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Energy and Phase Changes (part of Ch.1) Energy – ability to do work Kinetic Energy – energy of motion Potential Energy (PE) – stored energy Examples of Energy: heat, light, electricity Law of Conservation of Energy: - energy may be changed from one form to another, but the total amount is the same - energy is never destroyed - energy is absorbed or given off in chemical reactions - exothermic reaction: energy is GIVEN OFF - endothermic reaction: energy is ABSORBED Measuring Energy: - measured in joules - 1 kilojoule = 1,000 joules Formula to solve heat/calorie problems: - Calorie is another unit to describe heat energy q=mCΔT heat (in joules) = grams x specific heat x ΔT Note: specific heat of water is 4.18 joules/gramC (which means that it takes 4.18 joules to raise one gram of water 1 C) Ex: How much heat energy in joules is absorbed by 30 grams of water (H2O) when it is heated from 20C to 30C? Temperature: - a measure of the average kinetic energy of molecules - higher temperature means molecules have MORE kinetic energy - lower temperature means molecules have LESS kinetic energy RULE: heat flows from a body at higher temperature to a body at lower temperature until both temperatures are the same. Thermometers: - used to measure temperature - two fixed points on a Celsius thermometer - Boiling Point - Freezing Point 0˚ C = freezing point of water (a.k.a.: ice-water equilibrium temperature) 100˚ C = boiling point of water (a.k.a.: water-steam equilibrium temperature) Kelvin Temperature (Absolute Temperature): Kelvin = ˚C + 273 Ex: What is the Kelvin temperature for 0˚C? Ex: What is the Kelvin temperature for 20o C? Ex: Convert 100oC to Kelvin Heating Curve Diagram: - Heat is added at a uniform rate. - endothermic reactions/process 5 4 100 Temp (C) 3 2 0 1 Time (minutes) 1: Solid; increase in temperature means increase in Kinetic Energy (KE), Potential Energy (PE) remains constant. 2: Melting - Heat of Fusion (heat needed to change solid into liquid) - Temperature stays the same (no change in KE) - Increase in PE - endothermic reaction 3. Liquid; increase in temperature means increase in KE PE remains constant 4. Boiling - Heat of Vaporization (heat needed to change liquid into gas) - Temperature stays the same (no change in KE) - Increase in PE - endothermic reaction 4. Gas: increase in temperature means increase in KE PE remains constant Heat of Fusion: - the heat that ice takes in/absorbs (334 joules per gram) to become water - the amount of heat needed to change a solid to a liquid at constant temperature - while ice is melting, the temperature (average KE of molecules) stays the same - Opposite properties when cooling/ releasing energy Heat of Vaporization: - the heat that water takes in/absorbs (2260 joules per gram) to become water vapor - amount of heat needed to change liquid to gas at constant temperature - while water is boiling, the temperature (average KE of molecules) stays the same - Opposite properties when cooling/ releasing energy Solve: Ex. How much heat energy is needed to melt 100g of ice that is at the melting point? Ex. What is the Heat of Vaporization of a substance that takes 410 joules to boil 20 grams of the substance? Cooling Curve : - reverse of the heating curve - Heat is taken away at a uniform rate. - exothermic reactions/process Let’s Draw a cooling curve: For water: - gas changes to liquid and gives off 2260 joules of heat per gram (condensation) - liquid changes to solid and gives off 334 joules of heat per gram (freezing) Lower Plateau: Melting-Freezing Point: - solid and liquid are at equilibrium - temperature at which a solid changes to a liquid if heating/adding energy - temperature at which a liquid changes to a solid if cooling/removing energy Higher plateau = condensation/boiling point - liquid and gas are at equilibrium - Temperature at which a liquid changes to a gas if heating/adding energy - Temperature at which a gas changes to a liquid if cooling/ removing energy Intermolecular force and Hf/ Hv - The stronger the IMF the higher the Hf and Hv Sublimation: - change from a solid to gas without passing through a liquid phase - Ex: dry ice (CO2) and Iodine - Ice and snow can sublime from energy provided by the sun. Deposition: - change from gas to solid without passing through liquid phase. - Ex. Snow from water vapor. Evaporation: - water changes into a gas - Different from boiling because evaporation occurs on the surface of the liquid, when boiling vapor/gas is formed throughout the liquid. Boiling-Condensation Point: - water boils when vapor pressure equals atmospheric pressure (101.3 kPa or 1 atm) - Boiling Point of Water = 100C at 101.3 kPa (see Ref. Table H): - Vapor Pressure (101.3 kPa) = Atmospheric Pressure (101.3 kPa) Vapor Pressure: - in a closed container, the pressure exerted on the container and on the surface of the liquid by the gas phase of a liquid (water) - With an increase in water temperature, there is an increase in vapor pressure. - The weaker the intermolecular forces, the higher the vapor pressure Phase Change Animation: http://mutuslab.cs.uwindsor.ca/schurko/animations/waterp hases/status_water.htm