Engineering Chemistry S Datta

88 ENGINEERING CHEMISTRY
cyclic process ATP is hydrolysed to ADP, free energy is released which can again be coupled to
another biochemical reaction of our system that is non-spontaneous or requires energy. Thus,
the energy of glucose oxidation can be utilised to carry out other non-spontaneous reaction via
ATP.
Highlights:
• Intensive property is independent of the amount of substance present in the system
e.g., temperature, pressure, viscosity, surface tension, density, specific heat,
refractive index, etc.
• Hess’s law states: For a chemical process, the amount of heat evolved or absorbed
is the same whether the process takes place in one step or in several steps.
• Second law of thermodynamic states: It is impossible to convert heat completely
into work.
• ∆G is negative for a spontaneous reaction.
• ∆G is zero for reaction at equilibrium.
• ∆G is positive for non-spontaneous reaction.
• ∆S for a spontaneous process is ∆S > 0.
• The entropy of the universe is increasing.
• Third law of thermodynamics states: The entropy of a solid or a liquid is zero at
the absolute zero of temperature.
• Clapeyron-Clausius equation is:
dP
dT =
q
.
T(V2 − V 1)
Spontaneity of changes of a system
• Many of the spontaneous processes are accompanied with a decrease in enthalpy but
this statement is not always true, reactions with increasing enthalpy may also occur
spontaneously.
• A spontaneous process always has a net increase in entropy for the computation of the
change of entropy, the changes in entropy of both the system and the surrounding are
to be considered when a system is in equilibrium,
dS total
= 0
For any spontaneous irreversible process,
dS total
> 0
When a reversible process is at equilibrium,
dS + dS′ = 0
For a spontaneous process,
dS + dS′ > 0
where dS and dS′ are the changes of entropy of the system and the surroundings
respectively.
• For a spontaneous process at a definite T and P, we can calculate ∆G of the system, if
the ∆G is negative the change is spontaneous, if ∆G is zero the initial and final states of
the system will be at equilibrium, if ∆G is positive the process is non-spontaneous.