PHASE CHANGE GRAPH(12.4-12.5)

Tro's Introductory Chemistry, Chapter 12 1

Tro's Introductory Chemistry, Chapter 12 2

Temperature and Boiling• As you heat a liquid, its

temperature increases until it reaches its boiling point.

• Once the liquid starts to boil, the temperature remains the same until it all turns to a gas.

• All the energy from the heat source is being used to overcome all of the attractive forces in the liquid.

Tro's Introductory Chemistry, Chapter 12 3

Energetics of Evaporation• As it loses its high energy molecules through

evaporation, the liquid cools.• Then the liquid absorbs heat from its surroundings to

raise its temperature back to the same as the surroundings.

• Processes in which heat flows into a system from the surroundings are said to be endothermic.

• As heat flows out of the surroundings, it causes the surroundings to cool.As alcohol evaporates off your skin, it causes your skin to

cool.

Tro's Introductory Chemistry, Chapter 12 4

Energetics of Condensation• As it gains the high energy molecules through

condensation, the liquid warms.• Then the liquid releases heat to its surroundings to

reduce its temperature back to the same as the surroundings.

• Processes in which heat flows out of a system into the surroundings are said to be exothermic.

• As heat flows into the surroundings, it causes the surroundings to warm.

Tro's Introductory Chemistry, Chapter 12 5

Heat of Vaporization• The amount of heat needed to vaporize one mole of a

liquid is called the heat of vaporization.Hvap

It requires 40.7 kJ of heat to vaporize one mole of water at 100 °C.Always endothermic.

Number is +.

Hvap depends on the initial temperature.• Since condensation is the opposite process to evaporation,

the same amount of energy is transferred but in the opposite direction.Hcondensation = −Hvaporization

Tro's Introductory Chemistry, Chapter 12 6

Heats of Vaporization of Liquidsat Their Boiling Points and at 25 °C

LiquidChemicalformula

Normal boiling

point, °C

Hvap at boiling point,

(kJ/mol)

Hvap at 25 °C,

(kJ/mol)

Water H2O 100 +40.7 +44.0

Isopropyl alcohol

C3H7OH 82.3 +39.9 +45.4

Acetone C3H6O 56.1 +29.1 +31.0

Diethyl ether

C4H10O 34.5 +26.5 +27.1

Tro's Introductory Chemistry, Chapter 12 7

Temperature and Melting• As you heat a solid, its temperature

increases until it reaches its melting point.

• Once the solid starts to melt, the temperature remains the same until it all turns to a liquid.

• All the energy from the heat source is being used to overcome some of the attractive forces in the solid that hold them in place.

Tro's Introductory Chemistry, Chapter 12 8

Energetics of Melting and Freezing• When a solid melts, it absorbs heat from its

surroundings, it is endothermic.• As heat flows out of the surroundings, it causes the

surroundings to cool.As heat flows out of your drink into the ice cubes (causing

them to melt), the liquid gets cooler.

• When a liquid freezes, it releases heat into its surroundings, it is exothermic.

• As heat flows into the surroundings, it causes the surroundings to warm.Orange growers often spray their oranges with water when a

freeze is expected. Why?

Tro's Introductory Chemistry, Chapter 12 9

Heat of Fusion• The amount of heat needed to melt one mole of a solid

is called the heat of fusion.Hfus

Fusion is an old term for heating a substance until it melts, it is not the same as nuclear fusion.

• Since freezing (crystallization) is the opposite process of melting, the amount of energy transferred is the same, but in the opposite direction.Hcrystal = -Hfus

• In general, Hvap > Hfus because vaporization requires breaking all attractive forces.

Tro's Introductory Chemistry, Chapter 12 10

Heats of Fusion of Several Substances

LiquidChemicalformula

Melting point, °C

Hfusion, (kJ/mol)

Water H2O 0.00 6.02

Isopropyl alcohol C3H7OH -89.5 5.37

Acetone C3H6O -94.8 5.69

Diethyl ether C4H10O -116.3 7.27

Determine the amount of energy needed to heat 200 g of water

from -13oC to 120oC

Tro's Introductory Chemistry, Chapter 12 11

Hvap = 40.7 kj/molHfus = 6.02 kj/molC ice= 2.092 j/goCCsteam = 2.008 j/goC

200

120

100

0

-13

ENERGY