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Vapor pressure is the pressure of the vapor above the liquid at equilibrium. It is caused by evaporation at the surface of the liquid. The evaporated molecules fly around in the air and can rejoin the liquid. At equilibrium vapor pressure, there is no net change in the vapor pressure even though molecules are constantly evaporating and combining back into the liquid. High vapor pressures are caused by molecules with weak intermolecular forces and high energy.

Contents [hide] 1 Vapor Pressure Unlocked 2 Vapor Pressure Practice Problems 3 Answer and Explanation

 

Vapor Pressure Unlocked

The vapor pressure of a liquid or solid is the pressure of the vapor (gas) above the substance at equilibrium. It caused by molecules on the surface of the liquid or solid breaking off and forming gas molecules. These molecules circulate in the air and can fall back onto the liquid or solid. Equilibrium vapor pressure occurs when there is no net change in the amount of liquid (or solid) or vapor. Liquid molecules still turn into gas and gas molecules still turn into liquid, but the processes are equal. In general, when the temperature increases, vapor pressure increases. Vapor pressure also tends to be high for substances with weak intermolecular forces because their molecules break off easily.

Vapor pressure can be measured with a barometer. The common units for pressure are pascals (Pa), atmospheres (atm), and torrs. Liquids with high vapor pressures are called volatile. Raoult's law states that the vapor pressure of a solution can be determined by calculating the individual vapor pressures of each component and the mole fraction of each component: P = p1x1 + p2x2 where P is the vapor pressure, p is the vapor pressure of each component, and x is the mole fraction of the component.

Also, vapor pressure can be calculated using the clausius-clapeyron equation. Here is the equation:

P1 and P2 are respective pressures and T1 and T2 are respective temperature (in Kelvin).

R is the gas constant (8.314 J mol−1K−1)

ΔHvap is the molar enthalpy of vaporization.

 

Here is a sample vapor pressure calculation problem: (The explanations of the mathematics are below the calculations)

Explanation:

To calculate the vapor pressure of the water, we use the clausius-clapeyron equation, which allows us to find the vapor pressure of water at different temperatures using varying pressures.

We plug in the pressure at 25 degrees Celsius and both the initial and final temperature. We also use the given heat of vaporization of water and the R constant.

Simplify the right side.

To remove the natural log take the e^(left side) and e^(right side).

Simplify further.

Since we are looking for vapor pressure, the answer is in torr.

 

Vapor Pressure Practice Problems

1. Which does not affect the vapor pressure of a substance?

A. Strength of intermolecular forces.
B. Surface area of the substance.
C. Temperature of the substance.
D. Type of substance.

 

2. Which two processes cause vapor pressure in closed container of water at 25 degrees Celsius?

A. Boiling and condensation.
B. Evaporation and deposition.
C. Boiling and solidification.
D. Evaporation and condensation.

 

3. Do liquids in open containers still exhibit vapor pressure?

A. Yes, but they eventually evaporate because the equilibrium vapor pressure can only be achieved in a closed system.
B. Yes, because the processes of evaporation and condensation are still equal in open containers.
C. No, because the vapor pressure of a liquid in an open container is also dependent on the pressure of the surrounding air.
D. No, because it is impossible for molecules to condense back into the liquid in open containers.

 

4. The vapor pressure of water at 25 degrees Celsius is 23.8 torr, and the heat of vaporization of water at 25 degrees Celsius is 43.9kJ/mol. Calculate the temperature if the final vapor pressure is 250 torr.

5. The vapor pressure of water at 25 degrees Celsius is 23.8 torr, and the heat of vaporization of water at 25 degrees Celsius is 43.9kJ/mol. Calculate the vapor pressure at 60 degrees Celsius.

6. Short Answer Question: How is vapor pressure related to boiling points?

 

Answer and Explanation

1) Answer: B
The surface area of a substance does not affect its vapor pressure because vapor pressure is an equilibrium point where there is no net change between the pressure of the gaseous particles and the liquid. Liquids with smaller surface areas will retain the same vapor pressure in the air above.

 

2) Answer: D
Molecules evaporate off the surface of a liquid and condense back onto the surface at equilibrium vapor pressure. Boiling creates gas molecules, but occurs only at the boiling point of the substance.

 

3) Answer: A
Liquids in open containers still evaporate and create gas molecules. However, in an open container most (but not all) of these molecules will fly off instead of rejoining the liquid. More molecules then evaporate in an attempt to establish the equilibrium. This process continues until the liquid has completely evaporated.

4)

ln(P1/P2) = (H/R)(1/T2 - 1/T1)

P1 = 23.8 torr
P2 = 250 torr
T1 = 25 degrees Celsius = 298 K
H = 43.9k J/mol = 43000 J/mol
R = 8.3145 J/mol

ln(23.8/250) = (43000/8.3145)(1/T2 - 1/298)
-2.35 = 5172(1/T2 - 1/298)
-4.60 x 10^-4 = 1/T2 - 1/298
.00290 = 1/T2
T2 = 345 K = 72.4 degrees Celsius

Explanation:
To calculate the vapor pressure of the water, we use the clausius-clapeyron equation, which allows us to find the vapor pressure of water at different temperatures using varying pressures.

We plug in the temperature at 25 degrees Celsius and both the initial and final pressure. We also use the given heat of vaporization of water and the R constant.

Simplify the left side and right side.

Divide by 5172.

Simplify further and find the answer by taking the inverse of the left side.

Since we are looking for temperature, the answer is in Celsius or Kelvin.

 

5) ln(P1/P2) = (H/R)(1/T2 - 1/T1)

P1 = 23.8 torr
T1 = 25 degrees Celsius = 298 K
T2 = 60 degrees Celsius = 333 K
H = 43.9k J/mol = 43000 J/mol
R = 8.3145 J/mol

ln(23.8/P2) = (43000/8.3145)(1/333 - 1/298)
ln(23.8/P2) = 5172(1/333 - 1/298)
ln(23.8/P2) = -1.82
23.8/P2 = .16
P2 = 147.5 torr

Explanation:
To calculate the vapor pressure of the water, we use the clausius-clapeyron equation, which allows us to find the vapor pressure of water at different temperatures using varying pressures.

We plug in the intial and final temperatures and the initial pressure. We also plug in H and R.

Simplify the right side.

To remove the natural log, use e^(left side) and e^(right side).

Simplify further to find the answer.

The answer, P2, is in torr.

 

6) Answer: When the vapor pressure equals the surrounding pressure, liquids begin to boil. That means that the vapor pressure is high enough that the intermolecular bonds between the liquid are broken and the molecules fly off as a gas. This is why places with lower air pressures such as Denver, Colorado and Lhasa, Tibet (PRC) have lower boiling points for liquids such as water.