PHASE CHANGE GRAPH ( ) Tro's Introductory Chemistry, Chapter 12 1

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.  H vap It requires 40.7 kJ of heat to vaporize one mole of water at 100 °C. Always endothermic.  Number is +.  H vap 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.  H condensation = −  H vaporization

Tro's Introductory Chemistry, Chapter 12 6 Heats of Vaporization of Liquids at Their Boiling Points and at 25 °C Liquid Chemical formula Normal boiling point, °C  H vap at boiling point, (kJ/mol)  H vap at 25 °C, (kJ/mol) Water H2OH2O Isopropyl alcohol C 3 H 7 OH Acetone C3H6OC3H6O Diethyl ether C 4 H 10 O

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.  H fus 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.  H crystal = -  H fus In general,  H vap >  H fus because vaporization requires breaking all attractive forces.

Tro's Introductory Chemistry, Chapter Heats of Fusion of Several Substances Liquid Chemical formula Melting point, °C  H fusion, (kJ/mol) Water H2OH2O Isopropyl alcohol C 3 H 7 OH Acetone C3H6OC3H6O Diethyl ether C 4 H 10 O

Determine the amount of energy needed to heat 200 g of water from -13 o C to 120 o C Tro's Introductory Chemistry, Chapter H vap = 40.7 kj/mol H fus = 6.02 kj/mol C ice = j/g o C C steam = j/g o C

ENERGY