Chemical Thermodynamics

The study of movement of heat energy in a chemical reaction is known a Thermodynamics .

System : is The part of the universe which is under thermodynamic consideration. The thermodynamic consideration is always taken at room temperature (25o C = 298 K) Surrounding : is the part of universe other than the system. Boundary : separates the system and the surroundings.

System Configuration
Open System   : Transfer of Energy and matter is allowed

Closed System : Transfer of Energy is allowed but matter is not allowed

Isolated System :  Transfer of Energy and matter is not possible. (thermostat)

The part of the universe, apart from the system is called as Surroundings .

State
State , is the condition of the system. It is defined by using state variables. State variables are the properties of the system like temperature, pressure , energy ,etc.

When a state changes from one form to other, we say a process  has occurred.

Process
Isocoric       Volume remains constant

Isobaric       Pressure remains constant

Isothermal  Temperature remains constant

Adiabatic    There is no heat exchange

Cyclic          The final and initial state of the process is same  (All state variables should be restored) Reversible   The driving force is greater than the opposing force by infinitely small difference.

Extensive Property                                                                                                                                    These properties depend upon the amount in the system. e.g. Mass, volume , pressure , internal energy , heat capacity ,  entropy , enthalpy , gibbs free energy.

Intensive Property                                                                                                                                     These properties are independent of amount taken in a system. e.g. Density, Viscosity , refractive index , specific heat capacity , resistivity.

Functions
State Function :  The state variables do not depend on the pathway. They only depend on the state of the system. e.g. Potential Energy, Entropy.

Path Function :  The state variables do depend on the pathway. e.g. work, heat.

Thermodynamic Equilibrium : A system is said to be in equilibrium if the state variables do not change with time.

Heat and Work
The form of energy exchanged between two bodies at different temperatures is known as Heat energy. The amount of heat transferred is represented by q.

Heat is gained         q is positive

Heat is lost               q is negative

If a force applied on a body, displaces the body , then work is said to be done.

Pressure Volume Work
Isothermal Irreversible Process

W = - Pex ∆V

W = -∆nRT

Isothermal Reversible Process

W = - 2.303 nRT log10 (V2/V1)

= -2.303 nRT log10 (P1/P2)

NOTE : Work done = Area under PV curve and Work is a path function.

First Law of Thermodynamics
The First Law of Thermodynamics states that the energy lost from one part of the system evolves in another form, thus conserving total energy of the universe.

1) Th total energy of an isolated system remains constant.

2) Energy can be converted from one form to another but not destroyed.

3) The total energy of universe is always constant.

4) Whenever quantity of energy of one kind disappears, an exactly equivalent amount of energy of other kind must evolve.

Mathematical Form

∆U = q + W

First Law for different Processes

Isothermal Process


 * ∆U = 0 | ... 0 = q + W   | ... - q = W |

Isochoric Process


 * ∆V = 0  |... W = 0  | ... ∆U = q |

Isobaric Process


 * ∆U = q + W | But, W = - P∆V |.. ∆U = q - P∆V |

Adiabatic Process


 * q = 0 |... ∆U = W |

Enthalpy
∆H = ∆U + P∆V

∆H = ∆U + RT∆n

Enthalpy for Physical Processes
Enthalpy of Fusion

Enthalpy of Freezing

Enthalpy of Vaporization

Enthalpy of Condensation

Enthalpy of Sublimation

Enthalpy of ionization

Enthalpy of Electron Gain

Enthalpy of Atomization

Enthalpy of Dilution

Hess' Law
Hess' Law states that the change in enthalpy for a reaction is the same whether the reaction is carried out in 1 step or a series of steps. This means that Enthalpy is a State Function.

Born Haber Cycle
For NaCl, E = -411 kJ

Enthalpy of Sublimation : 109 kJ

Enthalpy of Ionization : 496 kJ

Enthalpy of Atomization : 122 kJ

Enthalpy of Electron Gain : - 348 kJ

Lattice Enthalpy : -790 kJ

Enthalpy of Solution
Solute - Solute Interaction : The enthalpy of lattice gives us an idea about the solute-solute interaction.

Solvent-Solvent Interaction :

Solute - Solvent Interaction : The Hydration Enthalpy gives us an idea about solute - solvent interaction.

Enthalpy of Chemical Reaction
Enthalpy of a Chemical Reaction is also called the heat of reaction.

... ∆H = ΣHproducts- ΣHreactants

Enthalpy of chemical reaction is expressed as joule or kilo joule. The individual enthalpies are expressed as Joule per Mole.

The enthalpy of a substance in its elemental state is 0. e.g. ∆H(O2) = 0, ∆H(Cl2) = 0 , ∆H(C) = 0

Standard Enthalpy 

The enthalpy change that accompanies a chemical reaction in which all the substances are in their standard forms.

Enthalpy of Formation 

The enthalpy change that accompanies a reaction in which the products are formed in their standard forms from its elements also in the standard form.

Enthalpy of Combustion

The enthalpy change that accompanies the complete combustion of one mole of a substance or its raction with oxygen.

Bond Enthalpy
The enthalpy change needed to break bonds in one mole of gaseous molecules is called as bond enthalpy. An exactly opposite energy is applied to break the bond.

Let BE be the Bond Enthalpy.

... ∆HoREACTION = Σ ∆Horeactants- Σ ∆HoProducts

Spontaneous Process
A spontaneous process is a process which occurs irreversibly without any need of external force. A Non-spontaneous Process is a process which does not occur on its own and is reversible.

For a reaction to occur spontaneously, the stability of product should be more than that of reactants. Thus, the energy of products should be less than that of reactants. ... The system should lose energy to perform a spontaneous process. Thus we can formulate :-

q is -ve ; reaction is spontanous

q is +ve ; reaction is Non-spontaneous

But, this rule is not universally applicable. For e.g. it does not hold true for the melting or fusion of solid to liquid, which is an endothermic reaction but still is a spontaneous reaction , as it occurs on its own at room temperature.

Entropy
Entropy is the measure of disorderness. The gaseous state is more disordered than liquid and liquid is more disordered than solid, due to their mobility of molecules.

Thus, the entropy of gaseous state is higher than liquid , higher than solid state. This shows that entropy change is directly proportional to the heat provided to it in a reversible manner. Now, at higher temperatures the change in heat won't have a big change in Entropy. At lower temperatures, a change in heat , causes greater change in entropy. This shows that Entropy change is inversely proportional to the temperature at which the system is already present.

... ∆S = qrevundefined/ T 

Here, we cannot calculate the entropy , but we can calculate the change in entropy.

Second Law of Thermodynamics
The Second Law of thermodynamics States that the spontaneous flow of heat is always unidirectional from higher temperature to lower temperature. Heat cannot be completely converted into an equivalent amount of work without producing permanent changes either in the system or its surroundings.

The total energy of the universe increases in a spontaneous process.

∆Suniverse = ∆Ssys + ∆Ssurr

Thus, if ∆S is+ve ; reaction is spontanous ; (i.e. the ∆S is increasing)

∆S is -ve ; reaction is non-spontaneous

Though this law is universally valid, it does not fall under thermodynamics because it involves the observation of surrounding , which is against the basic rule of thermodynamics. Thus, we have the third law of thermodynamics , in which the second law has been modified in terms of system properties only.

Gibbs Free Energy
Gibbs Energy was introduced to modify the second law of thermodynamics, so that it deals only with the system.

G = H - TS

∆G = ∆H - T∆S

Consider ∆Ssurr = qrev / T

∆Ssurr = - ∆H / T

Now, ∆Stotal= ∆S + ∆Ssurr

∆Stotal= ∆S - H/T

- ∆Stotal = H/T - ∆S

- T∆Stotal= H - T∆Sundefined

-T∆Stotal= H -T∆Sundefined

∆G =- T∆Stotal

G is -ve ; Reaction is Spontaneous

G is +ve ; Reaction is Non-Spontaneous

∆G = ∆Go+ RT ln Q  {For a condition of the reacting system, other than equilibrium}

∆G = ∆Go + RT ln K  {For a reacting system at Equilibrium}

But, ∆G = 0

... ∆Go = - RT ln K = - 2.303 RT log10 K

Third Law of Thermodynamics
The third law of thermodynamics states that the entropy of a perfectly ordered crystalline substance is zero at absolute zero of temperature. ... S = 0 ; T=0.

Derivations

 * 1) Work Done in isothermal irreversible process.
 * 2) Work done in isothermal reversible process
 * 3) Born-Haber Cycle
 * 4) Enthalpy Change
 * 5) Entropy
 * 6) Gibb's Energy