Internal energy of the system

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

It is clear from the entire matter described above that in chemical systems conservation of energy is usually handled in terms of three quantities (i) work done by the system on the surroundings which is taken as plus (+) w or the work done by the surroundings on the system which is taken as (-) w (ii) heat gained by the system (+q) and heat lost by the system (-q) and (iii) the increase (+A U) or decrease (—A (7) in the internal energy of the system as a result of the process carried through. 

If this reaction is implemented, for instance, in a calorimeter, an amount of heat, q, will be released and an amount of work, u/, will be carried out by the expansion of the hydrogen gas along with other volume changes. According to the first law of thermodynamics, the change in the internal energy of the system can be written as... 

Calculate the expectation values of the coordinates Xi and of the internal energy of the system. 

The internal energy of the system is composed of the contributions from each species. Therefore it is appropriate to represent U as... 

The average internal energy of the system for a given composition is now... 

For liquids the PV term is negligible compared to the 1/ term, and we use the time rate of change of the enthalpy of the system instead of the internal energy of the system. 

The internal energy of a system is a state function —i.e., the variation of the internal energy of the system when passing from state A to state B does not depend on the reaction path but only on initial and final state conditions (cf appendix 2). 

According to the First Law of Thermodynamics, Aq = AE -1- w where q is heat, E is the internal energy of the system, and w is work done by the system. If no heat is added to the system (heat was transferred to maintain the initial temperatnre of 300K), then... 

If the volume of the system is kept constant when the heat is added to a system then no work is done by the system. Thus the heat absorbed by the system is used up completely to increase the internal energy of the system. Again if the pressure of the system is kept constant when the heat is supplied to the system then some work of expansion is also done by the system in addition to the increase in internal energy. Thus if at constant pressure, the temperature of the system is to be raised through the same value as at constant volume, then some extra heat is required for doing the work of expansion. Hence Cp >C,... 

Heat Content or Enthalpy. A thermodynamic property closely related to energy. It is defined by H = E + PV where E is the internal energy of the system, P is the pressure on the system and V is the volume of the system. Often it is used in differential form as in. AH = AE + PAV for a constant pressure process... 

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