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Unformatted text preview: Spontaneity: A spontaneous reaction is a process that happens with no external intervention.  The spontaneity of a reaction depends not ONLY on the enthalpy of reaction but also a new property ENTROPYà༎ the amount of randomness in a system o For any spontaneous process, the entropy of the universe must increase. (ΔSuniverse > 0) o Work is organized energy transfer, while heat increases the disorder on a system, thus entropy must some how depend on q (heat) by calculation and substitution we can derive this formula relating entropy to heat: ΔS = qrev
T §༊ Note, there is no entropy change for reversible processes By integration we can further expand on the idea that heat is related to entropy. Further calculation gives us this equation relating the change in entropy to different temperature states of a substance: T2 ⎛ྎ T ⎞ྏ
CdT ΔS (T1 → T2 ) =
= C ln⎜ྎ 2 ⎟ྏ
⎜ྎ T ⎟ྏ
T ⎝ྎ 1 ⎠ྏ
T1 Phase Transitions  Phase transitions occur at a single temperature so we can use the simplified equation: ∫ ΔS =
Ex. qrev
T Knowing that DH = qp and restricting ourselves to constant pressure conditions, we can determine the entropy change for any phase transition ΔS v =
 ΔH vaporization
Tvaporization Similarly, for pressure changes, we have ⎛ྎ P ⎞ྏ
ΔS ( P → P2 ) = −nR ln⎜ྎ 2 ⎟ྏ
1
⎜ྎ P ⎟ྏ
⎝ྎ 1 ⎠ྏ Third Law of Thermodynamics  The entropy of a pure crystalline substance at absolute zero temperature (0 K) is zero.  We can determine and tabulate absolute entropy values, such as tables of entropy of formation ΔSOf Ethan Newton & Barry Zhang for SOS Winter 2012 29 Gibbs Free Energy  Gibbs free energy takes into account the entropy, enthalpy and the temperature of a system to show the spontaneity of the reaction.  Spontaneous reactions ALWAYS HAVE NEGATIVE GIBBS FREE ENERGY ( Δ G) o Δ G Negative à༎ system is spontaneous as written o Δ G zero à༎ system is at equilibrium o Δ G Positive à༎ system is non spontaneous as written (it may be spontaneous in the reverse direction)  Gibbs free energy can be calculated through the following two equations: ΔG = ΔH − TΔS o
o
ΔG = ∑ ΔG f (products) − ∑ ΔG o f (reactants) €
o NOTE: this latter equation is only good for DG at T=25ºC since that is the temp at which values of DGf° are tabulated. €   Since Δ G has to be negative for...
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 Winter '08
 carran
 Chemistry, Atom, Quantum Chemistry, Stoichiometry

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