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Learning objectivesAfter reviewing this presentation
learnerwill be able to • Explain entropy and enthalpy• Describe Gibb’s free energy • Derive a relation for Helmholtz free
energy.
Entropy
Entropy, S: Measure of dispersal or disorder. Can be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no
disorder and S = 0. If temperature change is very small, can
calculate entropy change, S = q/T (heat absorbed / T at which change occurs)
Sum of S can give total entropy at any desired temperature.
Entropy Examples (positive S) Boiling water Melting ice Preparing solutions CaCO3 (s) CaO (s) + CO2 (g)
Entropy Examples (negative S) Molecules of gas collecting Liquid converting to solid at room
temp 2 CO (g) + O2 (g) 2 CO2 (g) Ag+ (aq) + Cl-(aq) AgCl (s)
Entropy Generalizations
Sgas > S liquid > Ssolid
Entropies of more complex molecules are larger than those of simpler molecules (Spropane > Sethane>Smethane)
Entropies of ionic solids are higher when attraction between ions are weaker.
Entropy usually increases when a pure liquid or solid dissolves in a solvent.
Entropy increases when a dissolved gas escapes from a solution
Laws of Thermodynamics
First law: Total energy of the universe is a constant.
Second law: Total entropy of the universe is always increasing.
Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.
Calculating So system
So system = So (products) - So (reactants)
So surroundings = q surroundings / T
= - Hsystem / T
Calculating So universe
So universe = So surroundings + So
system
So universe =- Hsystem / T + So system
• Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure.
• Enthalpy is a state function.• If the process occurs at constant pressure,
EnthalpyEnthalpy
PVEH
VPE
PVEH
• Since we know that
• We can write
• When H is positive, the system gains heat from the surroundings.
• When H is negative, the surroundings gain heat from the system.
EnthalpyEnthalpy
VPw
P
P
P
q
VPVPq
VPwq
VPEH
)(
Gibbs Free Energy
Gibbs free energy is a measure of chemical energy.
All chemical systems tend naturally toward states of
minimum Gibbs free energy
G = H - TSWhere:
G = Gibbs Free Energy
H = Enthalpy (heat content)
T = Temperature in Kelvins
S = Entropy (can think of as randomness)
Gibbs Free Energy G is a measure of the
maximum magnitude of the net useful work that can be obtained from a reaction.
Gibbs Free Energy
Gsystem = - T Suniverse
= Hsystem - TSsystem
Gosystem = Ho
system - T Sosystem
Go
rxn = Horxn - T So
rxn
Gibbs Free Energy
Gosystem or Go
rxn If negative, then product-favoured. If positive, then reactant-favoured.
Go reaction = Gfo (products) - Gf
o (reactants)
Thermodynamics and KIf not at standard conditions,G = Go + RT ln Q (Equilibrium is characterized by the inability to do
work.)At equilibrium, Q = K and G = O
Therefore, substituting into previous equation gives
0 = Go + RT ln K and Go = - RT ln K (can use Kp or Kc)
Thermodynamics and K Understand relationship
between Go, K, and product-favoured reactions
Go<0 K>1 Product-favoured Go=0 K=1 Equilibrium Go>0 K<1 Reactant-favoured
The Helmholtz free energy is a thermodynamic potential that measures the “useful” work obtainable from a closed thermodynamic system at a constant temperature and volume.
Helmholtz Free Energy
The Helmholtz energy is defined as: A= U - TSwhereA is the Helmholtz free energy (SI: joules, CGS: ergs),U is the internal energy of the system (SI: joules, CGS: ergs),T is the absolute temperature (Kelvins),S is the entropy (SI: joules per Kelvin, CGS: ergs per kelvin).
Helmholtz Free EnergyFrom the first law of thermodynamics dU = δQ - δW,where U is the internal energy, δQ is the energy added by heating and δW is the work done by the system. From the second law of thermodynamics, for a reversible process we may say that δQ = TdS. Also, in case of a reversible change, the work done can be expressed as δW = pdV dU = TdS - pdVApplying the product rule for differentiation to d(TS) = TdS + SdT, we have: dU = d(TS) – SdT – pdVd(U-TS) = – SdT – pdV,and The definition of A = U - TS enables to rewrite this as: dA = – SdT – pdV