Fundamentals of thermodynamics:- 1st law, 2nd law and 3rd law short notes

·         Deals with energy and the energy changes.

Thermodynamics can only give information about a system when it is at equilibrium state; a time-invariant state.

When it is based on the concept of equilibrium it’s known as “Equilibrium thermodynamics”.

When it is based on the concept of time-invariant state it’s known as “Thermodynamics of Steady state” or “non-equilibrium thermodynamics”.

Equilibrium thermodynamics: - Only with closed and isolated systems.

System

·         Open: Both matter and energy can transfer between system and surroundings.

·         Closed: Only energy can transfer.

·         Isolated: Neither energy nor matter can transfer.

·         Homogenous system: Consists of a single phase.

·         Heterogeneous system: Consist of two or more phases.

Isothermal: Constant T

Isobaric: Constant P

Isochoric: Constant V

Adiabatic: No heat transfer between the system and the surroundings.

Heat and work do not belong to system and are NOT properties of thermodynamics.

They are operations which performed on the system to alter its energy.

Properties

·         Extensive: Describe and depend on the size of the system. (Mass, volume, pressure…)

·         Intensive: Does not depend on the size of the system. (Molar volume, Molecular weight, Temperature…)

First law

Introduces the concept of internal energy.

·         DQ = U + W

From 1^st law;

When isothermal: DU= 0

                              DQ = DW

At constant volume: DW = 0

                                  DQ = DU

W = - PDV   it is a minus value for a closed system in expansion

W=nRTln.V_f / V_i   Can be taken for an isothermal expansion of a gas

Heat capacity

C=dQ/dT

At constant P: C_p= (dH/dT)_p

At constant V: C_v= (dU/dT)_v

To know how the reaction proceeds we need to know;

·         Enthalpy-H

·         Entropy-S

Enthalpy

DH^o  = å n H^o_products  - å n H^o_reactants

DH = U + PV

DH = C_pdT (as above mentioned in heat capacity)

DH = mCDǾ

Second Law

Describes entropy. Entropy is an idea of randomness in a reaction.

S>0 reaction is spontaneous

S<0 reaction is non spontaneous

S=0 reaction is at equilibrium

·         DS  =  DQ/T

At constant pressure

DS  =  DH/T

Also;

DS_universe  =  DS_system  +  DS_surroundings

DS^o  =  å n S^o_products  -  å n S^o_reactants

Third law

Absolute Entropy, S, =  0  at 0  Kelvins for a perfect crystal of a pure substance.

DG,  Gibbs Free Energy

The maximum amount of energy available to do useful work on the surroundings.

·         DG  = D H – T D S

DG  <  0  (-) Spontaneous

DG  >  0 (+) Spontaneous in the opposite direction       DG  =  0  equilibrium

·         DG^o  =   DH^o  -  T DS^o

DS (+), DH (-) Spontaneous at all temperature

DS (+), DH (+) Spontaneous at high temperatures (where exothermicity is relatively unimportant)     

    

DS (-), DH (-) Spontaneous at low temperatures

            (where exothermicity is dominant)

DS(-), DH (+)  Process not spontaneous at any temperature (reverse process is spontaneous at all temperatures)

·         DG  = DG^o  +  RT ln Q

                       Q  =  Reaction Quotient.

Free energy at equilibrium

·         G  =  0

·         So DG^o  =  -RT ln Q_equilibrium

              Q_equiliibrium  =  K_p (gases)

                                 =  K_c  (solution)