Energy and Equilibrium: The Laws of Thermodynamics (Read Chapter 7.7 p.494 511)

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SCH 4U

Energy and Equilibrium: The Laws of Thermodynamics

(Read Chapter 7.7 p.494


511)



Thermodynamics:

T
he study of energy transformations
.


Three empirically derived, fundamental laws of nature:


First Law of Thermodynamics:

The total amount of
energy in the universe is constant. Energy can neither be created nor destroyed, but can be
transferred from one object or place to another, or transformed from one form to another.


Second Law of Thermodynamics:


All changes, either directly or indirectl
y, increase the entropy of the universe.




Δ S
univ erse

> 0



symbol for entropy:

S
(measure of disorder or
randomness)


Third Law of Thermodynamics:

The entropy of a perfectly ordered pure crystalline substance is zero at
absolute zero.


S = 0

when T

= 0 K


Why do some physical & chemical changes occur spontaneously while others do not?


Spontaneous rxn

-

one that, given the necessary activation energy, proceeds without continuous outside input



e.g. rxn of sodium with water, burning of paper, rust
ing of iron, dissolving of NaCl.

e.g. of non
-
spontaneous rxns: photosynthesis, frying an egg, building a house.


Spontaneity




determined by
enthalpy

(
ΔH)

and





entropy

(
ΔS)

We know that exothermic rxns (
ΔH is


ve, i.e. products have less energy than reactants) tend to be
spontaneous over endothermic rxns
(
ΔH is + ve).


However, some endothermic rxns






water

e.g. NH
4
Cl(s) + heat


NH
4
Cl(aq)



are spontaneous even at low temperatures.


What’s happe
ning?

Another driving force of rxns:




The Tendency to Grea
ter Disorder
:
Entropy


Δ S = S
products



S
reactants

Δ S > 0 when S
products

> S
reactants

(i.e. disorder has ↑)

Δ S < 0 when S
products

< S
reactants

(i.e. disorder has ↓)



Note: the universe
favours greater disorder!



Read together:



Last 2 paragraphs p.496



Predicting the signs of Δ S p.497



Sample Problem p.498



Practice p.498


Recall: 2
nd

Law of Thermodynamics (Δ S
univ erse

>0)

Sometimes we may think we “beat” this Law of the Universe because
our “system” in question has achieved
greater order.

But to do so the “surroundings” must have ↑ in disorder.


i.e.

Δ S
univ erse

= Δ S
sy stem

+ Δ S
surroundings








See worksheet “Spontaneity of
Chemical Reactions”



Read together p.500


501 from “The second law has ...”



Read p.500


502



Read “Equilibrium and Δ G” p.508


509




Note: you do not need to know how to do the specific numerical calculations from p.506


511


Gibbs free energy (or sim
ply “free energy”)



In the late 1800’s American physicist Josiah Willard Gibbs helped formulate an equation that linked
together the concepts of energy change (Δ H), entropy change (Δ S), and temperature of a reaction &
that predicts if it will occur
spontaneously


Gibbs
-
Hemholtz equation:





















hwk. p.512 #1
-
6, 10

May be ↓
=
But will be ↑ by an even
greater=amount
=