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Thermodynamics and Equilibirum

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University of Guelph
CHEM 1050
Dennis Baker

Chapter 18 Thermodynamics and EquilibriumThermodynamics is the study of the relationship between heat and other forms of energy involved in a chemical or physical process 181 First Law of Thermodynamics EnthalpyNote the change in enthalpy equals the heat of reaction at constant pressureFirst Law of Thermodynamics essentially the law of conservation of energy applied to thermodynamic systemsInternal energy U the sum of the kinetic and potential energies of the particles making up the system The kinetic energy includes the energy of motion of elections nuclei and molecules The potential energy results from the chemical bonding of atoms and from the attractions between molecules Internal energy is a state function a property of a system that depends only on its present state which is completely determined by variables such as temperature and pressure When a system changes from one state to another its internal energy changes from one definite value to another You can calculate the change in internal U from the initial value of the internal energy U and the final value of the iinternal U UUU Usually you measure the changes in internal energy exchanges fifof energy between the system and its surrounding 2 kinds of exchanges heat and work Heat is energy that moves into or out of the system because of a temperature difference between the system and its surroundings Work is the energy exchange that results when a force F moves an object through a distance d WFdEX Supposed the temperature of the surroundings is raised and as a result heat passes from the surroundings to the vessel If you find that the energy of the surroundings decreases by 165 J in this way you know from the law of conservation of energy that the internal energy of the system must have increase by 165 J ThusAs the q165 Jtemperature of the gas in the vessel increases the gas pressure increases at fixed volume In lifting the weight the system does work The energy gained by the weight equals the force of gravity on the weight times the height to which the weight was raised Suppose this energy is 92 J Because the surroundings which include the weight have gained 92 J of energy the system must have lost 92 J of energy You write W92 J adhering to the sign convention that work done on the system is positive energy is added to the system and work done by the system is negative energy is subtracted from the system First law of Thermodynamics Net change of internal energy is heat plus work UqWHeat of Reaction and Internal Energy As hydrogen is evolved work must be done by the system to push back the atmosphere Image that the atmosphere is replaced by a piston and weights whose downward force from gravity F creates a pressure on the gas equivalent to that of the atmosphere WPV Thus you can calculate the work done by a chemical reaction carried out in an open vessel by multiplying the atmospheric pressure P by the change in volume of the chemical system V UqW q PVppEnthalpy and Enthalpy Change
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