Quiz Question

$\Delta S = \frac{q_{rev}}{T}$

Daniela Contreras 3E
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Quiz Question

I know its a little late but in what cases do I use each of the ''s'' equations

Kristienne Edrosolan
Posts: 19
Joined: Wed Nov 18, 2015 3:00 am

Re: Quiz Question

Hi Daniela,

S=KblnW is used for entropy on a molecular level. Kb stands for Boltzmann's constant and W stands for the number of ways of achieving a given energy state, microstates.

Delta S = qrev/T is used for isothermal reversible reactions and for phase changes (when heat goes into a system but the temperature remains constant). Thus for phase changes, qrev would be the enthalpy of vaporization, fusion, etc. and T would be the temperature of the melting or boiling point.

One can use delta S = n x R x lnV2/V1 when the reaction is isothermal but volume is changing or delta S = n x R x lnP1/P2 when the reaction is isothermal but the pressure is changing. (Note its P1 not P2 because pressure is inversely related to volume and thus the opposite of the first equation!)

Lastly, use DeltaS = n x C x lnT2/T1 for calculating the entropy when the reaction undergoes a change in temperature. The equation will tell you if the reaction is at constant volume or constant pressure. Thus if at constant volume, use C = Cv and for constant pressure, use C = Cp. As found on your periodic table, for a monatomic ideal gas, Cv = 3/2R and Cp = 5/2R.

Hope that helps!

Return to “Concepts & Calculations Using Second Law of Thermodynamics”

Who is online

Users browsing this forum: No registered users and 0 guests