大学化学课件-国外原版教材.ppt
ThermochemistryChapter 5David P.WhiteUniversity of North Carolina,WilmingtonCopyright 1999,PRENTICE HALL1Chapter 5The Nature of EnergyKinetic and Potential EnergyFrom Physics:Force is a push or pull on an object.Work is the product of force applied to an object over a distance:w=F dEnergy is the work done to move an object against a force.Kinetic energy is the energy of motion:Copyright 1999,PRENTICE HALL2Chapter 5The Nature of EnergyKinetic and Potential EnergyPotential energy is the energy an object possesses by virtue of its position.Potential energy can be converted into kinetic energy.Example:a ball of clay dropping off a building.Copyright 1999,PRENTICE HALL3Chapter 5The Nature of EnergyEnergy UnitsSI Unit for energy is the joule,J:We sometimes use the calorie instead of the joule:1 cal=4.184 J(exactly)A nutritional Calorie:1 Cal=1000 cal=1 kcalCopyright 1999,PRENTICE HALL4Chapter 5The Nature of EnergySystems and SurroundingsSystem:part of the universe we are interested in.Surroundings:the rest of the universe.Copyright 1999,PRENTICE HALL5Chapter 5First Law of ThermodynamicsInternal EnergyInternal Energy:total energy of a system.Cannot measure absolute internal energy.Change in internal energy,E=Efinal-EinitialCopyright 1999,PRENTICE HALL6Chapter 5Relating E to Heat and WorkEnergy cannot be created or destroyed.Energy of(system+surroundings)is constant.Any energy transferred from a system must be transferred to the surroundings(and vice versa).From the first law of thermodynamics:when a system undergoes a physical or chemical change,the when a system undergoes a physical or chemical change,the change in internal energy is given by the heat added to or change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the absorbed by the system plus the work done on or by the system:system:E E=q q+w w First Law of ThermodynamicsCopyright 1999,PRENTICE HALL7Chapter 5Relating E to Heat and WorkFirst Law of ThermodynamicsCopyright 1999,PRENTICE HALL8Chapter 5Relating E to Heat and WorkFirst Law of ThermodynamicsCopyright 1999,PRENTICE HALL9Chapter 5Endothermic and Exothermic ProcessesEndothermic:absorbs heat from the surroundings.Exothermic:transfers heat to the surroundings.An endothermic reaction feels cold.An exothermic reaction feels hot.First Law of ThermodynamicsCopyright 1999,PRENTICE HALL10Chapter 5State FunctionsState function:depends only on the initial and final states of system,not on how the internal energy is used.First Law of ThermodynamicsCopyright 1999,PRENTICE HALL11Chapter 5State FunctionsFirst Law of ThermodynamicsCopyright 1999,PRENTICE HALL12Chapter 5Enthalpy,H:Heat transferred between the system and surroundings carried out under constant pressure.Can only measure the change in enthalpy:H=Hfinal-Hinitial=qPEnthalpyCopyright 1999,PRENTICE HALL13Chapter 5For a reaction Hrxn=H(products)-H(reactants)Enthalpy is an extensive property(magnitude H is directly proportional to amount):CH4(g)+2O2(g)CO2(g)+2H2O(g)H=-802 kJ2CH4(g)+4O2(g)2CO2(g)+4H2O(g)H=-1604 kJWhen we reverse a reaction,we change the sign of H:CO2(g)+2H2O(g)CH4(g)+2O2(g)H=+802 kJChange in enthalpy depends on state:H2O(g)H2O(l)H=-88 kJEnthalpies of Reaction Copyright 1999,PRENTICE HALL14Chapter 5Heat Capacity and Specific HeatCalorimetry=measurement of heat flow.Calorimeter=apparatus that measures heat flow.Heat capacity=the amount of energy required to raise the temperature of an object(by one degree).Molar heat capacity=heat capacity of 1 mol of a substance.Specific heat=specific heat capacity=heat capacity of 1 g of a substance.q=(specific heat)(grams of substance)T.Be careful of the sign of q.Calorimetry Copyright 1999,PRENTICE HALL15Chapter 5Heat Capacity and Specific HeatCalorimetry Copyright 1999,PRENTICE HALL16Chapter 5Constant-Pressure CalorimetryAtmospheric pressure is constant!H=qPqrxn=-qsoln=-(specific heat of solution)(grams of solution)T.Calorimetry Copyright 1999,PRENTICE HALL17Chapter 5Bomb Calorimetry(Constant-Volume Calorimetry)Reaction carried out under constant volume.Use a bomb calorimeter.Usually study combustion.Calorimetry Copyright 1999,PRENTICE HALL18Chapter 5qrxn=-CcalorimeterT.Bomb Calorimetry(Constant-Volume Calorimetry)Calorimetry Copyright 1999,PRENTICE HALL19Chapter 5Hesss Hesss lawlaw:if a reaction is carried out in a number of steps,H for the overall reaction is the sum of H for each individual step.For example:CH4(g)+2O2(g)CO2(g)+2H2O(g)H=-802 kJ2H2O(g)2H2O(l)H=-88 kJCH4(g)+2O2(g)CO2(g)+2H2O(l)H=-890 kJHesss Law Copyright 1999,PRENTICE HALL20Chapter 5In the above enthalpy diagram note that H1=H2+H3Hesss Law Copyright 1999,PRENTICE HALL21Chapter 5Enthalpies of FormationIf 1 mol of compound is formed from its constituent elements,then the enthalpy change for the reaction is called the enthalpy of formation,Hof.Standard conditions(standard state):1 atm and 25 oC(298 K).Standard enthalpy,Ho,is the enthalpy measured when everything is in its standard state.Standard enthalpy of formation:1 mol of compound is formed from substances in their standard states.If there is more than one state for a substance under standard conditions,the more stable one is used.Copyright 1999,PRENTICE HALL22Chapter 5Enthalpies of FormationStandard enthalpy of formation of the most stable form of an element is zero.Copyright 1999,PRENTICE HALL23Chapter 5 Hrxn=H1+H2+H3Enthalpies of FormationUsing Enthalpies of Formation to Calculate Enthalpies of ReactionWe use Hess Law to calculate enthalpies of a reaction from enthalpies of formation.Copyright 1999,PRENTICE HALL24Chapter 5Enthalpies of FormationUsing Enthalpies of Formation to Calculate Enthalpies of ReactionFor a reaction:Copyright 1999,PRENTICE HALL25Chapter 5Foods and FuelsFoods Fuel value=energy released when 1 g of substance is burned.1 nutritional Calorie,1 Cal=1000 cal=1 kcal.Energy in our bodies comes from carbohydrates and fats(mostly).Intestines:carbohydrates converted into glucose:C6H12O6+6O2 6CO2+6H2O,H=-2816 kJFats break down as follows:2C57H110O6+163O2 114CO2+110H2O,H=-75,520 kJFats:contain more energy;are not water soluble,so are good for energy storage.Copyright 1999,PRENTICE HALL26Chapter 5Foods and FuelsFoods Copyright 1999,PRENTICE HALL27Chapter 5Foods and FuelsFuels U.S.:1.0 x 106 kJ of fuel per day.Most from petroleum and natural gas.Remainder from coal,nuclear,and hydroelectric.Fossil fuels are not renewable.Copyright 1999,PRENTICE HALL28Chapter 5Foods and FuelsFuels Fuel value=energy released when 1 g of substance is burned.Hydrogen has great potential as a fuel with a fuel value of 142 kJ/g.Copyright 1999,PRENTICE HALL29Chapter 5End of Chapter 5ThermochemistryCopyright 1999,PRENTICE HALL30Chapter 5
