MU #1 The concepts of kinetic and potential energy can be used to explain physical processes that include: fusion (melting), solidification (freezing), vaporization (boiling, evaporation), condensation, sublimation, and deposition.

EXPERIMENT #1: A sample of H₂O(s) at -20^oC is heated continuously.

At A, T or KE increases and PE is constant.

At B, the ice starts to melt at 0^oC. T or KE is constant and PE increases until all the ice is melted.

At C, T or KE increases and PE is constant.

At D, the water starts to boil at 100^oC. KE is constant and PE increases until all the water evaporated.

At E, T or KE increases and PE is constant.

EXPERIMENT #2: A sample of H₂O(g) at 150^oC is cooled continuously.

At E, T or KE decreases and PE is constant.

At D, the water vapor starts to condense. T or KE is constant and PE decreases until all the gas is condensed.

At C, T or KE decreases and PE is constant.

At B, the water starts to freeze at 0^oC. T or KE is constant and PE decreases until all the water is frozen.

At A, T or KE decreases and PE is constant.

Fusion (melting) is the change from solid to liquid phase. The opposite of melting is solidification or freezing, or also crystallization. The melting point of a substance is the temperature at which the solid and liquid phases exist in equilibrium. The opposite is the freezing point.

Example

The melting point and the freezing point of water is 0^oC.

Vaporization (evaporation, boiling) is the change from liquid to gas. The opposite is condensation.

Evaporation happens when only some liquid particles have acquired enough energy to break their intermolecular attractions to change into gas particles.  Evaporation takes place at all temperatures. 

Boiling happens when all liquid particles have acquired enough energy to break their intermolecular attractions to change into gas particles. The boiling point of a substance is the temperature at which the liquid and gas phase exist in equilibrium.

Example

Water boils at 100^oC.

Sublimation is the apparent direct change from solid to gas phase. Deposition is the opposite of sublimation.

Example

At room temperature dry ice CO₂ (s) and iodine crystals I₂ (s) sublime

Vapor pressure is the pressure exerted by gaseous particles over a substance in a close system at constant temperature and pressure. See table H

Example 

A liquid boils when its vapor pressure is equal to the pressure exerted on the liquid.

Example 

The vapor pressure of water at 100^oC is 101.3 kPa or 1 atm. Therefore, water boils at 100^oC when the pressure exerted on it is 1 atm.

In an open container the pressure exerted on water is the atmospheric pressure.

Note: The normal boiling point of water is 100^oC. The term normal refers to standard pressure or 1 atm, or also 101.3 kPa.

Problems 

What is the boiling point of propanone, ethanol, and ethanoic acid at standard pressure and at 60 kPa ?

Solids (CO₂ and I₂) that sublime at room temperature and standard pressure have high vapor pressures because their intermolecular forces are weak.

Example

TEST YOUR UNDERSTANDING

6/02

As ice melts at standard pressure, its temperature remains at 0^oC until it has completely melted. Its potential energy            

            (1) decreases                (2) increases                 (3) remains the same

8/02

1 The solid and liquid phases of water can exist in a state of equilibrium at 1 atmosphere of pressure and a temperature of

            (1) 0^oC       (2) 100^oC   (3) 273^oC        (4) 373^oC

2 Given the equation: H₂O(s) <--> H₂O(l) At which temperature will equilibrium exits when the atmospheric pressure is 1 atm?

            (1) 0 K     (2) 100 K    (3) 273 K         (4) 373 K

3 The table below shows the normal boiling point of four compounds.

Which compound has the strongest intermolecular forces? (1) HF(l)          (2) CH₃Cl(l)    (3) CH₃F(l)         (4) HCl(l)

4 The vapor pressure of a liquid is 0.92 atm at 60^oC. The normal boiling point of the liquid could be (1) 35^oC   (2) 45^oC   (3) 55^oC  (4) 65^oC

5 As the temperature of a liquid increases, its vapor pressure  (1) decreases    (2) increases     (3) remains the same

6 As the pressure on the surface of a liquid decreases, the temperature at which the liquid will boil

            (1) decreases    (2) increases     (3) remains the same

7 A sample of water is heated from a liquid at 40^oC to a gas at 110^oC.

a) On the heating curve diagram provided above, label each of the following regions:

Liquid, only ; Gas, only; Phase change

b) For section QR of the graph, state what is happening to the water molecules as heat is added.

c) For section RS of the graph, state what is happening to the water molecules as heat is added.  

1/03

1 Base your answers on the graph below, which shows the vapor pressure curves for liquids A and B.

        1. What is the vapor pressure of liquid A at 70^oC? Your answer must include correct units.

        2. At what temperature does liquid B have the same vapor pressure as liquid A at 70^oC? Your answer must include correct units.

        3. Which liquid will evaporate more rapidly? Explain your answer in terms of intermolecular forces.

2 Which phase change results in the release of energy?

(1) H₂O(s) à H₂O(l)       (2) H₂O(s) à H₂O(g)       (3) H₂O(l) à H₂O(g)        (4) H₂O(g) à H₂O(l)

3 The graph below represents the heating curve of a substance that starts as a solid below its freezing point.

What is the melting point of this substance? (1) 30^oC (2) 55^oC  (3) 90^oC  (4) 120^oC

6/03

1 Which change is exothermic? (1) freezing of water (2) melting of iron  (3) vaporization of ethanol (4) sublimation of iodine

2 According to Reference Table H, what is the vapor pressure of propanone at 45^oC? (1) 22 kPa  (2) 33 kPa  (3) 70. kPa (4) 98 kPa

3 The freezing point of bromine is (1) 539^oC (2) -539^oC  (3) 7^oC (4) -7^oC

8/03

Base your answers to questions 59 through 62 on the information below.
Given the heating curve where substance X starts as a solid below its melting point and is heated uniformly:

59 Identify the process that takes place during line segment DE of the heating curve.
60 Identify a line segment in which the average kinetic energy is increasing.
61 Using "o" to represent particles of substance X, draw at least five particles as they would appear in the substance at point F.

62 Describe, in terms of particle behavior or energy, what is happening to substance X during line segment BC.

1/05

38 According to Reference Table H, what is the boiling point of ethanoic acid at 80 kPa?

                (1) 28°C (2) 100°C (3) 111°C (4) 125°C

Base your answers to questions 54 and 55 on the heating curve below, which represents a substance starting as a solid below its melting point and being heated at a constant rate over a period of time.

54 What is happening to the average kinetic energy of the particles during segment BC?

55 How does this heating curve illustrate that the heat of vaporization is greater than the heat of fusion?

6/05

17 In which process does a solid change directly into a vapor? 

                    (1) condensation (2) sublimation (3) deposition (4) solidification 

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