Enthalpy of Vaporization

Purpose of Enthalpy of Vaporization Lab

To use the relationship between temperature and vapor pressure to determine the molar heat of vaporization (Hvap) of water

Properties of Liquids

1. Vapor Pressure

2. Surface Tension

3. Viscosity

Factors Affecting Vapor Pressure

Determined by the size of the intermolecular forces in the liquid, temp, and amount of solutes:

Weaker intermolecular forces, higher vapor pressure

higher temperature, higher vapor pressure

More Solutes, lower vapor pressure in solvent

Surface Area

Quantity measure of energy/per unit area

Viscosity

Liquid’s resistance to flow

Ex: Honey has a higher viscosity than water

Equilibrium vapor pressure

Rate of evaporation = rate of condensation

Vapor Pressure

The pressure exerted by vapor above a liquid at equilibrium when rate of evaporation = rate of condensation

How do intermolecular forces (IMF’s) influence boiling point?

The stronger the intermolecular force, the higher the boiling point

Molar Heat of Vaporization

A measure of the strength of IMF forces in a liquid:

Energy required to vaporize 1 mole of liquid (kJ/mol)

Volatility

A measure of how easily a liquid evaporates

Higher volatility → higher vapor pressure.

Convert 1 atm to to mmHg/torr

1 atm = 760 mmHg = 760 torr

Clausius-Clapeyron Equation

ln (P) = –ΔHvap/R x (1/T) + C (R = -8.314)

Y = m (x) + b

In a a plot of ln (P) vs. (1/T), what is the slope?

there’s a negative slope: –ΔHvap/R

Ideal Gas Law

PV = nRT

P = Pressure (atm)

V = Volume (Liters)

N = Moles

R = 0.08206 (atm/ mol)⋅K

T = Temperature (Kelvin)

Percent Error

(Experimental Value - Accepted Value)    x 100

              Accepted Value

Dalton’s Law of Partial Pressures

The total pressure exerted by a liquid is equa to the sum of the individual partial pressures of each gas in the mixture.

P_{total  /} P_atm   =  P₁+ P₂+P_{3 ….}

Evaporation

When surface particles obtain enough energy to break away from bulk particles

Condensation

When gaseous particles lose energy and are “captured” by the liquid

What is the normal boiling point?

Temperature where vapor pressure = 1 atm (760 torr)

What equation relates vapor pressure and temperature?

Clausius–Clapeyron Equation

What are intermolecular forces?

Attractive forces between molecules that affect properties like boiling point, vapor pressure, and viscosity.

What is partial pressure?

Pressure exerted by one gas in a mixture

If IMF’s increase, what happens to vapor pressure?

Vapor pressure decreases

As IMF’s increase, it’s harder to undergo a phase change. So there’s more energy needed to break the hydrogen bonds to allow the molecules to become vapor. With less vapor molecules, the overall vapor pressure decreases.

If intermolecular forces increase, what happens to boiling point?

Boiling point increases.

More energy is required for molecules to escape the liquid.

What happens to vapor pressure when temperature increases?

Vapor Pressure increases.

Higher temperature → higher kinetic energy → more molecules escape the liquid.

If the slope of ln(P) vs 1/T is steeper (more negative), what does it mean?

The liquid has a larger ΔHvap and stronger intermolecular forces

aka requires more energy to transform from a liquid to a gas

Why do we assume water vapor pressure is negligible at 0°C in the experiment?

Because very little water evaporates at 0°C, so almost all pressure is from the trapped air.

Two liquids have the same temperature. One has higher vapor pressure. What does this mean about intermolecular forces?

The liquid with higher vapor pressure has weaker intermolecular forces

POST LAB QUESTION: Why does the partial pressure of air change with temperature?

As temperature increases, the air molecules move faster, spread out, and become less dense, often reducing the number of molecules in a given area and lowering pressure

POST LAB QUESTION 2: Would you expect the vapor pressure of methanol to be smaller or larger than that of water at a particular temperature below the boiling points

The vapor pressure of methanol is larger than water at any given temperature below their boiling points because it has weaker intermolecular forces

Why does a correction to the volume need to be made for an inverted cylinder?

inverting the cylinder changes the meniscus, causing the measured volume to be slightly less than the actual volume.

How to prepare the apparatus:

1. Beaker with water

2. Inverted graduated cylinder with a 2.2mL bubble clamped to ring stand

3. Thermometer rubber banded around graduated cylinder

4. On top of a Hot Plate

Procedure Steps

1. Prepare apparatus

2. Invert graduated cylinder and remove beaker quickly to trap 2.2mL air bubble

3. Added ice into the beaker until it reached 0°C

4. THEN heated water bath to 85°C

5. Removed beaker and took temperature readings every 0.2mL until lowered to 60°C

6. Took volume-temperature readings between 60 degrees celsius and 50°C every 0.1mL.

7. Stopped when thermometer reached 50°C (experiment over)

Excel Calculations: Correcting volume

Measured Volume - 0.15mL

Divide by 1000 because corrected volume was in liters!

8.4mL →(8.4 - 0.15)/1000 = 0.00825

Calculating Molar Heat of Vaporization of Water with the Graph:

ΔH_vap = Slope x - R

because the slope of the graph = -ΔH_vap /R so multiply by - R to get a positive answer for ΔH_vap.

Excel Calculations: Solving for N (mols of trapped air bubble)

Moles of Trapped Air Bubble  (N_air )

N_air   =

P(atm) x V

       R x T₀ 

P = 30.2 inhg(given, barometric pressure reading) 

V = 2.2mL (measured bubble) 

R = 0.08206 L atm/ mol K  

T₀ = 273.15 (0 degrees celsius because we assume no air escaped here)

= 9.90 ×10^-5  mol of trapped air after substituting & solving

 

Excel Calculations: Solving for P air

P_{air =} (n_air x RT) / V

n_{air =} 9.90 × 10^-5 mol of trapped air (see other card)

R = 0.08206 L atm/mol * K constant

T = (Any Recorded Temp from table) = 85℃/ 358.15k

V = (Corrected Volume from table) = 0.00825L

P_air =

9.90 × 10^-5 mol x 0.08206 L atm/mol * K ( 358.15K)

divided by 0.00825

=0.353 atm

Excel Calculations: P H2O

P_total   =  P_atm   =  P_air + P_water 

1.009atm =  0.349atm +  P_water  

 P_water   = 1.009atm  - 0.353atm = 0.656atm\

What’s the value of R in the Clausius Clapyeron Equation?

R = 8.314

What’s the value of R in the ideal gas law?

R = 0.08206

What’s the value of the temperature calculating N moles of air

273.15