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Chemistry LibreTexts

21.1: Entropy Increases With Increasing Temperature

  • Page ID
    14476
  • Entropy versus temperature

    S&McQ
    853

    We can put together the first and the second law for a reversible process with no other work than volume work and obtain:

    dU= δqrev + δwrev
    δqrev= TdS
    δwrev= -PdV

    Thus:

    dU= TdS -PdV for reversible changes

    This is a very interesting expression because we no longer have any path functions in it, as U, S and V are all state functions. This means this expression must be an exact differential.

    (We can generalize the expression to hold for irreversible processes, but then it become an inequality

    dU≤ TdS - PdV

    We will develop this later, because we still have an item on the wish list)

    Natural variables

    S&McQ
    897

    Consider:

    dU= TdS -PdV

    This equality expresses U in two variables U(S,V). They play a special role and are called the natural variables of U.

    Entropy and heat capacity

    However, there is nothing to stop us from expressing U in other variables (see example 21-1), e.g. T and V. If fact we can derive some interesting relationships if we do:

    S&McQ
    854
    Image:CH431_Image80.gif
    1. We write U out in T and V
    2. We write U in its natural variables
    3. We rearrange 2) to find an expression for dS
    4. We substitute 1) into 3) and rearrange
    5. This is the definition of Cv
    6. We write out S in T and V

    Comparing the three bottom formulas it becomes clear that

    Image:CH431_Image81.gif

    A less elegant expression for the partial derivative of S versus V is also found.

    We can play the same game for our other state function H= U + PV

    As dH= dU +d(PV)= dU + PdV + VdP

    we find for the reversible case

    dH = dU + PdV + VdP= TdS -PdV + PdV + VP = TdS + VdP

    The natural variables of the enthalpy are therefore S and P (not: V).

    A similar derivation as above departing from the enthalpy shows that the temperature change of entropy is related to the constant pressure heat capacity:

    Image:CH431_Image82.gif

    This means that if we know the heat capacities as a function of temperature we can calculate how he entropy changes with temperature. Usually it is a lot easier to do this on basis of data that were obtained for P constant than for V constant, so that the route with Cp is the more common one.