October 24 Objectives Describe what happens with PE and KE as phase changes Identify MP and BP from heating curve MP=FP and PB=condensation point Define heat of fusion and heat of vaporization Calculate the Hfus and Hvap from the heating curves Cooling curves

States of Matter

States of Matter .

Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during the phase change.

Types of energy POTENTIAL ENERGY : STORED ENERGY. The energy inside the substance. KINETIC ENERGY : Associated with motion. Average KE = TEMPERATURE

Endothermic Phase Changes If the substance is melting or boiling, heat is being absorbed, and is being used to change the state of matter. THE AVERAGE KINETIC ENERGY DOES NOT CHANGE!!! THE POTENTIAL ENERGY INCREASES.

Melting Point and Freezing Point The temperature at which a substance melts. Is the same temperature at which the substance freeze. Boiling Point and Condensation Point are the same temperature. Normal boiling point is the temperature at which a liquid boils at normal pressure.

Exothermic changes If the substance is undergoing condensation or freezing then heat energy is being released. The potential energy is decreasing and the TEMPERATURE REMAINS CONSTANT!!!

Problems How much heat is needed to completely melt 10 g of ice at 0 0 C ? How much heat is needed to vaporize 10 g of water at 100 0 C ?

VAPORIZATION IS ENDOTHERMIC In hot climates drinking water is cooled by evaporating water from the surfaces of porous clay pots. As water evaporates it ABSORBS heat from the water inside the container which is maintained cool. Like cooling yourself off on a hot day by pouring water over your body. As water evaporates it absorbs heat

FREEZING IS EXOTHERMIC In freezing weather, citrus crops are sprayed with water to protect the fruit from frost damage. As the water freezes (around the fruit-outside the fruit!) it releases heat, which helps to prevent the fruit from freezing.

October 21 How to calculate the amount of heat that a substance absorbs while it is being heated up. DO NOW 1 .-Calculate the amount of heat needed to increase the temperature of 200g of water from 0 0C to 100 0C 2.-How much heat is needed to completely vaporize 200 g of water at 100 0C

q (amount of heat ) table T q = m C D T

Heat of Fusion Hfus Amount of heat needed to completely melt 1 gram of substance at its melting point. At the melting point the heat is calculated Heat = mass x Hfus

Heat of Vaporization Amount of heat needed to completely boil off 1 gram of substance at its boiling point. At the boiling point the heat is calculated Heat = mass x Hvap