Home > Chemistry > Chemistry 1A Part Deux: Lecture 25 Notes

Chemistry 1A Part Deux: Lecture 25 Notes


Today is another 49 minute lecture on entropy, going into Gibbs Free Energy.


Reactions that are exothermic and increase entropy favor products.  Other reactions that are either exothermic or increase entropy may also favor products.

The Laws of Thermodynamics

1st Law – Conservation of Energy

2nd Law – dS universe = dS sys + dS surr >=0

3rd Law – Entropy at 0K is 0

4Fe + 3O2 = 2Fe2O3

This is a negative dHo, a positive dSo, and the product is favored at 300K.

dSo is positive when entropy is created; it is negative when the system becomes more ordered.

dS system = -dH sys/T

G = H – TS

dS universe >=0

If G is negative, the reaction is spontaneous.  If G is 0, the reaction is at equilibrium.  If G is greater than zero, the reactants are favored.

dGo reaction measures the difference between the free energies of the reactants and products when all components of the reaction are present at standard state conditions.

Barium Hydroxide and Ammonium Thiocyanate

Ba(OH)2*8H2O + 2NH4SCN = Ba(SCN)2 + 2NH3 + 10H2O

dH sys = +102 kJ/mol

dS sys = +495 J/mol K

Therefore, dG sys = negative at most temperatures.

This reaction absorbs heat from the environment.

Dehydration of Sucrose

C12H22O11 = 12C + 11H2O

dHf = -2221 kJ/mol

So = +360

dGf = -1544

She adds sulfuric acid to a glass full of sugar at 40 minutes in.  The glucose then “snakes” out of the glass.  This reaction is driven by entropy.  You need to add half, stir, then add the other half.

y = mx + b

G = -TS + H

You can make a very nice graph here with G versus T.


PV = nRT

IGL = random motion, point source, elastic collisions, high temperatures, low pressure

K = products/reactants

Bronstead acids donate protons, bases accept protons.

Lewis acids accept electrons, lewis bases donate electrons

s 2, p 6, d 10, f 14

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