Transcript Thermodynamics

Thermodynamics
(the other part)
Topic 19 in Red Book
Chapter 16 in textbook
Why does change occur?
• DH – change in enthalpy (heat transferred
btwn system and surroundings)
• DS – change in entropy (randomness or
disorder of the system)
• Systems tend to move towards lower enthalpy
(releasing heat) and higher entropy (more
disorder)
Spontaneous Processes
• reaction that proceeds on its own with no
outside help
• if a reaction is spontaneous in one direction, it
is nonspontaneous in the other direction
• does not necessarily mean that the reaction
happens quickly, just that it happens
Enthalpy
• DH = heat absorbed by the system
– exothermic = -DH
– endothermic = +DH
• 1st law of thermo = energy is conserved, heat
lost by system must equal heat gained by
environment & vice versa
• remember the formula for calculating DH of a
rxn:
DHorxn = SDHof products – SDHof reactants
Entropy & 2nd Law
• measure of randomness (gases are more
random than liquids, so gases have higher S)
• DS is positive for solid  liquid  gas changes
and negative for the reverse
• DS is higher for more complex molecules
(propane has higher S than methane)
• 2nd law of thermo states that any spontaneous
change is always associated with an overall
+DS
3rd Law
• states that S of a pure crystalline solid at 0K is
always zero.
• As the temperature is warmed above 0K, S
increases gradually
• At phase changes, S increases a lot
Standard Molar Entropies
• So = measures order/disorder of pure
substances at 1 atm & 298 K
• units are J/mol K
• So is largest for gases and heavier/more
complex substances
Entropy Changes
• looks just like the equation for DHo
DSorxn = SSo products – SSo reactants
• What is the DSorxn for the following rxn?
2NO(g) + O2(g)  2NO2(g)
Gibbs Free Energy
• DG = amt of useful work/energy that can be
obtained from a process at constant T&P
DG = DH – TDS
DGorxn = SGof products – SGof reactants
• G & H are in kJ/mol, S is in J/molK so it must
be converted to kJ
What does DG tell you?
If DG is…
negative
positive
zero
The process is…
spontaneous
nonspontaneous
at equilibrium
What is the standard free energy
for the reaction:
2NO(g) + O2(g)  2NO2(g)
Free Energy & Temperature
DH DS -TDS DG The rxn is…
- +
- spontaneous at all T
+ +
+ nonspontaneous at all T
+ +/- Spontaneous only at low T
+ +
- +/- Spontaneous only at high T
KNOW THIS TABLE!!!
Is DG for a burning campfire positive or
negative? Explain citing the positive or negative
signs of DH and DS for the process. Is there a
temperature at which DG for a burning campfire
will change signs? Explain.
Free Energy & Equilibrium
• you can calculate free energy at non-standard
conditions using the following formula:
DG = DGo + RT lnQ
• have to use R = 8.314 J/molK
• At equilibrium, Q = K & DG = 0, so the eqn
becomes:
DGo = -RT lnK
Using the std. free energies of formation for the
reaction:
2H2S(g) + 3O2(g)  2H2O(g) + 2SO2(g)
Calculate DGo and DG for a mixture at 25oC with
the following composition:
PH2S = 1.00 atm, PO2 = 2.00 atm, PH2O = 0.500
atm, PSO2 = 0.750 atm
The standard free energy change for the
following reaction at 25oC is -118.4 kJ/mol:
KClO3(s)  KCl(s) + 3/2 O2(g)
Calculate Kp for the reaction and the equilibrium
pressure of O2 gas.