UNIVERSITY OF ALBERTA
INTRODUCTORY UNIVERSITY CHEMISTRY II (CHEM 102)
Many chemical reactions or processes are spontaneous (occurs by itself without outside
• A ball rolls down a hill spontaneously but never rolls back up the hill
• Heat flows from hot object to cooler one. The reverse process never occurs
• Iron rusts spontaneously as shown by the following chemical equation
4Fe (s) + 3O (g) → 2Fe O (s)
2 2 3
But iron oxide does not spontaneously change to iron metal and oxygen
• Methane burns to form carbon dioxide and water but the reversed reaction is not
CH (4) + 2O (g2 → CO (g) 2 2H O (l)2
• Below 0 C, water spontaneously freezes and at temperatures above 0 C, ice melts
Limitations of the First Law of Thermodynamics
The First Law of Thermodynamics is good for bookkeeping of energy in chemical
reactions such as:
• How much energy is involved in the chemical reaction
• Does energy flow in or out of the system
The First Law of Thermodynamics does not make sense as to the direction of a reaction.
Spontaneous processes and entropy
A process is said to be spontaneous if it occurs without outside intervention.
A non-spontaneous process will not occur unless some external action is continuously
A spontaneous process does not mean instantaneous and has nothing to do with how long
the process takes to occur.
CH (g) + 2O (g) → CO (g) + 2H O (l)
4 2 2 2
It won’t happen unless a spark is applied.
Remember: How fast a
Reactants reaction occurs is determined
Energy Thermodynamics by the domain of kinetics – the
Extent of a chemical reaction
What is the law that predicts direction of a chemical reaction?
Since many spontaneous reactions are exothermic, it was thought that exothermicity was
the criterion for spontaneity.
CH (4) + 2O (g)2→ CO (g) + 22 O (l) 2 ∆H = -802 kJ
2Fe (s) + 3/2O (g) → Fe O (s) ∆H = -826 kJ
2 2 3
Almost all exothermic reactions are spontaneous.
However, some endothermic reactions are also spontaneous at 25 C.
H 2 (s) → H O (2) ∆H = +6.02 kJ (melting of ice)
H 2 (l) → H O 2g) ∆H = +44.0 kJ (evaporation of water)
Disorder and entropy
In all spontaneous processes, the driving force is an increase of the entropy (S) of the
The entropy is viewed as a measure of randomness or disorder: The natural progression
of things is from order to disorder (i.e. from lower entropy to higher entropy)
Entropy is a thermodynamic state function that describes the number of arrangements
(microstates) that are available to a system existing in a given state. Entropy is therefore
closely associated with probability.
Expansion of an ideal
gas into a vacuum
Fig 20.2, p885
2 The gas molecules fill the other bulb spontaneously because there is an increase of
If there are four molecules A, B, C and D are in bulb A, the number of possible
microstates available after expanding into bulb B are:
Bulb A Bulb B
A, B, C, D
B, C, D A
A, C, D B
A, B, D C
A, B, C D
A, B C, D
A, C B, D
A, D B, C
There are a total of 8 x 2 = 16 microstates available for mixing 4 molecules.
From the Boltzmann equation, S = k lnW where,
S = entropy of a system
k = Boltzmann’s constant = R / NA(gas constant / Avogadro’s number)
W = number of microstates
• A system with more microstates would be expected to have more disorder and
• Entropy is a state function and so ∆S is dependent on the initial and final values.
∆S = S finalSinitial
Which of the following changes would result in an increase of entropy (∆S > 0)?
CO 2s) → CO (g2
N 2P = 1.0 x 10 atm) → N (P2= 1.0 atm)
(NH 4 2r 2 7s) → N (2) +Cr O 2s3 + 4H O 2g)
NaCl (s) + H2O (l) → NaCl (aq)
3 Third Law of Thermodynamics
Unlike internal energy (U) and enthalpy (H), the absolute entropy value (S) can be
The Third Law of Thermodynamics states that a perfect crystal has zero entropy at a
temperature of absolute zero.
Ssys= 0 at 0 K
Standard molar entropy S of different compounds can be determined (Appendix D,
A18 - A24)
entropy at 25 C
Entropy of vaporization
> Entropy of fusion
Rapid rise in
entropy due to
phase change –
Entropy of fusion
Fig 20.7 p890
-1 -1 o
From the standard molar entropy values (unit = J mol K ), ∆S can be calculated for
o o o
∆S = ∑n products products ∑n reactantreactants
Calculate the standard entropy for the following reaction:
N 2g) + 3H (g2 → 2NH (g) 3
Absolute entropy values S are found from the table
o o o o
∆S = 2 mol x (S NH ) – 33mol x (S H ) – 1 mo2 (S N ) 2
= 2 mol x 192.5 J mol K - 3 mol x 130.7 J mol K - 1 mol x 191.6 J mol K -1 -1
= -198.7 J K .
4 The result is consistent with the equation with 4 gas molecules react to give 2 gas
Entropy and the Second Law of Thermodynamics
At -10oC, the freezing of water is spontaneous:
H2O (l) → H 2 (s)
However, ∆S < 0 because ice is more order than liquid water.
Therefore, entropy change of the system ∆S sys cannot be used as a criterion for
The Second Law of the Thermodynamics: In any spontaneous process there is always
an increase in the entropy of the universe
To be spontaneous:
∆S = ∆S + ∆S > 0
univ surr sys
We therefore need to take the entropy change of the surroundings into consideration for
The mathematical expression for entropy change of a system is given by:
Where q = heat
T = temperature, K
At constant pressure, q = q = ∆H
And so ∆S =
But ∆H = - ∆H
Therefore, ∆H surr - ∆Hsys
- ∆H sys
∆Suniv= + ∆Ssys
Multiplied both sides by T gives
T∆S = - ∆H + T∆S
univ sys sys
5 - T∆S univ= ∆H sys- T∆S sys
∆G = ∆H - T∆S
sys sys ∆G = Gibbs Free Energy = - T∆S univ
The Gibbs Free Energy is another important thermodynamic state function. Its absolute
value cannot be determined.
G = T x S = J K -1K = J
Therefore G has a unit of J or kJ
Since measurement of ∆S surris difficult, we use ∆G as a criterion for spontaneity.
• ∆G < 0 (spontaneous)
• ∆G > 0 (non-spontaneous)
• ∆G = 0 (equilibrium)
The three factors (∆H, ∆S and T) that affect spontaneity are summarized below:
Free energy change and the work done of a system
▯ For a spontaneous process, the free energy is the maximum work obtainable from
the system as the process takes place.
∆G = w (max)
Burning of gasoline
C 8 18) + 25/2 O (g)2 8CO 2g) + 9 H O2(g) ∆G = - 1596.2 kJ /mol
6 This amount of free energy is used mainly as the kinetic energy of the motor car.
However, most of the free energy is wasted as heat increasing the entropy of the
▯ For a non-spontaneous process, the free energy is the minimum work that must be
done to the system to make the process take place.
In reality, the energy supplied to a system to make a non-spontaneous reaction to occur is
always more than the minimum because some of the free energy change is lost a