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THERMODYNAMICS
EQUILIBRIA
KENYATTA UNIVERSITY
SPONTANEITY AND REVERSIBILITY
 Spontaneity is the tendency of a process to occur naturally
 The reverse process which does not proceed to its own its referred to a
as non-spontaneous process
Example rolling of a ball down an incline
If the steepness of incline is released the tendency of the ball to roll
also becomes less and the spontaneous of the process decreases
When the ball is on a level section the spontaneous is reduced to zero
and at this time the ball is reversible or in a balanced state
A small change in a slope one way or the other will make a process occur
spontaneously one way or the other
...
e no small change in the factor that affects the tendency of the reaction to
proceed will make the opposite direction the spontaneous one
...

A balanced or reversible process in one which the driving force is zero and
its system is in equilibrium
...
e
...

EXAMPLE OF SPONTANEOUS PROCESS
i
...

This requires work to transfer heat from one place to the other
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ii
...


iii
...


Criteria of spontaneity
A
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fFor a spontaneous change to occur, time is not a factor because a
spontaneous criteria can take place so rapidly or very slowly
C
...

D
...
To take away the system from equilibrium some external work must be done
on the system
F
...
This implies that only exothermic rxns can occur but the melting
of ice and evaporation of rain water are endothermic process which proceed
spontaneously similarly for rain water to be evaporated
Therefore there is some factor in additional to enthalpy which governs
spontaneously and this is the 2nd law of thermodynamics which introduces the
new factor called entropy
By definition entropy is a thermodynamic state quantity which is a measure of
randomness or disorder molecules in a system
...


The 2nd law of thermodynamics state that whenever a spontaneous change or process
occurs it is accompanied by increase in the total energy of the universe
...

The second law tells us that an irreversible spontaneous process occurs, the entropy of the
system and the surroundings must increase i
...
ΔS universe >o
But when a reversible process occurs the entropy of the sysem remains constant and
therefore Δ s increase =o
i
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Δs system + Δ s surrounding = o
therefore Δ s system = -Δ s surrounding for a reversible process
...


State of the 3rd law
The entropy of a substance varies directly with temperature
...

Further cooling leads to the formation of ice crystals that are highly ordered and the
entropy of the system is very low
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At absolute zero, all molecular vibrations seize and the water molecules are said to
be perfectly ordered and the entropy of the system is zero and this leads to the
statement of 3rd law of thermodynamics which states that at absolute zero the
entropy of a pure crystal is also zero i
...
s=o at T=ok
By numerical definition of entropy was first introduces by scientist called Clausius
(1850)
According to him the entropy of a system, not undergoing any chemical or physical
changes is a constant quantity when there is no communication of heat
...

𝑞𝑞

𝑇𝑇

Heat flowing out of the system produces a corresponding decrease of -

𝑞𝑞

𝑇𝑇

Therefore for a reversible change taking place at a field temperature, T, the change
is entropy ΔS=heat absorbed or evolved divided by temperature T i
...

𝑞𝑞

Δs = 𝑇𝑇

If heat is absorbed the Δs is positive, then there will be an increase in entropy whereas if
heat is evolved, the Δs is negative and a decrease in entropy
...
184 Eu

Standard entropy
From the 3rd the entropy of a pure crystal is zero at absolute K and therefore it is
possible by measurement and calculations to find the actual amount of entropy that a
substance posses at any temperature above zero kelvin this is referred to as
absolute entropy
...
CO2 (g)+2NH3(g)

Because the product formed a more gaseous compounds to reactants entropy will
be a true value
...

Substance

So(cal k-1 mol-1)

CO(NH2)2

41
...
73

CO2(g)

51
...
01

Δso=∑So(products)………
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06+92
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55+16
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08-58
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81 ca k-1mol-1
Calculate the standard entropy of formation of carbon dioxide gas given that the entropy of
CO2(G), C(S) and O2(G) are 213
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74, 205
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CO2(g)
SOF=SO(COMPOUNDS) -∑SO(ELEMENTS)
SOf =(SO(CO2)-[∑so(c(s) )+ SOO2(S) ]
213
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79+205
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86 JK-1

DEFINITIONS
1
...
HEAT ENGINE
The flow of heat from a hot body to cold body is a spontaneous process a machine
which can do work by using heat that flows at continuously from a higher
temperature source to a low temperature source is called heat engines
...
e
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Efficiency of heat engine
The rate of work obtained in a cyclic process(w) to the heat taken from high
temperature reservoir (q) is referred to as efficiency of a heat engine i
...
efficiency
𝒘𝒘
=
𝒒𝒒

no heat engine can convert all the heat from high temperature body into work
because such an engine would be 100% efficiency
...
Carnot imaginary engine would perform
a series of operation between temperature
...
These cycles of process
which occurred under reversible conditions is referred to as Carnot cycle
The medium employed is operating Carnots engine was 1 mole of an ideal gas
which could be imagined to be contained in a cylinder fitted with a frictionless piston

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