Entropy

Introduced to this in AP Chemistry

Entropy of the Surrounding §

$△$ S${}_{surr}$ (Entropy of the surrounding) depends on:

1. Heat flow of the rxn
   • Exothermic rxn - flow to the surrounding $△$ S${}_{surr}$ = (+)
   • Endothermic rxn - flow to the system $△$ S ${}_{surr}$ = (-)
2. Temp. of surroundings:
   • Impact of the transfer of a given quantity of heat energy, will be great at lower temps (of the surroundings)
   • small change in entropy at high temps

$△$ S${}_{surr}$ (Entropy of the surrounding) = $\frac{quantityofheat}{temp}$ = $\frac{J}{K}$

At constant pressure, heat flow = $△$ H

$△$ S${}_{surr}$ = - $\frac{△H}{T}$

Gibbs Free Energy: $△$ G §

use to determine if a rxn is thermodynamically favored

$△$ S${}_{univ}$ = $△$ S${}_{surr}$ + $△$ S${}_{sys}$

$△$ S${}_{univ}$ = - $\frac{△H}{T}$ + $△$ S

-T $△$ S${}_{univ}$ = $△$ H - T $△$S

$△$ G	Reaction
-	thermodynamically favored (spontaneous)
zero	equilibrium
+	not favored (reverse rxn is favored)

$△$ G = $△$ H - T$△$S

Provides us a thermodynamically favored reaction ALWAYS

Thermodynamic and Kinetic Control (9.4) §

• Common misconception: Thermodynamically favored reactions occur quickly
• Many favored reactions occur incredibly slowly (example: conversion of diamonds to graphite)
• These types of reactions are often slow because they have a high activation energy

Example §

C (diamond) $\to$ C (graphite)

• $△$G${}^{°}$ = -3 kJ
• Occurs spontaneously
• Does not require the input of any external energy to occur
• Incredibly slow, as in thousands of years slow
• Kinetically controlled

Coupled Reactions (9.6) §

• Thermodynamically unfavorable reactions do not occur on their own
• To make an unfavored reaction occur is through use of coupled reactions
• Couple reactions - a combination of a non favored reaction and a favored reaction that have a common intermediate

Example §

Fe${}_2$O${}_3$ + 3CO $\to$ 2Fe + 3CO${}_2$:

Fe${}_2$O${}_3$ $\to$ 2Fe + 3/2O${}_2$

• $△$G${}^{°}$ = 742.2 kJ

CO + 1/2O${}_2$ $\to$ CO${}_2$

• $△$G${}^{°}$ = -283.5 kJ