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Chapter: 11th Chemistry : UNIT 7 : Thermodynamics

Summary: Thermodynamics(Chemistry)

Chemistry : Thermodynamics: Summary


The branch of science which deals the relation between energy, heat and work is called Thermodynamics. The main aim of the study of chemical thermodynamics is to learn (i) transformation of energy from one form into another form (ii) Utilization of various forms of energies.


System: A system is defined as any part of universe under consideration. There are three types of thermodynamic systems. They are (i) isolated system (ii) closed system and (iii) open system.


Surrounding: Everything in the universe that is not the part of the system is called surrounding.


Boundary: Anything which separates the system from its surrounding is called boundary.


Thermo dynamic Properties: Any quantity that depends only on the state of system is called thermodynamic property of a system. There are two kinds of thermodynamic properties called (1) intensive - independent of the quantity of material and (2) extensive - directly proportional to the quantity of material. There are five basic thermodynamic properties. (U,H,S and G)


Thermodynamic Processes:

Adiabatic process in which no heat transfer takes place (q = 0)

Isothermal process in which temperature remains constant (dT =0).

Isobaric process in which pressure remains constant(dP =0).

Isochoric process in which volume remains constant(dV =0).

Cyclic process in which the system returns to its original state after completing a series of changes.


Internal energy (U): Internal energy of a system is equal to the energy possessed by all its constituents namely atoms, ions and molecules. The energy of a system of molecules is equal to the sum of its translational energy, vibrational energy, rotational energy, bond energy, electronic energy and energy due to molecular interactions.


Heat: Heat is regarded as the energy in transit across the boundary separating a system from its surrounding. Heat is a path function. The SI unit of heat is joule (J)


Work : Work is defined as the force (F) multiplied by the displacement -w=F.x, work is measured in Joules, i.e the SI unit of work is Joule. During expansion or compression of a gas the work done is calculated by the relation w= -PΔV.


The sign conventions for heat and work are as follows:


If heat is absorbed by the system : +q

If heat is evolved by the system                                                                                    : -q

If work is done by the system : -w

If work is done on the system : +w


Laws of Thermodynamics:


Zeroth law : When two systems are separately in equilibrium with a third system, they are in equilibrium with each other.


First law: "Energy can neither be created nor destroyed, but may be converted from one form to another".


ΔU = q + w.


Enthalpy is a thermodynamic property of a system. Enthalpy H is defined as the sum of the internal energy and pressure volume work.


H=U+PV. Enthalpy change, ΔH = ΔU + ΔngRT.


Hess's law: It states that “the enthalpy change of a reaction either at constant volume or constant pressure is the same whether it takes place in a single or multiple steps”. Hess's law can be applied to calculate enthalpies of reactions that are difficult to measure.


Heat capacities: [Cp and Cv]


Heat capacity is defined as the amount of energy required to increase the temperature of one unit quantity of material by one degree, under specific conditions. It can be measured under two different conditions, namely,


a. constant pressure Cp = dH/dT

b. constant volume  Cv = dU/dT


Second law of thermodynamics:


The second law of thermodynamics helps us to predict whether the reaction is feasible or not and also tells the direction of the flow of heat.


To predict spontaneity of a process, a new thermodynamic quantity namely entropy (S) was introduced. Entropy is a measure of the randomness or disorderliness of the system.


Entropy statement: “whenever a spontaneous process takes place, it is accompanied by an increase in the total entropy of the universe”.


Kelvin-Planck statement: It is impossible to take heat from a hotter reservoir and convert it completely into work by a cyclic process without transferring a part of that heat to a colder reservoir.


Clausius statement: This statement recognises that heat flows spontaneously from hot objects to cold objects and to get it flow in the opposite direction, we have to spend some work.


Gibbs Free Energy (G):


is expressed as G=H-TS, free energy change of a process is given by the relation ΔG= ΔH-TΔS.


Standard free energy change and equilibrium constants are related by the equation ΔG0 = -RTlnKeq


Third law:

The entropy of a perfectly crystalline material at absolute zero is zero.

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