Aim: How to measure the amount of heat released during a phase change

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Presentation transcript:

Aim: How to measure the amount of heat released during a phase change Do Now: Is ice melting an exothermic or endothermic process? Explain.

Heat of Solidification The amount of heat needed to change a liquid to a solid at constant temperature Hsolidification= - Hfusion q=mHsol The heat of solidification for water is -334 joules per gram

Heat of Condensation The amount of heat needed to convert a gas to a liquid at constant temperature. Hcondensation = -Hvaporization q=mHcon The heat of condensation for water is -2260 joules per gram

Practice What is the heat of solidification for 3.4 g of water? 2. How much energy is released to the environment by 50.0 grams of condensing water vapor? 3. Is freezing endothermic or exothermic? Explain.

Temperature (oC) H2O Gas Gas - Liquid Liquid Liquid - Solid Solid 130 100 Gas - Liquid 3 Temperature (oC) H2O Liquid 4 5 Liquid - Solid Solid -40 Heat Released

Cooling Curve The cooling curve is the reverse of the heating curve. The temperature of the gas is lowering; kinetic energy is decreasing. When gas changes to liquid (condensation), the temperature stays the same. There is a change in potential energy; potential energy is decreasing. 3. The temperature of the liquid is lowering; kinetic energy is decreasing.

Cooling Curve 4. When liquid changes to solid (solidification), the temperature remains the same. There is a change in potential energy; potential energy is decreasing. 5. The temperature of the solid is lowering; kinetic energy decreases.

Stearic Acid is a waxy solid at room temperature. It comes from animal and vegetable fats and oils. It is used as an ingredient in soap, candles, and cosmetics. A sample of stearic acid was placed in a heat resistant test tube and heated to 95 Celsius, which point the stearic acid was completely liquified. The test tube was placed in a beaker of ice water, and the temperature monitored until it reached 40 Celsius

Answer the following: 1. What is the freezing temperature of stearic acid? What is its melting temperature? 2. Between which two points is stearic acid freezing? ***3. Compare the melting temperature of stearic acid to the melting temperature of water. Which substance has stronger intermolecular forces? How do you know?