Rest

What does it mean if a liquid has high vapor pressure? 1. More molecules are found in the vapor phase. 2. Does not require a lot of energy to vaporize the liquid. 3. Weak inter molecular attractive forces holding the molecules together in their liquid state.

A comparison Open system Closed system Rate of vaporization cannot equal rate of condensation Rate of vaporization can equal rate of condensation Dynamic equilibrium not reachable Dynamic equilibrium reachable Heat

A comparison Open system Closed system As the temperature is increased, We can send more and more molecules in to Vapor phase Heat There can only be a fixed number of molecules that can be in the vapor phase

A comparison Open system Closed system The maximum vapor pressure that can be reached here is atmoshpheric pressure Heat The maximum vapor pressure that can be reached here is more than the atmoshpheric pressure

What is VAPOR PRESSURE OF WATER? It is the pressure (technically, partial pressure) exerted by the water molecules in the vapor phase (gas, water vapor) above the surface of a liquid at equilibrium at that temperature. Heat Vapor phase or Water vapor Liquid phase P vap

Air bubble at the top of a inverted test tube Surface of water should cover test tube. Magnetic stir-bar (position carefully) Stirrer/hot plate EXPERIMENTAL SETUP

0 1 2 3 4 5 6 7 8 Note: This scale is unit less and does not correspond to inches or centimeters. Cut the scale out from the manual Paste the scale on a 100 mm test tube as shown, with the scale facing inside of the tube

Air bubble at the top of a inverted test tube Surface of water should cover test tube. Magnetic stir-bar (position carefully) Stirrer/hot plate EXPERIMENTAL SETUP

What are we going to do? 1.Measure the volume of the bubble at different temperatures Low temperatureHigher temperature As temperature increase, the volume increases (at constant P,n) Charle’s Law

What are we going to do? 2. Calculate the pressure contribution from N 2 and O 2 in the bubble using the ideal gas equation. P total = PN 2 + PO 2 + Pwvap PN2 and PO2 are much greater than Pwvap P total = P NO + Pwvap P total inside the bubble = P atmosphere But, So, we have P atmosphere = P NO + P wvap P wvap = P atmosphere - P NO 3. Calculate the vapor pressure of water.

How to convert the change in the number of divisions to change in volume? Have to find out the relationship between the number of divisions to number of mL

Burrette Filled with water ViVi VoVo VNVN Unscaled Volume, V u= v o -v i S=4.5

Water Temp ( o C) T * Scale Value S * Water Temp (K) T K Bobble Vol. (mL) V B Press. N 2 & O 2 (atm) P NO Water Vap P (atm) P W Vap Ln P W Vap 1/TK1/TK DATA Please use 3 sig. figs., and scientific notation

HEAT OF VAPORIZATION It is defined as the heat required to vaporize (converting a liquid to gas) one mole of a substance at constant pressure and temperature. The heat of vaporization is expressed in kJ/mol. `  vap DEFINITION

HEAT OF VAPORIZATION Clausius-Clapeyron equation R=8.314 J/K mol

1/T k, (k -1 ) LnP wvap (unitless) Best-fit line x x 2 1 X1X1 x2x2 Y2Y2 Y1Y1 x y Equation of the best-fit line: Y=mX + z m = slope = intercept LnP wvap = m(1/T) + Z R=8.314 J/K mol