Thermodynamics
Thermodynamics :
The science which deals with the study of different forms of energy and quantitative relationship.
System & Surroundings :
The part of the universe for the study is called system and the remaining portion is surroundings.
State of system & state function :
The state of a system is described in terms of T, P, V, etc. The property which depends only on the state of the system not upon path is called state function eg. P, V, T, E, H, S, etc.
Extensive & Intensive Properties :
Properties which depend on the quantity of matter called extensive prop. eg. mass, volume, heat capacity, enthalpy, entropy, etc. The properties which do not depend on matter present depends upon nature of substance called Intensive properties. eg. T, P, density, refractive index, viscosity, bp, pH, mole fraction, etc.
Internal energy :
The total energy with a system. i.e. U = Ee En + Ec + Ep + Ek + ------ ∆ U = U2 – U1 or UP – UR & U is state function and extensive properly. If U1 >U2 energy is released.
Heat (q) :
It a form of energy which is exchanged between system and surrounding due to difference of temperature. Unit is Joule (J) or Calorie (1 Calorie = 4.18 J).
First Law of Thermodynamics :
It is law of conservation energy. Energy can neither be created not destroyed, it may be converted from one from into another. Mathematically ∆U = q + w, w = –p. V∆ (work of expansion) ∆U = q – p. ∆ V or q = ∆ U + p. ∆V, q,w are not state function. ∆But U is state function.
Enthalpy (H):
At constant volume ∆V = 0,qv =∆So H = U + p. ∆V, qp = H2 H1 = ∆H ➱ ∆H = ∆ U + P.∆V.
Relationship between qp, qv i.e. ∆H& ∆U It is ∆ H= ∆U+ ∆ng.RT or qp = qv + ∆ ng.RT
Exothermic and Endothermic reactions :
∆H = –Ve for exothermic and ∆H = +Ve for endothermic reaction i.e. evolution and absorption of heat.
Eg C+O2 → CO2 + 393.5 KJ, H = –393.5 KJ (exothermic) N2 + O2 → 2NO – 180.7 KJ, H = 180.7 KJ (Endothermic)
Enthalpy of reaction ( ∆rH) :
The amount of heat evolved or absorbed when the reaction is completed.
Standard Enthalpy of reaction ( ∆ rHo) at 1 bar pressure and specific temp. (290K) i.e. standard state.
Different types of Enthalpies of reactions:
| ( i ) Enthalpy of combustion (∆cH) | ( ii) Enthalpy of formation (∆fH) |
| ( iii ) Enthalpy of neutralization | ( iv ) Enthalpy of solution |
| ( v ) Enthalpy of atomization (∆aH) | ( vi ) Enthalpy of Ionisation (∆iH) |
| ( vii ) Enthalpy of Hydration (∆hyolH) | ( viii ) Enthalpy of fusion (∆fusH) |
| ( ix ) Enthalpy of vaporization (∆vapH) | ( x ) Enthalpy of sublimation (∆subH) |
(∆subH) = ∆fus(H) - ∆vapH) --------
Bond enthalpy :
It is amount of energy released when gaseous atoms combine to form one mole of bonds between them or heat absorbed when one mole of bonds between them is broken to give free gaseous atoms. Further ∆ rH = ∑B.E. (Reactants) - ∑B.E. (Products)
Spontaneous & Non Spontaneous Processes :
A process that can take place by itself is called the spontaneous process. A process which can neither take place by itself or by initiation is called in
Driving forces for the spontaneous process :
(i) The tendency for minimum energy state.
(ii) The tendency for maximum randomness.
Entropy (S) :
It is a measure of randomness or disorder of the system.i.e. Gas>Liquid>Solid.
Spontaneity in term of ( ∆S ) ∆S(total) = ∆S(universe) = ∆S(system) + ∆S(surrounding) If ∆S(total) is +ve, the process is spontaneous If ∆S(total) is –ve, the process is non spontaneous.
Second Law of thermodynamics :
In any spontaneous process, the entropy of the universe always increases. A spontaneous process cannot be reversed.
Gibb’s free energy (G) :
defined as G = H – T.S & ∆G = ∆H – T.∆S (Gibb’s Helmholts equation) it is equal useful work .e. - ∆G = W(useful) = W(max).If G = ve, process is spontaneous.
Effects of T on the spontaneity of a process :
∆G = ∆H – T. ∆ S.
(i) For endothermic process may be non-spontaneous at low temp.
(ii) For exothermic process may be non-spontaneous at a high temp. and spontaneous at low temp.
Calculation of ( ∆ rGo)
∆rGo = ∑∆fGo (p) - ∑ ∆fGo (r)
Relationship between ( ∆rGo) & equilibrium constant (k)
∆G = ∆Go + RTlnQ & ∆Go = –2.303RT logk.
Calculation of entropy change:
∆rSo = ∑∆ So (p) - So (r)
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