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Lecture 4 Predicting Spontaneous Change

Lecture 4 Predicting Spontaneous Change - Someofthe...

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Thermochemistry we have seen so far (CH6, 9.4) Internal Energy (E) is transferred as heat and work ( ). 1 st law of thermodynamics : energy of the universe is conserved. Enthalpy (H) is a useful, measurable chemical quantity. Enthalpy is a state function , so H depends only on the difference between the final and initial values of H. At constant pressure, Ways to calculate H: 1) Hess’ Law: 2) Enthalpies of formation: 1 ∆ࡱ ࢛࢔࢏࢜ࢋ࢙࢘ࢋ = ∆ࡱ ࢙࢙࢚࢟ࢋ࢓ + ∆ࡱ ࢙࢛࢘࢘࢕࢛࢔ࢊ࢏࢔ࢍ࢙ = 0 ∆ࡱ = + H < 0 exothermic H > 0 endothermic Some of the H =  H f (Products) –  H f (Reactants) H = H step 1 + H step 2 + …
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Today We start Ch. 20 – Thermodynamics : Entropy, Free Energy and the Direction of Chemical Reactions CHM139 F2011 Jockusch , Thermodynamics L4 2 Today’s question: How do we predict whether or not a reaction is spontaneous?
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Spontaneous Change CHM139 F2011 Jockusch , Thermochemistry L4 3 A spontaneous change is one that occurs without a continuous input of energy from outside the system. The chemical change may require energy ( activation energy ) to start, but once a spontaneous process has begun, no further input of energy is needed. A nonspontaneous change occurs only if the surroundings continuously supply energy to the system. If a change is spontaneous in one direction, it will be nonspontaneous in the reverse direction.
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Spontaneous Change CHM139 F2011 Jockusch , Thermochemistry L4 4 If a change is spontaneous in one direction, it will be nonspontaneous in the reverse direction.
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Spontaneous Change CHM139 F2011 Jockusch , Thermochemistry L4 5 H 2 H O A spontaneous, exothermic reaction
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