3 Enthalpy and Adiabatic Changes

3 Enthalpy and Adiabatic Changes - Enthalpy and Adiabatic...

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Enthalpy and Adiabatic Changes Sections 2.5-2.6 of Atkins (7th & 8th editions) Enthalpy Definition of Enthalpy Measurement of Enthalpy Variation of Enthalpy with Temperature Relation Between Heat Capacities Adiabatic Change Work of Adiabatic Change Heat Capacity and Adiabats Chapter 2 of Atkins: The First Law: Concepts Last updated: Sept. 30, 2009: figure on slide 8 has changed; previous one (from text) was incorrect
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Enthalpy H ' U % pV For a system that changes volu me , the internal energ y i s no t equ al to the h eat supplied , as for a fixed volume system Some energy supplied as heat to the system returns to surroundings as expansion work: dU < dq , because dU = dq + dw When heat is supplied to the system at a constant pressure (e.g., reaction containers open to atmosphere), another thermodynamic state function known as enthalpy , H , can be measured accurately: Change in enthalpy is equal to heat supplied to the system at constant pressure dH ' dq ) H ' q p Enthalpy is a convenient state function, since it lumps together changes in energy in the system as well as changes in energy resulting from volume changes.
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Enthalpy: Why does ) H = q p ? 1. Infinitessimal change in state of system: U changes to U + dU , p changes to p + dp , V changes to V + dV , so H = U + pV becomes H % dH ' ( U % dU ) % ( p % dp )( V % dV ) ' U % dU % pV % pdV % Vdp % dpdV 2. The product of two infinitessimal quantities, dpdV , disappears. Since H = U + pV , we write H % dH ' H % dU % % dH ' dU % % 3. Substitute in dU = dq + dw dH ' dq % dw % % 4.System is in mechanical equilibrium with surroundings at pressure p , so there is only expansion work ( dw = - pdV ) dH ' dq % 5. Impose condition that heating is done at constant pressure, so dp = 0 dH ' dq (constant p , w e = 0)
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Measurement of Change in Enthalpy, ) H An adiabatic bomb calorimeter or an
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3 Enthalpy and Adiabatic Changes - Enthalpy and Adiabatic...

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