Microstates entropy is also known as a measure of

Microstates
Entropy is also known as a measure of randomness.
A greater mess means a greater entropy.
General Rule: The more available microstates the higher the
entropy
S= klnW
k=Boltzmann constant
W= the number of microstates
A single microstate (ln1) means Entropy is 0!

ΔSsys = qsys/T
It takes 6.01 kJ of energy to melt 1mol of ice at 0 ° C
Therefore (1mol)(6.01*103J/mol)/273K = ΔSsys
=22 J/K
Heat flows into ice and therefore qsys is positive
Because thermodynamics says that when a system gains energy,
the surroundings must lose energy. Thus, you can make more
complex questions.
See next Slide.

Melting 1mol of ice at 0°C on a tabletop at 20°C. Calculate the total
entropy in this situation.

Spontaneous Processes must mean an increase in the
entropy of the universe.
Δsuniv > 0
Some common spontaneity rules:
Higher state means more spontaneous
Gas>liquid>solid
More moles of a gas or liquid on the product side
means Δsuniv > 0

Examples
H2O (l)  H2O (g) >0
NH3(g) + HCl(g)  NH4Cl(s) <0
Ag+(aq) + Cl-(aq)  AgCl(s) <0
C12H22O11(s)  C12H22O11 (aq) >0

Absolute Entropy
Absolute Entropy is for pure substances
Standard Molar Entropy (S°)
Calculate the standard entropy change for the following reaction:
2H2(g) + O2 (g) 2H2O (g)
2(190) – [2(130)+205)]
= -85J/K
Non-spontaneous

Involving Δssurr
To calculate Δsuniv we need to sum Δssys and Δssurr
ΔSsys is the same as ΔS°
ΔSsurr is calculated differently. ΔSsurr is not equal to (-)
ΔSsys
ΔSsurr = ΔHsurr /T = -ΔHsys/T
ΔHsys = ΔH°

Example
Calculate Δsuniv: N2(g) + 3H2(g)  2NH3 (g)
ΔSuniv = ΔSsys + Δssurr
ΔSsys =ΔS°
[2(192.5)] – [(191.5)+3(130.6)]
= -198.3 J/K

ΔSsurr = ΔHsurr /T = -ΔHsys/T
ΔHsys = ΔH°
[2(-46.1)] – [0 + 3(0)]
= 92.2kJ
-92.2 * 103 J/ 298K = 309 J/K
ΔSuniv = ΔSsys + Δssurr
-198.3 J/K + 309 J/K = 111 J/K

Gibbs Free Energy (ΔG)
Free Energy is the maximum amount of energy available to
do work
ΔG = ΔH – TΔS
Fundamental Equation for this segment of Thermo
ΔG < 0 means a process is spontaneous

Standard Free Energy of Formation
(ΔG°)
ΔG°rxn = Σnproducts ΔG°products – Σnreactants
ΔG°reactants
Calculate the ΔG°
ΔG = ΔH – TΔS
= -104.9 kJ – 298K(-290.2 J/K)
=-104.9 * 103 J – 298K(-290.2 J/K) = -18.4 kJ

Final Example