Causes of Change – Calorimetry



Causes of Change – CalorimetryHow Does Energy Affect Change?Heat vs. TemperatureHEATTEMPERATUREDefinition:Depends on:Examples:Heat is energy and is measured in joules (J) or kilojoules (kj). The symbol for heat is ?H.How does heat energy travel? From _________ to _________, NEVER from _________ to _________.Examples:We always talk in terms of HEAT, not COLD.Heat of Physical ChangesRecall that one example of a physical change is when a substance goes through a phase change (solid to liquid, liquid to solid, liquid to gas, etc.)LEAST Energy“Middle” EnergyMOST ENERGY Least Heat “Middle” Heat Most HeatGiving Off HeatAbsorbing HeatWe can calculate how much heat __________ a substance absorbs __________ or gives off __________.We just need 3 categories of information:1.2.3. What happened to substance?Heat is…Sign of ?H…..It boiled…..Temp. change from 23oC to 38 oC…..It condensed…..It melted…..Temp. changed from 400 oC to 130 oC…..It frozeWhat happens to the temperature when a substance is changing phase (boiling, melting, etc.)?Categories 1 and 2 (at bottom of previous page) are easy to handle. About Category 3…Since all materials are unique, all materials have unique numbers that relate to heat energy.For example, pure water boils ___________________ at 100oC and melts ____________________ at 0oC.Other substances change phases at other specific temperatures.Examples:WATERWhen a substance changes from a solid to a liquid,a specific amount of energy per gram of substance must be absorbed by the substance from its surroundingsThis same amount of energy is released when the substancechanges from a liquid into a solid.Cf = 333 J/g for H2OWe call this value, which is constant for any particularsubstance, the heat of fusion. Cv = 2256 J/g for H2OWhat does the heat energy absorbed or released during a phase change depend on? 1. 2.The formula for calculating heat energy gained or lost during a phase change is:Example 1: A 65 g sample of ice at 0oC melts to become water at 0oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?+ 21, 645 JExample 2: A 154 g sample of water at 0oC freezes into ice at 0oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?What is the change in temperature?How many joules (J) of energy are needed?- 51,282 JWithin a phase, however, what happens when the substance…ABSORBS energy?RELEASES energy?For finding the heat energy absorbed or released within a phase (i.e., with a ____________________ change), what does the amount of heat depend on?1.2.3. The formula for calculating heat within a phase is:We call the constant Cp, the specific heat. For example:Cp (ice) 2.077 J/(g oC)Cp (water) 4.184 J/(g oC)Cp (steam) 1.87 J/(g oC)Example 3: A 35 g sample of ice foes from a temperature of –5oC to –23oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?- 1309 JExample 4: A 46 g sample of water goes from a temperature of 34 oC to 78 oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?+ 8460 JWe can visually sum up the energy changes experience by a substance using a diagram.Example 5: A 80 g sample of ice at –10oC turns into water at 55oC. Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?46,694 JExample 6: A 34 g sample of steam at 120oC turns into water at 4oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?-91,619 JExample 7: In an insulated container, 50 g of water (initially at 45 oC are mixed with 90 g of water (initially at 82oC). What is the final temperature of the system?68.8 oCCauses of Change – Calorimetry KEYHow Does Energy Affect Change?Heat vs. TemperatureHEATTEMPERATUREDefinition: Total kinetic energy of particles in a substanceAverage kinetic energy of particlesIn a substanceDepends on: 1) particles average kinetic energy1) particles average kinetic energy 2) number of particles (mass)Examples: Large beaker of warm water and small beaker have same temperature, but larger sample has More heat – it can melt more ice cubes.Heat is energy and is measured in joules (J) or kilojoules (kJ). The symbol for heat is ?H.How does heat energy travel? From HOT to COLD, NEVER from cold to hot.Examples: 1. You jump into cold swimming pool, your body heat leaves and you feel cold.(“cold” from water does not go into you.)Tap water, throw in ice cubes, heat from water goes into cold cubes; “cold” from cubes does not go into warm water.We always talk in terms of HEAT, not COLD. 50 g Ice Cube 50 g Water 50 g Steam “Less” Heat “More” Heat “Even More” HeatHeat of Physical ChangesRecall that one example of a physical change is when a substance goes through a phase change (solid to liquid, liquid to solid, liquid to gas, etc.)LEAST Energy“Middle” EnergyMOST ENERGY SOLID LIQUID GAS Least Heat “Middle” Heat Most Heat(solid liquid) MELTING (liquid gas) BOILINGFREEZING (liquid solid) CONDENSING (gas liquid)Giving Off HeatAbsorbing HeatWe can calculate how much heat ?H a substance absorbs (+)??H or gives off (-)??H .We just need 3 categories of information:1. MASS of substancem 2. beginning (Ti) and final (Tf) TEMPERATURE ?T = Tf - Ti 3. CONSTANTS associated with the substance Cp = specific heatWhat happened to substance?Heat is…Sign of ?H…..It boiledheat + H2O(l) H2O(g)Added (+)…..Temp. change from 23oC to 38 oCAdded (+)…..It condensedH2O(g) H2O(l) + heat Removed (-)…..It meltedheat + solid liquidAdded (+)…..Temp. changed from 400 oC to 130 oCRemoved (-)…..It frozeliquid solid + heatRemoved (+)What happens to the temperature when a substance is changing phase (boiling, melting, etc.)?It stays the same.Categories 1 and 2 (at bottom of previous page) are easy to handle. About Category 3…Since all materials are unique, all materials have unique numbers that relate to heat energy.For example, pure water boils and condenses at 100oC and melts and freezes at 0oC.Other substances change phases at other specific temperatures.Examples: Iron melts (and freezes) at… ~1520oCGold melts (and freezes) at… ~1040oCHelium boils (and condenses) at… ~ -268oCNeon boils (and condenses) at.. ~ -246oCWATER Heat from environment When a substance changes from a solid to a liquid, is absorbed by ice.a specific amount of energy per gram of substance ice at 0oCmust be absorbed by the substance from its surroundings HEAT water at 0oC Heat from water is HEAT released into freezer.This same amount of energy is released when the substancechanges from a liquid into a solid.(liquid solid)Cf = 333 J/g for H2OCf = heat of fusionWe call this value, which is constant for any particularsubstance, the heat of vaporization. (liquid gas)Cv = 2256 J/g for H2OWhat does the heat energy absorbed or released during a phase change depend on? 1. heat of fusion, Cf (or heat of vaporization, Cv) 2. mass of sampleThe formula for calculating heat energy gained or lost during a phase change is:Example 1: A 65 g sample of ice at 0oC melts to become water at 0oC. Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?+ 21, 645 JExample 2: A 154 g sample of water at 0oC freezes into ice at 0oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?What is the change in temperature? There is NO change in temperature.How many joules (J) of energy are needed?- 51,282 JWithin a phase, however, what happens when the substance…ABSORBS energy? Its temperature increases RELEASES energy? Its temperature decreasesFor finding the heat energy absorbed or released within a phase (i.e., with a temperature change), what does the amount of heat depend on?1. how big the temperature change is2. how big the sample is (mass)3. materials constantsThe formula for calculating heat within a phase is:We call the constant Cp, the specific heat. For example:Cp (ice) 2.077 J/(g oC)Cp (water) 4.184 J/(g oC)Cp (steam) 1.87 J/(g oC)Example 3: A 35 g sample of ice goes from a temperature of –5oC to –23oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?- 1309 JExample 4: A 46 g sample of water goes from a temperature of 34oC to 78oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?x = 8460 J+ 8460 JWe can visually sum up the energy changes experience by a substance using a diagram.Example 5: A 80 g sample of ice at –10oC turns into water at 55oC. Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?46,694 JExample 6: A 34 g sample of steam at 120oC turns into water at 4oC.Does the sample absorb or release energy?Will the sign for energy be (+) or (-)?How many joules (J) of energy are needed?-91,619 JExample 7: In an insulated container, 50 g of water (initially at 45oC are mixed with 90 g of water (initially at 82oC). What is the final temperature of the system?68.8oC ................
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