The First law of thermodynamics
The First law of thermodynamics CHEM 3423
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This 4 page Class Notes was uploaded by Nicole Takam on Sunday October 4, 2015. The Class Notes belongs to CHEM 3423 at University of Oklahoma taught by Dr. Chaubin Mao in Summer 2015. Since its upload, it has received 103 views. For similar materials see Physical Chemistry in Chemistry at University of Oklahoma.
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Date Created: 10/04/15
First law of Thermodynamics The First law says that the energy of universe remains constant Concepts to understand State Function Certain macroscopic properties have fixed values for a particular state of the system For instance 1g of a vessel of water at T 25 degrees Celsius and P 1 bar Once the state of the system is specified by giving some values to the state functions values of the state functions are fixed That is if P T n are known then Volume is fixed These quantities specify the state of the system and are called state functions or state variables For example mass Pressure temperature volume etc Whenever at these conditions water is in the same state the energy of the molecules is the same When the state of any system is changed the state function depend on only the initial and the final states and not on the path taken Equilibrium state the state functions have constant values throughout the system The chemical mechanical properties and temperature must be constant and uniform in time For any system to be in equilibrium the Force exerted on the piston must exactly balance the Pressure P of the gas So F PA if F increases the gas is compressed If F decreases the gas expands Reversible process means maintaining equilibrium in an infinitely slow process For this process to occur Psystem Pext on time So if Psystemi Pext the process is irreversible First Law DeltaU the internal energy change of the system q heat absorbed by the system w Work done on the system U is contributed by kinetic motion of the molecules the potential energy due to the interaction between the molecules kinetic and potential energy of the nuclei and electrons within the individual molecules Sign convention Workdone by the system is positive and heat absorbed by the system is positive Workdone by the system is negative and heat lost by the system is negative U depends only on the state of system not on how the system achieved its particular state 0 For a particular change in state AU qw is independent of the way in which the change is brought about 0 Heat q and work w are not state functions because the change can be brought about by various divisions of the energy between heat and work only the sum qw is fixed Work can39t be a state function because it is proportional to the distance an object is moved which depends on the path used to go from the initial to the nal state 0 Heat is not a state function Different paths will result in different heats during the process 0 The thermodynamic properties of a system that are state functions are usually symbolized by capital letters T V P U and so on Thermodynamic properties that are not state functions are often described by lowercase letters q and W Whether or not a property is a state function is related to mathematical concept of exact and inexact differentials The definite integral of a state function such as U2 U1AU is an exact differential because it has a value of U2 U1AU which is independent of the path Reversible work Work clone in a reversible process where Force is increased by an in nitesimal amount dF when P constant Reversible work done on the system wav FMI Perv FIGURE 22 The reversible work done by a ocnslaim pressure P moving a pie an A simple way for a gas to be i constant pressure Is to have a vapor in equilibrium with its liquid t ilr s 39 r 39l 3 39 39v r r Area of cross 391 ion F PA wm F Wm PAW When P is not constant wrev P AV So dwrev PdV v2 V PdV V1 Wre For Free expansion expansion into vaccum Pext0 so Work done on system w 0 Expansion against a constant pressure P work done on system w P V2V1 PAV For the above equation Expansion AVgt0 then wlt0 Compression AVlt0 then wgt0 In genera I the work done by the system in a reversible expansion from A to B represents the maximum work that the system can perform in changing from A to B Looking the first law for different processes In Isochoric process Constant Volume AU q w So w P dV but since V is constant dV 0 and w 0 Therefore AU qV AU U2 U1 qV the internal energy change at constant volume is equal to the heat Increase decrease of internal energy of a system in a rigid container ie constant V Is equal to heat that is applied to system released from system in Isobaric Process Constant Pressure AUqwwPdV dqp dU PdV qp U2 U1 PV2 Vl U2 PV2 U1 PV1 H2 H1 qp AH heat supplied the increase in enthalpy at constant P If AH gt 0 heat is absorbed by the system endothermic AH lt 0 heat is lost to the surroundings exothermic Heat Capacities Heat capacity C the amount of heat required to raise the temperature of any substance by 1 K ie 10C K1 Speci c heat capacity the amount of heat required to raise the temperature of unit mass of a substance by 1 K ie 10C K1kg1 Molar heat capacity the amount of heat required to raise the temperature of 1 mol of a substance by 1 K ie 10C J K mol d v a U At Constant volume CV dT a T at constant volume Z dq 8 H Cp 5 d T a T at constant Pressure Z Relationship between CW and GP m qvlm Cg mT2 AU qprm CP HJTQ T1 AH lFor liquids and solids Definition of Enthalpy APv0 A lPV not appreciable as E AHFH H E U Fig dc dU P all For ideal gas A PW is appreciable Pv RT 1 dq mm m m 1quotquot m use 1 d1 diRT i dT T CPE d d C av 11quot 6T 1 quot EFl PFu f39u V Standard States Standard state of a pure substance is the form solid liquid gas in which it is most stable at T25 oC 29815K and P1 bar 105Pa eg 02 g C s graphite standard state of C If reaction involve only liquid and solids Solids and liquids APV negligible AH zAU lf reaction involves gas APV not negligible AH and AU differ signi cantly Standard molar enthalpy of formation Enthalpy change when the compound in its standard state is formed from the elements in their standard states For element it is zero For a Compound it iS not zero it iS AHH products Hreactants Equations to know At Constant P qp AH PVmRT n1 mol de nition of H UPV At Constant volume qV AU For adiabatic compression P1V1 A V P2V2 A V