PChem312 f20 w13

Monday, Nov 9, 2020

We will now highlight how to calculate entropy (ΔS) for four processes involving ideal gases:

1) Reversible Isothermal Expansion or Compression

V_iT_i --> V_fT_i

2) Constant P, V temperature change

V_iT_i --> V_iT_f

P_iT_i --> P_iT_f

3) Changes in both V and T, constant P

V_iT_i -->V _fT_f

4) Changes in both P and T, constant V

V_iT_i -->V _fT_f

Example Problem 1

(Please write out your complete solution and email to bes by next class period)

Consider a gas undergoing the following change under constant pressure (1.00 bar) conditions; calculate the change in entropy (ΔS): V_i = 10.0 L; T_i = 298 K; T_f = 398 K

Example Problem 2

(Please write out your complete solution and email to bes by next class period)

Consider a gas undergoing the following change under constant volume (10.0 L) conditions; calculate the change in entropy (ΔS): P_i = 1.00 bar; T_i = 298 K; T_f = 398 K

Wednesday, Nov 11, 2020

Review contributions to ΔS due to ΔT, ΔV, ΔP for ideal gas/closed system.

Entropy of Phase Changes...

...continuing numbering from above...

5) Solid --> Liquid phase change (fusion/melting/freezing)

- under constant pressure (ΔH)

- under constant volume (ΔU)

For processes involving solids and liquids, ΔH ~ ΔU, hence ΔS is the same for both constant P and constant V.

6) Liquid --> gas phase change (vaporization/boiling/condensation)

- under constant pressure (ΔH)

- under constant volume (ΔU)

For processes involving gases, ΔH ≠ ΔU, hence ΔS is not the same for constant P and constant V processes.

At constant P;

At constant V;

where,

and Δn is the change in moles of gas (Products - reactants; ie. increase in the number of moles of gas)

Example Problem 3

(Please write out your complete solution and email to bes by next class period) Poly aromatic hydrocarbons (<-- wiki link here) represent and interesting set of compounds.

Calculate the entropy of fusion for 5 PAHs.

Example Problem 4

(Please write out your complete solution and email to bes by next class period) Calculate the entropy of vaporization for methanol, ethanol, and propanol under both constant pressure and constant volume conditions.