Chemical Thermodynamics

1. CHM 313
CHEMICAL THERMODYNAMICS
DR. (MRS.) OSUNDIYA, M.O.
LECTURE: C19-1

2. COURSE OUTLINES
• The First Law of themodynamics- Law of conservation of Energy
• Mathematical Formulation of the
first law of thermodynamics
• The Second Law of Thermodynamics----- Spontaneous Processes
• Relationship between Energy,
Enthalpy, and Entropy
• The combination of the first and the second laws of
thermodynamics
• The Third Law of Thermodynamics—Physical significance of the
third law of thermodynamics
• Entropy changes

3. THE LAWS OF CONSERVATION OF ENERGY
1ST LAW OF THERMODYNAMICS
The law has been stated in various forms, but the fundamental implication is that
although energy may be transformed from one form into another, it can neither be
created nor destroyed.
Mathematical formulation of the 1st law of thermodynamics. Consider a system
in site A(Fig 1) with internal energy 𝐸𝐴. It absorbs from the surroundings a certain
amount of heat , Q and undergoes a change in its opposition to B where its energy is
𝐸𝐵 , The change in energy of the system ∆𝐸 is given by:
∆𝐸 = 𝐸𝐵 − 𝐸𝐴 (1)

4. • Note that the change is independent of the path or manner in which the change
has been brought about.. If w is the work involved in this transformation, the
net gain of energy is 𝑄 + 𝑤, from the -
1
𝑠𝑡
law
∆𝐸 = 𝐸𝐵 − 𝐸𝐴 = 𝑄 + 𝑤 2𝑎
∆𝐸 − 𝑤 = 𝑄 (2𝑏)
For an infinitesimally change 𝛿𝑄 = 𝛿𝐸 − 𝛿𝑤 (2𝑐)

5. Equations (2𝑏) and (2𝑐) are the mathematical form of the 1st law of
thermodynamics. The heat absorbed is equal to increase in energy of the system
plus the work done by the system. If the system loses heat to the surrounding, its
energy decreases, ∆𝐸 will be negative and work will be done on the system by
the surrounding. The above change can be brought about by a large number of
paths (Fig 1) and since E is a state function, its magnitude depends on the state
of the system.The change in ∆𝐸 will be independent of the path followed.

6. A
B
v
p
iii
ii
i
Fig 1: Energy change independent of the path
The number of paths involved for A system to move from point A to point
B

7. ENERGY CHANGE IN AN ISOLATED SYSTEM
If the transformation is carried out under adiabatic condition, such that heat
neither enters nor leaves the system, then 𝑄 = 0, and therefore
∆𝐸 = 𝑤 2𝑐
∆𝐸 − 𝑤 = 0 2𝑑
The work done by the system would be at the cost of the internal energy, and is
equal to the decrease in its energy content or the algebraic sum or difference of
the changes in energy and the work performed in an isolated system is zero.

8. ENERGY CHANGE IN A CYCLIC PROCESS
If the system after undergoing a change in its state is brought back to its initial
state as in the case of Carnot cycle.
∆𝐸 = 0 2𝑒
𝑄 = 𝑤 (2𝑓)
The heat absorbed by the system from the surrounding is exactly equal to the
work done by the system on the surrounding. Equation (2𝑓) establishes the
impossibility of a perpetual motion of the first kind, viz, work cannot be
produced without withdrawing heat from an external source.

9. THE 𝟐𝒏𝒅 LAW OF THERMODYNAMICS
The primary and essential interest of thermodynamics to Chemists is to use
it as a criterion of feasibility of physical and chemical changes under a
given set of conditions. The first law merely sums up of experience
regarding the energy changes and states that if a process is to occur then
the total energy before and after the transformation is constant, But
whether such a process is possible or not , the law gives no information.
The two functions 𝐸, 𝑎𝑛𝑑 𝐻 introduced in the first law can predict the
feasibility of an exothermic process in which ∆𝐸, 𝑎𝑛𝑑 ∆𝐻 decrease.

10. However, there are many endothermic processes in which ∆𝐸, 𝑎𝑛𝑑 ∆𝐻 increase,
and yet the processes are feasible. Hence these functions are clearly insufficient
in predicting the direction and feasibility of a process under specified conditions
The first law is therefore inadequate, provide no information concerning the
feasibility of a process and an additional law is required.
The law provides the necessary criterion of feasibility of a process in terms of
additional thermodynamic functions and other related expressions.

11. SPONTANEOUS PROCESS
Processes which take place of their own without the external
intervention of any kind are known as spontaneous processes. All
the processes that takes place in nature are spontaneous in
character, proceeds only in one direction and are, therefore
thermodynamically irreversible. They can be reversed only with
the aid of an external agency. The following examples of
spontaneous changes can help understand the irreversible nature
and the conditions which determine their feasibility.

12. 1. When two metal blocks A and B at temperature 𝑇𝐴 and 𝑇𝐵 such that 𝑇𝐴 >
𝑇𝐵 are brought in contact with each other, heat flows from A to B. The flow
of heat is spontaneous and continues until they attain a uniform temperature
and a thermal equilibrium is established. The reverse process in which A and
B initially at the same temperature attaining a state in which A becomes
warmer at the expense of B is never observed. The reverse process is thus
non spontaneous.
2. Flow of liquids from a higher to a lower level is a spontaneous process, and
the flow continues until the two levels are equal and a mechanical
equilibrium is attained. The reverse process is not observed to occur.

13. 3. If a vessel containing a gas at higher pressure is connected by
means of a valve to another vessel filled with the gas at lower
pressure, then on opening the valve the gas spontaneously passes
from higher pressure to the lower pressure side until the combined
system attains a uniform pressure i.e. a mechanical equilibrium is
established. The process of mixing is spontaneous and the reverse
change in which the concentration of the gas suddenly becomes
greater in one part is never observed.