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General Enthalpy Question

  1. Nov 24, 2013 #1
    1. The problem statement, all variables and given/known data

    I am studying enthalpy and heat transfer and I have a question that has been bugging me.

    It is said the that the enthalpy change is the enthalpy of the products minus the enthalpy of the reactants. However, I understand that it is not possible to measure the total enthalpy of a system, so what exactly is meant when they say "enthalpy of the products"? Does it mean the total enthalpy of the products?!

    Thank you
     
  2. jcsd
  3. Nov 24, 2013 #2

    jfizzix

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    They mean standard enthalpy of products. Since we can't measure absolute enthalpy, we measure enthalpy relative to a standard state, in this case, pure elements. We can measure changes of enthalpy though, because at contant pressure, that's just the net heat of reaction.
     
  4. Nov 24, 2013 #3
    I'm not sure I follow.
     
  5. Nov 24, 2013 #4
    To elaborate on what jfizzix said, the enthalpy of the elements that comprise a given compound under standard conditions of temperature and pressure (usually 298K and 1 atm) are each taken to be zero. The enthalpy of the particular compound under standard conditions is then the heat of formation of the compound (change in enthalpy at constant standard temperature and pressure) when the elements react to form the compound. These heat of formation (enthalpy) values for many, many, compounds are tabulated in many references. The standard heat of reaction for reactions involving a group of reactant compounds is equal to the sum of the enthalpies of the products minus the sum of the enthalpies of the reactants (weighted in terms of the stoichiometric coefficients). If the reaction occurs at a different temperature, the enthalpies of the reacting and product compounds have to be modified using the heat capacity for each compound to get the enthalpy at temperature. Stuff like this is discussed in much greater detail in virtually all books that treat chemical thermodynamics.
     
  6. Nov 25, 2013 #5

    jfizzix

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    there is no way to measure absolute enthalpy just like there is no way to measure absolute energy. It depends on one's reference frame and so forth. What we can do is define a standard state, and look at the differences of the enthalpy relative to that standard state.
     
  7. Nov 25, 2013 #6
    So basically, because it is not possible to measure absolute enthalpy, we "assign" a standard state enthalpy of 0 to each element. The heat released by that element during a reaction is then taken to be its "enthalpy", correct?
     
  8. Nov 25, 2013 #7
    Not exactly. The enthalpies of all compounds and elements are referenced to the same standard state of temperature and pressure. The enthalpies of the elements in this standard state are taken to be zero (with no loss of generality), and the enthalpies of compounds are equal to the amounts of heat that must be added in forming the compound from its elements at the standard temperature and pressure. So compounds like water that are formed by an exothermic reaction between elemental hydrogen (actually molecular H2 is assigned zero enthalpy) and elemental oxygen (actually molecular O2 is assigned zero enthalpy) has a negative enthalpy at standard conditions.

    I recommend that you read the relevant sections of Introduction to Chemical Engineering Thermodynamics by Smith and Van Ness to learn all the particulars of how this all works.
     
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