ENG_sgwhk04

# ENG_sgwhk04 - The statement of the first law of...

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1 The statement of the first law of thermodynamics Δ U = Q + W (closed system or d U = δ Q + δ W (closed system δ Q and δ W are process quantities tiny quantities, they could not be differentiated W 0 The work done by surroundings to the system (System gains energy in work form) W 0 The work done by system to its surrounding System loses energy in work form) Q 0 The system absorbs heat from its surroundings (System gains energy in form of heat) Q 0 The system releases heat to its surroundings (System loses energy in form of heat)

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2 Understanding the thermodynamic energy at a microscopic point of view U The energy inside the system T+V of all particles in the system T+V inside particles The internal energy is a state function and extensive quantity. For a process from A to B Δ U = U ( B ) -U ( A ) U Internal energy ( 内能） or Thermodynamic energy ( 热力学能 )
3 Enthalpy: Any uniform single phase system at equilibrium, the state function U + pV is defined as enthalpy of the system, and is denoted H 1 Enthalpy is an extensive property 2 Enthalpy is one of properties of a system at equilibrium. It is independent of process, and its change equals Q p only for a closed system undergoing an isobaric process. Δ H = H 2 -H 1 = U 2 +pV 2 - U 1 +pV 1 = Q p def HU p V +

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4 1 All heat absorbed by a system in a isobaric process without other work is used to increase the enthalpy; 2 The absolute value of enthalpy can not be determined since the internal energy is unknown; 3 In all processes the enthalpy has defined value, but does not equal to Q p ; 4 H differs from U for that it can be changed in an isolated system H 2 , O 2 H 2 O Δ H=U 2 -U 1 +p 2 V-p 1 V = V ( p 2 - p 1 )<0 Important points:
5 For any object (or system) the heat capacity is the heat absorbed when the temperature of the object has risen an unit degree. It is the thermodynamic response function . 2.4 Heat capacity ( 热容 ) The definition dT Q T T Q T C T T δ δ lim ) ( 0 0 = = Average C from T to T 0 unit J.K -1 -4 th lecture-

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6 Molar heat capacity T Q n T C d 1 def ) ( m δ unit: J.K -1 .mol -1 Isochoric molar heat capacity ( 等容摩尔热容 ) V V V V T U n T Q n n T C T C = = 1 d δ 1 ) ( def ) ( m , Fixed composition no chemical reaction no phase change closed system isochoric process W ′＝ 0 Definition δ Q = d U - δ W
7 Isobaric molar heat capacity( 等压摩尔热容 ) p p p p T H n T Q n n T C T C = = 1 d δ 1 ) ( def ) ( m , Fixed composition no chemical reaction no phase change closed system isobaric process W ′＝ 0 δ Q p = d H Definition

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8 1 C is not a state function of system and depends on process 2 C V,m ( T,V ), C p,m ( T,P ) are intensive properties of system, and relating to T p V Important points
9 The above equations are valid for calculation of Δ U Δ H in pure change of temperature of gas under conditions of constant-volume and

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ENG_sgwhk04 - The statement of the first law of...

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