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Science Forum

  1. Subscriber joe shmo
    Strange Egg
    31 Mar '11 02:32 / 1 edit
    below a table in my thermo book is given the standard molar chemical exergy (kJ/kmol) of selected substances at 298K and p_0

    The table contains Model I & Model II

    the difference between the models is

    "in model I p_0= 1.019 atm. This model attempts to impose a criterion that the reference environment be in equilibrium. The reference substances are determined assuming restricted chemical equilibrium for nitric acid and nitrates and unrestricted thermodynamic equilibrium for all other chemical components of the atmosphere, the oceans,and a portion of the Earths crust. The chemical composition of the gas phase of this model approximates the composition of the natural atmosphere."

    " in model II, p_0 =1 atm. in developing this model a reference substance is selected for each chemical element from among substances that contain the element being considered and that are abundantly present in the natural environment, even though the substances are not in completely mutual stable equilibrium. An underlying rationale for this approach is that substances found abundantly in nature have little economic value. On an overall basis, the chemical composition of the exergy reference environment of model II is closer than model I to the composition of the natural environment, but the equilibrium criterion is not always satisfied."

    so, is the a certain model that should be used in a given situation based on the notes above?
  2. 11 Apr '11 13:02
    Model one is a theoretical equilibrium given all components in chemical equilibrium. It is thus applicable to closed, controlled systems where we can maintain stable conditions.

    Model two allows for local conditions forcing the equilibrium away from the stable state, and is therefore applicable to atmospheric studies where we have a closed (almost) system but subject to considerable variations in terms of pollution, temperature and pressure.

    NB that it was a while since I studied chemistry so I might be a little off.
  3. Subscriber joe shmo
    Strange Egg
    11 Apr '11 18:10
    Originally posted by mortisdead
    Model one is a theoretical equilibrium given all components in chemical equilibrium. It is thus applicable to closed, controlled systems where we can maintain stable conditions.

    Model two allows for local conditions forcing the equilibrium away from the stable state, and is therefore applicable to atmospheric studies where we have a closed (almost) sys ...[text shortened]... re and pressure.

    NB that it was a while since I studied chemistry so I might be a little off.
    This is for an engineering thermodynamics course, so If you have done any serious studying on the models in chemistry I'll take your word for it.

    Thanks for the help.

  4. 16 Apr '11 03:39
    I don't remember anything like that in Thermo class.
    Is this a ChemE class you are taking or Chemistry?
  5. Subscriber joe shmo
    Strange Egg
    16 Apr '11 14:14
    Originally posted by mlprior
    I don't remember anything like that in Thermo class.
    Is this a ChemE class you are taking or Chemistry?
    Neither, its Applied Thermodynamics. The text is "fundementals of Engineering Thermodynamics" Im studying to be a mechanical engineer.