# L20 - E = Q W E = Q PV The internal energy of a system can...

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Dr. P. Lucas U of A MSE 110 Intro to thermo •F i r s t l a w : E = Q + W E = Q - P V The internal energy of a system can only be changed through transfer of heat or work from the surrounding. This implies conservation of energy. Energy is neither created nor destroyed, it is only transferred. For a heat transfer at constant V: E = Q V Enthalpy : H = E + PV For a heat transfer at constant P: H = Q P The enthalpy change H correspond to the heat transferred during a reaction at constant P, which is the case of most practical reaction. H = E + P V The total heat H absorbed at constant P is mostly used to increase the internal energy E of the system (kinetic and potential energy of the molecules) and a small part is used to push back against the ambient pressure (P V). Heat capacity : The heat capacity defines the T change of a material for a given heat transfer. The isothermal enthalpy change during a phase transition correspond to a peak in Cp which is used to detect transition temperature experimentally. SUMMARY FROM LAST CLASS P P dT dH C = V V dT dE C

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Dr. P. Lucas U of A MSE 110 Spontaneous process: INTRO TO THERMODYNAMICS During this process, the internal energy E= 3 / 2 kT is constant, hence T is constant and since PV=NRT, PV is also constant: P 1 V 1 =P 2 V 2 . The enthalpy change of that reaction is then: H= E+ (PV)=E 2 -E 1 +P 2 V 2 -P 1 V 1 = 0 + 0 H= E=0 This reaction proceed spontaneously and irreversibly even though it involves no change in internal energy and enthalpy. What is the driving force? We associate the spontaneity of that process with an increase in disorder. The driving force for that spontaneous process is an increase in disorder. 2
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L20 - E = Q W E = Q PV The internal energy of a system can...

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