Post on 22-Dec-2015
Chemical Thermodynamics: Chemical Thermodynamics: Entropy, Free Energy and Entropy, Free Energy and
EquilibriumEquilibriumChapter 18Chapter 18
18.1-18.618.1-18.6
Chemical ThermodynamicsChemical Thermodynamics
Science of interconversion of energyScience of interconversion of energy Heat into other forms of energyHeat into other forms of energy Amount of heat gained/released from a systemAmount of heat gained/released from a system Spontaneity of a reactionSpontaneity of a reaction Gibbs free energy functionGibbs free energy function Relationship between Gibbs Free Energy and Relationship between Gibbs Free Energy and
chemical equilibrium chemical equilibrium
Spontaneous ProcessesSpontaneous Processes
Main objectiveMain objective Spontaneous Reaction- Spontaneous Reaction- a reaction does occur a reaction does occur
under specific conditionsunder specific conditions Non-spontaneous Reaction-Non-spontaneous Reaction- a reaction does a reaction does
not occur under specific conditionsnot occur under specific conditions
Spontaneous ProcessesSpontaneous Processes
A waterfall runs downhill A lump of sugar dissolves in a cup of coffee At 1 atm, water freezes below 0ºC and ice
melts above 0ºC Heat flows from a hotter object to a colder
object Iron exposed to oxygen and water forms rust
Spontaneous ProcessesSpontaneous Processes
Does a decrease in enthalpy mean a reaction proceeds spontaneously?
Spontaneous reactions
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H0 = -890.4 kJ
H+ (aq) + OH- (aq) H2O (l) H0 = -56.2 kJ
H2O (s) H2O (l) H0 = 6.01 kJ
NH4NO3 (s) NH4+(aq) + NO3
- (aq) H0 = 25 kJH2O
EntropyEntropy
To predict spontaneity we need:To predict spontaneity we need: Change in enthalpyChange in enthalpy EntropyEntropy Entropy- Entropy- a measure of the randomness or disorder a measure of the randomness or disorder
of a system.of a system. ↑ ↑ Disorder = ↑ EntropyDisorder = ↑ Entropy
EntropyEntropy
New Deck OrderNew Deck Order Shuffled Deck OrderShuffled Deck Order ProbabilityProbability Ordered stateOrdered state Disordered StateDisordered State
Microstates and EntropyMicrostates and Entropy
Boltzmann, 1868Boltzmann, 1868 S = S = kk ln W ln W
kk = 1.38 x 10 = 1.38 x 10-23-23 J/K J/K ↑ ↑ W = ↑ EntropyW = ↑ Entropy
∆ ∆ S = SS = Sff – S – Sii
∆ ∆ S = S = kk ln ln WWff
WWii
Wf > Wi then S > 0
Wf < Wi then S < 0
Entropy and DisorderEntropy and Disorder
If the change from initial to final results in an increase in randomness
Sf > Si S > 0
For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state
Ssolid < Sliquid << Sgas
Entropy and DisorderEntropy and Disorder
How does the entropy of a system change for each of the following processes?
(a) Forming sucrose crystals from a supersaturated solution
Randomness decreases Entropy decreases (S < 0)
(b) Heating hydrogen gas from 600C to 800C
Randomness increases Entropy increases (S > 0)
Standard EntropyStandard Entropy
The standard entropy of reaction (S0 ) is the entropy change for a reaction carried out at 1 atm and 250C.
rxn
The Second Law of The Second Law of ThermodynamicsThermodynamics
The entropy of the universe increases in a The entropy of the universe increases in a spontaneous process and remains unchanged spontaneous process and remains unchanged in an equilibrium process. in an equilibrium process. Importance?Importance?
Spontaneous process: Suniv = Ssys + Ssurr > 0
Equilibrium process: Suniv = Ssys + Ssurr = 0
Entropy Changes in the SystemEntropy Changes in the System
To calculate To calculate ΔΔSSunivuniv, we need both , we need both ΔΔSSsyssys ΔΔSSsurrsurr
ΔΔSSsyssys
aA + bB cC + dD
S0rxn nS0(products)= mS0(reactants)-
Entropy Changes in the SystemEntropy Changes in the System
When gases are produced (or consumed)
• If a reaction produces more gas molecules than it consumes, S0 > 0.
• If the total number of gas molecules diminishes, S0 < 0.
• If there is no net change in the total number of gas molecules, then S0 may be positive or negative BUT S0 will be a small number.
Entropy Changes in the Entropy Changes in the SurroundingsSurroundings
ΔΔSSsurrsurr = = --ΔΔHHsyssys TT
Using the information from Example 18.2, determine whether or not the Using the information from Example 18.2, determine whether or not the reaction is spontaneous.reaction is spontaneous.
NN22(g) + 3H(g) + 3H22(g) (g) → → 2 NH2 NH33(g)(g) ΔΔHºHºrxnrxn = -92.6 kJ/mol = -92.6 kJ/mol
ΔΔSSsyssys = -199 J/K ∙ mol = -199 J/K ∙ mol
ΔΔSSsurrsurr = = -(-92.6 x 1000) J/mol-(-92.6 x 1000) J/mol 298 K298 K
ΔΔSSsurrsurr = 311 J/mol = 311 J/mol
ΔΔSSunivuniv = = ΔΔSSsyssys + + ΔΔSSsurrsurr
ΔΔSSunivuniv = -199 J/K ∙ mol + 311 J/mol = -199 J/K ∙ mol + 311 J/mol
ΔΔSSUNIVUNIV = 112 J/K ∙ mol = 112 J/K ∙ mol
The Third Law of Thermodynamics The Third Law of Thermodynamics and Absolute Entropyand Absolute Entropy
Third Law of Third Law of Thermodynamics-Thermodynamics- the the entropy of a perfect entropy of a perfect crystalline substance is crystalline substance is zero at the absolute zero zero at the absolute zero of temperature.of temperature.
Gibbs Free EnergyGibbs Free Energy
Predicts the direction of a spontaneous Predicts the direction of a spontaneous reaction.reaction.
Uses properties of the system to calculate. Uses properties of the system to calculate. For a constant pressure-temperature process:For a constant pressure-temperature process:
G = Hsys -TSsys
G < 0 The reaction is spontaneous in the forward direction.
G > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction.G = 0 The reaction is at equilibrium.
Standard Free-Energy ChangesStandard Free-Energy Changes
The standard free-energy of reaction (G0 ) is the free-energy change for a reaction when it occurs under standard-state conditions.
rxn
Standard free energy of formation (G0) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states.
f
G0rxn nG0 (products)f= mG0 (reactants)f-
Free Energy and Chemical Free Energy and Chemical EquilibriumEquilibrium
G = G0 + RT lnQ
R is the gas constant (8.314 J/K•mol)
T is the absolute temperature (K)
Q is the reaction quotient
At Equilibrium
G = 0 Q = K
0 = G0 + RT lnK
G0 = RT lnK