Download Thermo notes Part II

Thermodynamics
Part II
Entropy, S
Entropy is a measure of the disorder or randomness of a
system.
The entropy of the universe is increasing.
When:
DS is positive disorder increases (favors spontaneity)
DS is negative disorder decreases (disfavors spontaneity)
In general: Sgas> Sliquid > Ssolid
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 1) Melting of ice at room temperature
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 1) Melting of ice at room temperature
Entropy increases: a phase change from an organized solid
to a more disorganized liquid
∆S (+)
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 2) Condensation of water vapor on a window
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 2) Condensation of water vapor on a window
Entropy decreases: a phase change from a disorganized gas
to a more organized liquid
∆S (-)
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 3) An iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s)
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 3) An iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s)
Entropy decreases: there are 4 moles of a solid reacting
with 3 moles of a gas to make only 2 moles of the
product
∆S (-)
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 4) Baking soda reacting with vinegar
NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g)
Predicting ∆S
• Determine whether the entropy value increases or
decreases for the following situations
Ex. 4) Baking soda reacting with vinegar
NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g)
Entropy increases: a solid and liquid yield an aqueous soln, a
liquid, and a gas
(Also, there are 2 moles of reacts making 3 moles of product)
∆S (+)
• Entropy changes for reactions can be determined
similarly to DH for reactions. As with DH, entropies
have been measured and tabulated in for So298.
o
DS298
 n
o
Sproducts
 n
o
Sreactants
Ex. 5) Find the ∆S for liquid ethanol, C2H5OH,
undergoing combustion:
• C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
• Does this reaction create order or disorder?
Enthalpy and entropy can sometimes reinforce each other – this
makes the reaction really go or really not go.
Ex. Dynamite has a neg Δ H & a pos Δ S so the rxn really goes
once started…
If the signs don’t reinforce does a rxn occur? This is where Gibbs
Free energy addresses the spontaneity of rxns.
Ex. Liquid water to water vapor Δ H is + and Δ S is +
Δ G is – (neg),the reaction will go on its own once started to
make the products.
Ex. gas burning.
Δ G is + (pos), the rxn won’t go on it’s own, wants to stay as
reactants.
Ex. batter  cake
The change in the Gibbs Free Energy is a reliable indicator of
spontaneity of a physical process or chemical reaction.
When:
DG is > 0 reaction is not thermodynamically favored
(nonspontaneous) reactant favored
DG is < 0 reaction is thermodynamically favored
(spontaneous) product favored
Changes in free energy obey the same type of relationship we have described for
enthalpy and entropy changes.
o
DG298
=
o
 nDG products
o
  nDG reactants
Ex. 6) Find the ∆Grxn for liquid ethanol, C2H5OH,
undergoing combustion:
• C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
• Is this reaction spontaneous or nonspontaneous?
Using ∆H and ∆S to solve ∆G
The relationship between ∆H, ∆S and temperature can help
describe the spontaneity of a system. Whether the reaction
is thermodynamically favored or
not thermodynamically favored
DG = DH - TDS
(at constant T & P)
• Ex. 7) Determine if a reaction is spontaneous at
room temperature, 25oC, when ∆H = -176 kJ and
∆S = -285 J/K
– Units must match!!!
• Ex. 8) What happens if the reaction is now at
550oC, both ∆H = -176 kJ and ∆S = -285 J/K are
the same.
– Units must match!!!
• Ex. 8) What happens if the reaction is now at
550oC, both ∆H = -176 kJ and ∆S = -285 J/K are
the same.
– Units must match!!!
* At what temperature would ∆G = 0?
This general relationship
DG = DH - TDS
gives us 4 possibilities among the signs
DH
+
+
DS
+
+
-
DG Therefore
forward rxn spontaneous at all T’s
?
forward rxn spontaneous at low T’s
?
forward rxn spontaneous at high T’s
+
forward rxn nonspontaneous at all T’s
Copy this into your notes
State function
+
-
ΔH
Endothermic
(taking in heat)
ice melting
*Towards disorder
*Exothermic
(giving off heat)
dynamite
Towards order
not thermodynamically
favored
nonspontaneous
*thermodynamically
favored
spontaneous
ΔS
ΔG
Enthalpy (heat changes) H
Entropy (order and disorder) S
Gibbs Free energy (spontaneity) G
The universe prefers exothermic reactions that cause
disorder and are spontaneous