The New Property: Entropy

Ch 7, Lesson B, Page 3 - The New Property: Entropy

• S is the symbol we use for our new state variable, entropy.
• Entropy is defined by the equation in the yellow box.  dS = dQ over T along a reversible path.
• Entropy is an extensive property and it has units such as kJ/K or Btu/oR.
• Just like enthalpy and internal energy, entropy has intensive forms as well: specific entropy and molar entropy.
• So, other than the fact that the definition of entropy involves differential, there isn’t anything unusual about it.
• So, how do we evaluate entropy ?
• Well, just like U and H, we are most interested in the change in the entropy of a system as it undergoes a process.  That’s what we call DS.
• Consider the process shown in this PV Diagram.
• When we integrate dS from state 1 to state 2, we get S2 – S1 or ΔS.
• This is no surprise, right ?  Whenever we integrate the EXACT differentials that are associated with state variables, we get the change in that state variable.  The integral of dH from 1 to 2 is H2 minus H1 or ΔH.
• OK, so the left-hand side of the equation is pretty straightforward.  What about the right-hand side ?
• If T wasn’t inside the integral, the left-hand side would be easy too.
• So, when does T pop out of the integral ?  Flip the page and let’s see.