Chapter 23 Thermodynamics

acridboneMechanics

Oct 27, 2013 (3 years and 10 months ago)

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Chapter 23 Thermodynamics

What is the driving force for every
process in the universe?

Review of equilibrium


What you should know at this point


If the Keq > 1, the equilibrium favors the
products.


If the Keq < 1, the equilibrium favors the
reactants.



Spontaneous Processes


Spontaneous process
-

A process that occurs on its
own, without any outside intervention.


Spontaneous process may occur slowly (over
thousands of years) or fast (in less than a second).


The amount of heat given off or absorbed is not what
determines how fast a reaction takes place;
however, exothermic reactions do tend to be
spontaneous.

∆ H (enthalpy
-

heat) is a

measure of this energy change.



Not all spontaneous reactions are exothermic;
therefore, heat (∆H) is not the only factor that
determines spontaneity.


Examples: ice melts (spontaneous and
endothermic)


Some materials absorb heat as they dissolve in
water (spontaneous and endothermic).


A sports cold pack


What is it that causes a reaction to be
spontaneous?


What causes every reaction (and process) in
the universe to take place?


Lower energy is not the only driving force
-

there is another factor.

Entropy (S)
-

a measure of the disorder
(randomness) of a system.


The greater the disorder, the greater the S



S
gas

>


S
liquid

> S
solid


Less order more order


Δ
S = S
products



S
reactants


If
Δ
S is positive the system goes to more
disorder.



To gases from liquids and solids
Δ
S is positive


To solutions from solids and liquids
Δ
S is
positive


If temperature is increases
Δ
S is positive

Second Law of Thermodynamics


In any process the overall entropy of the
universe always increases.


Δ
S
universe

=
Δ
S
process

+
Δ
Ssurroundings

Must be + one of these can be


if the



other is + by a greater amount

Entropy change for a process and
surroundings


Process
Δ
S
process

Δ
H
process

Δ
S
surroundings
Δ
Sproc. +
Δ
Ssurr. spontaneous?



Exothermic +


-


+


+


yes


Entropy increase


Endothermic +


+


-


+ or
-


depends


Entropy increase


Exothermic
-

-

+ + or
-

depends


Entropy decrease


Endothermic
-


+


-


-


no


Entropy decrease


Spontaneity of a reaction depends on two
factors (entropy and enthalpy).


When these two factors oppose each
other, the spontaneity depends on which is
larger


Gibbs discovered the relationship between
entropy, enthalpy and spontaneity


He found that the relationship is
dependent on absolute temperature.

This concept is called Gibbs Free Energy (G)


∆G = ∆H
-

T ∆S


The change in Gibbs free energy (∆G) equals
the enthalpy change (∆H) minus the
absolute temperature times the change in
entropy.


Free Energy (∆G) is the amount of energy
available to do work.


More importantly, it is a measure of
spontaneity.

∆G = ∆H
-

T∆S


If
∆G is negative the reaction is spontaneous.


If ∆G is positive the reaction is not
spontaneous and will require a high energy
input to force it to occur.


If ∆G is zero, the reaction is at equilibrium.


The spontaneity of a system is dependent on
its temperature.

∆G = ∆H
-

T∆S


Case 2


If both
∆H and ∆S are positive, the
reaction will proceed at high temperatures
and will not happen at low temperatures.
(study equation)


Example: melting ice


How are ∆H and T∆S related at 0
o
C?


The two terms will be equal because the
system is at equilibrium at 0
o

∆G = ∆H
-

T∆S


Case 3
-

∆H and ∆S are both negative.


If the temperature is low enough, T∆S
becomes less in magnitude than the negative
∆H and the process will proceed even
though there is a more ordered arrangement.


Example: freezing of water


See the chart at the top of page 762.


If a process is spontaneous it can be made to do
work: however, the work from a reaction can never
exceed
∆G.


Free energy


the amount of energy “free” to do
work. The remainder is unavailable because it is
“lost” to the environment to meet the criterion that
the entropy of the universe must increase (the 2
nd

law of thermodynamics).


The total energy of the universe is constant;
however, the energy is continually dispersed, like the
winding down of a clock.