Thermodynamics

Terms used frequently in thermodynamics

• System• Surroundings• Isolated system• Closed system• Open system• State of a system• State variables

Extensive propertiesAn extensive property of a system is that which depends upon the amount of the substances present in the system.Examples: Mass, volume, energy

Intensive propertiesAn intensive property of a system is that which is independent of the amount of the substances present in the system. Examples: T, P, d, Conc, viscosity, refractive index, surface tension

Pocesses and Their types

The operation by which a system changes from one state to another is called a process.

• Isothermal process• Adiabatic process• Isobaric process• Reversible and Irreversible

Internal Energy

The total of all the possible kinds of energy of a system, is called its internal energy.

Δ E = E f – E in

-State function- Extensive property

First Law of Thermodynamics

StatementEnergy can neither be created nor destroyed, although it can be transformed from one form to another.

- Law of conservation of energy

E=mc2

Modified statement:The total mass and energy of an isolated system remains unchanged though it may change from one form to another.

- Perpetual motion machine

Mathematical statement

Δ E = q – w, where q = the amount of heat supplied to the system,w= work done by the system

Enthalpy

The total heat content of a system at constant pressure is equivalent to the internal energy E plus the PV energy.

H = E + PV- Relation between ΔH and ΔE- Enthalpy of Vaporisation- Enthalpy of Fusion

Molar heat capacity

The amount of heat required to raise the temperature of onemole of the substance (system) by 1 K.

- Molar heat capacity at constant volume (Cv)- Molar heat capacity at constant pressure (Cp)

- Relation between Cp and Cv

Joule-Thomson Effect

The phenomenon of producing lowering of temperature when a gas is made to expand adiabatically from a region of high pressure into a region of low pressure, is known as Joule-Thomson effect or Joule-Kelvin effect.

Joule-Thomson Coefficient

The number of degrees temperature change produced peratmosphere drop in pressure under constant enthalpy conditions on passing a gas through the porous plug, is called Joule-ThomsonCoefficient.

μ = dT/dP,

μ = positive cooling μ = negative warming

Thermochemistry

- Enthalpy of Reaction- Exothermic and Endothermic Reactions

Variation of heat of reaction with temperature (Kirchoff’s equations)

The heat of reaction changes with change in temperature ofa gas due to variation in its specific heat.

Different types of heat (enthalpy) of reaction• Heat of Formation• Heat of Combustion• Heat of Solution• Heat of Neutralization• Heat of Fusion• Heat of Vaporization• Heat of Sublimation• Heat of Transition

Hess’s Law

If a chemical change can be made to take place in two or more different ways whether in one step or two or more steps, the amount of total heat change is same no matter by which method the change is brought about.

- Bond Energy

Second Law of Thermodynmics

- Limitation of First Law- Spontaneous process- Entropy

Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously

Entropy

- Concept of Entropy

Entropy is thermodynamic state quantity that is measure of the randomness or disorder of the molecules of the system.

• Like total energy, E, and enthalpy, H, entropy is a state function.

• Therefore, S = Sfinal Sinitial

Second Law of Thermodynamics

Whenever a spontaneous process takes place, it is accompanied by an increase in the total energy of the universe.

Reversible (ideal):

Numerical Definition of Entropy

For a reversible change taking place at a fixed temperature (T),the change in entropy (ΔS) is equal to heat energy absorbed orevolved divided by the temperature (T).

ΔS = q / T

Entropy change for An Ideal Gas

- T and V as variables- P and T as variables

Free Energy

Concept of Free Energy

Variation of Free energy with temperature and pressure

Gibb’s Helmholtz Equations

The Third Law of Thermodynamics

At the absolute zero of temperature, the entropy of every substance may become zero and it does become zero in the case of a perfectly crystalline solid.

Multiple Choice Questions

Question 1Mixing of two or more gases is a a) Spontaneous processb) Non-spontaneous processc) Reversible processd) None of these

Key: (a)

Question 2The Free Energy function (G) is defined asa)G = H + TSb) G = H – TSc) G = TS – Hd) None of the above

Key: (b)

Question 3Which out of the following is not a state functiona) Free energyb) work functionc) Entropyd) work done

Key: (d)

Question 4When water is cooled to ice, its entropya) Increasesb) decreasesc) remains the samed) becomes zero

Key: (b)

Question 5In a process Δ H = 100 kJ and Δ S = 100 JK-1 at 400 K. The value of Δ G will bea) Zerob) 100 kJc) 50 kJd) 60 kJ

Key: (d)

Question 6Which of the following always has a negative value?a) Heat of formationb) Heat of reactionc) Heat of combustiond) Heat of solution

Key: (c)

Question 7The heat of neutralization of a strong acid and strong base is alwaysa) Zerob) constantc) Positived) changing

Key: (b)