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Lecture 24

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Brandeis University

Chemistry

CHEM 11B

Novak Claudia

Spring

Description

Thermodynamics
Spontaneity
- Things that are nonspontaneous is not impossible
First law of thermodynamics
- You can’t win
- Law of conservation of energy
- Energy is a constant
Second law of thermodynamics
- You can’t break even
- Something pays the price
- Lose a “transaction fee”
- Entropy
- All spontaneous processes are accompanied by an increase in the entropy of the
universe.
- Some of your energy investment must be lost so that the entropy of the universe
increases.
- Can’t get as much out as you put in
- All spontaneous processes are accompanied by an increase in the entropy of the
universe.
Third law of thermodynamics
- You can’t quit
- Perfect crystalline solid at absolute zero is the only thing with zero entropy
System, surroundings, universe
Enthalpy (ΔH): the measure of heat exchanged under conditions of constant pressure
Spontaneous physical processes
Phase changes and entropy
Increasing entropy: solid, liquid, gas
S = k ln W
S - entropy
K - Boltzmann’s constant: R/NA= 1.38 × 10-23 J/K
W - The number of energetically equivalent ways to arrange the components of a system
A state in which a given amount of energy is more highly dispersed (or more highly randomized)
has more entropy than a state in which the same energy is more highly concentrated. Entropy: Symbols and Signs
In thermodynamic calculations, the entropy term is given the symbol S.
Entropy is a state function: the change in entropy for a process can be calculated as the final
state minus the initial state:
- ΔS = Sfinal – Sinitial
or
- ΔS = Sproducts – Sreactants
Increase in entropy: ΔS > 0
Decrease in entropy: ΔS < 0
Calculating the Standard Entropy Change (ΔS°rxn) for a Reaction
ΔS°rxn = Σ np S° (products) - Σ nr S° (reactants)
The sum of the standard molar entropies of the products, each multiplied by its stoichiometric
coefficient (np), minus the sum of the standard molar entropies of the reactants, each multiplied
by its stoichiometric coefficient (nr).
All spontaneous processes are accompanied by an increase in the entropy of the universe.
ΔSuniv = ΔSsys + ΔSsurr
an exothermic process increases the entropy of the surroundings (ΔHsys < 0 g ΔSsurr > 0)
an endothermic process decreases the entropy of the surroundings (ΔHsys > 0 g ΔSsurr < 0)
When water freezes at temperatures below zero °C the entropy of the universe increases, and
the process is spontaneous
Gibbs free energy: G = H − TS
G = free energy
ΔGsys = ΔHsys − TΔSsys → the gibbs equation
The lower the temperature, the more the entropy of the surroundings is affected by a given
amount of heat emitted or absorbed by the system.
In order for a

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