Heat of Vaporization

Heat of Vaporization

Heat of vaporization: the energy required to vaporize one mole of a liquid at a pressure of one atmosphere.

Related topic: Topic 7 Chemical Energetics

How do we implement the concept of heat of vaporisation for better understanding?

Question 1

A liquid uses 50 Joules of heat to vaporize one mole of liquid, then what would be the enthalpy of vaporization?

SOLUTION
Liquid use 50J to vaporise 1 mole.
The heat in the process is equal to the change of enthalpy, which involves vaporization in this case
qtotal = Δvap
so
qtot = 50J = Δvap
So the enthalpy of vaporization for one mole of substance is _____ J.

Question 2

How do we know the enthalpy of vaporization for one mole of elements:
Period 2
1 mole of Lithium
1 mole of Beryllium
1 mole of Boron
1 mole of Carbon
1 mole of Nitrogen
1 mole of Oxygen
1 mole of Fluorine
1 mole of Neon
Period 3
1 mole of Sodium
1 mole of Magnesium
1 mole of Aluminium
1 mole of Silicone
1 mole of Phosphorus
1 mole of Sulphur
1 mole of Chlorine

1 mole of Argon

* use this calculator

* use this references

Question 3

How do we know the enthalpy of vaporization for one mole of molecules:
1 mole of Nitrogen gas
1 mole of Oxygen gas
1 mole of Fluorine gas
1 mole of Neon gas

Question 4

2.055 liters of steam at 100°C was collected and stored in a cooler container. What was the amount of heat involved in this reaction? The ΔHvap of water = 40.65 kJ/mol.

SOLUTION

First, convert 100°C to Kelvin.

°C + 273.15 = K
100.0 + 273.15 = 373.15 K

Second, find the amount involved (in moles).

n water = PV/RT

= [(1.0 atm) (2.055L)] / [(0.08206 L atm mol-1 K-1) ( 373.15K)]

= ________ mol

Third, find ΔHcond

ΔHcond = ΔHvap

so

ΔHcond=−40.65 kJ/mol

Fourth, Using the ΔHcond of water and the amount in moles, calculate the amount of heat involved in the reaction. To find kJ, multiply the ΔHcond by the amount in moles involved.

(ΔHcond)(n water)=(40.65 kJ / mol) (________ mol)=________ kJ

Question 5

How many phases are there in milk?

Hint: ________
Skill –
Define phase and enumerate the number of phases in a system.
Milk is a colloid.

Question 6

Which one of the following systems has three phases that are at equilibrium with one another?
A. a sealed container full of unsaturated NaCl solution.
B. a sealed container full of oversaturated sugar solution.
C. a sealed container full of colloid made by suspending solid in liquid.
D. a sealed container half full of water and oil.
E. an open container half full of oil and water and ice.
Hint: ________
Skill –
Identify the number of phases in a system.

Question 7

How many phases are present in a closed flask if it contains ice, water and air?
Hint: ________
Discussion –
What will be the temperature for a system describe here?

Question 8

How many phases are present in a closed flask if it contains dry ice and air?
Hint: Two phases: ________
Skill –
Define phase and enumerate the number of phases in a system.

Question 9

A sealed 5-L flask contains 1.0 L of solution, some NaCl crystals, and some ice crystals. The rest of the space is filled with air. How many phases are present in such a system?
A. 1
B. 2
C. 3
D. 4
E. 5
Hint: d. Four phases: liquid, NaCl , ice, gas
Skill –
Enumerate the number of phases in a system.
Do you know what the temperature is for such a system to be at equilibrium? It’s 0 degree F (the temperature of equal weight mixture of snow and salt).

Question 10

A 750-mL bottle contains 300 mL clear colorless alcoholic beverage. What are the phases present in the bottle?
Hint: ________
Skill –
Identify the phases in a system.

Question 11

What is the name for the process of phase transition by which a gas is converted to a solid?
Hint: ________
Skill –
Know the names of all phase transition processes.

Question 12

The vapor pressure of a solid
A. increases as the volume of the solid increases.
B. decreases as the temperature of the solid increases.
C. increases as the temperature of the solid increases.
D. decreases as the volume of the solid increases.
Hint: ________
Skill –
This is a test of common sense.
A scientific way of looking at the vapor pressure of a solid as a function of temperature is given by the Clausius-Clapeyron Equation:
Pvap = Ae^(−Hvap/RT) where Hvap is the heat of vaporization, and R T are the gas constant and temperature respectively.

Question 13

Water ALWAYS boils at 100 degree C: true or false?

Hint: ________
Skill –
This is a test of common sense.
Only when the atmospheric pressure is 1.00 atm will the boiling temperature be 100 deg. C, which is the normal boiling point of water.

Question 14

The volume of 18 grams of water increases as its temperature increases from 0 to 4 deg. C: true or false?
Hint: ________
Skill –
This is a test of common sense. You know that the density of water is the highest at 4 deg. C, and its volume for a mole is the lowest at 4 deg C.
One mole of water is 18 g, and its volume is called the molar volume. Water is a strange substance!

Question 15

The volume of 18 grams of water increases as its temperature increases from 10 to 84 deg. C: true or false?
Hint: ________
Skill –
Analyze the problem carefully. Most substances expand when heated.

Question 16

One mole water of which phase occupies the largest volume: solid, liquid or gas?

Hint: ________
Discussion –
Temperature is not specified here, but you may assume a temperature at which all three phases are stable. Under the circumstance, the pressure of vapor phase is much lower than 1 atm. For the same quantity of water, ice has a larger volume than water.

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