Chapter 6: Thermochemistry: Energy Flow and Chemical Change
Chapter 6: Thermochemistry: Energy Flow and Chemical Change CH 121
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This 0 page Class Notes was uploaded by Amelia Notetaker on Tuesday December 1, 2015. The Class Notes belongs to CH 121 at University of Alabama - Huntsville taught by Pamela D Twigg (P) in Fall 2015. Since its upload, it has received 19 views. For similar materials see GENERAL CHEMISTRY I - 90514 - CH 121 - 02 in Chemistry at University of Alabama - Huntsville.
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Date Created: 12/01/15
Chapter 6 Lecture Notes 0 Transfer and interconversion of energy 0 Thermodynamics is the study of energy and its transformations 0 Thermochemistry a branch of thermo that deals with the heat involved in chemical and physical changes Energy 0 Ability to do work or transfer heat 0 Work energy used to cause an object that has mass to move 0 Heat energy used to cause the temperature of an object to rise The system and its surroundings o A meaningful study of any transfer of energy requires that we rst clearly de ne both the system and its surroundings System and surroundings universe 0 The heat and energy transformations studied by thermo take place in a system de ned by the investigator connected to the surroundings rest of universe 0 The internal energy E of a system is the sum of the potential energy and kinetic energies of all the particles present 0 The total energy of the universe remains constant a charge in E of the system must be accompanied by an equal and opposite change in the energy of the surroundings Energy classi cation 0 The internal energy E of an system is the sum of the potential energy and kinetic energy of all the particles present in the system Internal energy of a chemical system depends on c Number of particles Type of particles 0 Temperature 0 Internal energy E o By de nition the change in internal energy delta E system is the nal energy of the system minus the initial energy 0 Delta E E nal Einitial Measuring E is not generally possible o The higher the temperature the higher the E 0 So we use changes in temperature delta T to monitor delta E Energy changes 0 Lower energy states are more stable and are favored over high energy states 0 Energy is neither created nor destroyed lts conserved Can be converted from 1 form to another 0 Potential and kinetic energy 0 Potential energy of an object possessed by virtue of its position or chemical composition 0 Chemical energy resulting from attractions of electrons and nuclei molecules Gravitational energy such as holding an object above the floor Electrostatic energy positive and negative ions a small distance apart 0 Kinetic energy an object possessed by virtue of its motion KE 12mvquot2 Thermal energy atoms and molecules 0 Mechanical energy macroscopic objects like a moving tennis ball 0 Electric energy electrons moving through conductor 0 Sound energy correspond to compression and expansion of the spaces between molecules 0 Different forms of energy can be interconverted Units of energy 0 1 calorie heat required to raise temperature 0 100 g of H20 by 1 degree C o 1000 cal 1 kilocalorie 1 kilacal o 1 kcal 1 calorie a quotfood caloriequot 0 Use unit called the Joule 1 cal 4184 joules o 1 1 kgmquot2squot2 First law of thermo O 0 Total energy in universe Eu is neither created nor destroyed Eu Esystem Esurroundings gt constant Energy and chemistry 0 O 0 All of thermo depends on the law of conservation of energy Total energy is unchanged in chemical reactions If potential energy of products is less than reactants the difference must be released as kinetic energy Changes in internal energy 0 Work Heat If delta Esystem gt 0 E nal gt E initial 0 System absorbed energy Endergonic or endothermic If deltaEsystem lt 0 E nal lt E initial 0 System released energy Exergonic or exothermic When energy is exchanged between the system and the surroundings it s exchanged as either work w or heat q 0 Delta Esystem qw Energy used to move an object over some distance W force distance When a process occurs in an open container commonly the only work done is a change in volume of gas pushing on the surrounding or being pushed on by the surrounding Can measure the work done by the gas if the reaction is done in a vessel that has been tted with a piston W P delta V Energy can also be transferred as heat 0 Causes temperature of object to rise 0 Heat ows from warmer to cooler 0 Heat is not the same as temperature 0 Delta Esystem q w and their signs 0 Q positive gains heat Negative loses heat 0 W positive done on the system Negative done by the system 0 deltaE positive gains energy Negative loss of energy Directionality of heat transfer 0 Heat always transfers from hotter objects to cooler o Exothermic heat transfer from system to surroundings Energy change of a system can take different paths 0 Even though q and w for the 2 paths are different the total deltaE is the same 0 State functions 0 Property of a system that is determined by specifying the system39s condition or state The value of a state functions depends only on the present state of the system not the path system took to reach the state 0 Usually no way of knowing the internal energy of system nding value too complex 0 E is independent of the path state function 0 Depends only on present state of system not the path taken deltaE depends only on Einitial and E nal Speci c heat capacity 0 Related to sample mass change in temperature and property of the material 0 Speci c heat heat lost or gained JmassT change in K 0 Heat transfer 0 No change in state 0 Q transferred speci c heatmassdeltaT Chemical reactivity o What drives chemical reaction and how Thermo and kinetics 0 Driving forces of productfavored reactions 0 Formation of precipitate Gas formation H20 formation acidbase reaction 0 Energy transfer allows us to predict reactivity Exothermic productfavored Enthalpy o deltaE qw Most common chemistry at work is PV work 0 Work done when the volume of a system changes in the presence of an external pressure 0 Enthalpy H is de ned as E PV deltaH is about equal to deltaE o Occur at constant P P qpdeltaH DeltaH H nal Hinitial o H nal gt Hinitial deltaH is positive gt endothermic o H nal lt Hinitial deltaH is negative gt exothermic o detaHf degree standard moar enthalpy of formation When I moe compound formed under standard conditions Zero for elements in standard states 0 deltaHdegreesrxn sum of deltaHfdegrees products sum of deltaHfdegrees reactants Hess39s Law 0 Adding deltaH of different compounds andor elements to get the total enthalpy of a reaction
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