These notes for CBSE Class 11 Chemistry, Chapter 6 on Thermodynamics, explain fundamental concepts of energy and its transformations. It defines thermodynamics as the study of energy forms and their quantitative relationships. Key topics include systems, surroundings, state functions (like P, V, T), and extensive/intensive properties. The notes detail internal energy (U), heat (q), and work (w), introducing the First Law of Thermodynamics as the conservation of energy (?U = q + w). Enthalpy (H) is explained, along with its relation to internal energy and its significance in exothermic and endothermic reactions. The concept of spontaneity is explored through entropy (S), the measure of disorder, and the Second Law of Thermodynamics, which states that the entropy of the universe increases in spontaneous processes. Gibbs free energy (G) is introduced as a criterion for spontaneity (?G = ?H – T?S), and its relationship with equilibrium constant (K) is also covered. These notes are ideal for revising core thermodynamics principles for exams.
Last optimized 10 Aug 2026
Jump to solutions, books, papers, or another subject — without leaving NCERT Help.
Science which deals with study of different forms of energy and quantitative relationship.
The part of universe for study is called system and remaining portion is surroundings.
State of system is described in terms of T, P, V etc. The property which depends only on state of system not upon path is called state function eg. P, V, T, E, H, S etc.
Properties which depends on quantity of matter called extensive prop. eg. mass, volume, heat capacity, enthalpy, entropy etc. The properties which do not depends on matter present depends upon nature of substance called Intensive properties. eg. T,P, density, refractive index, viscosity, bp, pH, mole fraction etc.
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.
It I 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).
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.
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
?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)
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.
| ( 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) --------
It is amount of energy released when gaseous atoms combines to form one mole of bonds between them or heat absorbed when one mole of bonds between them are broken to give free gaseous atoms. Further ? rH = ?B.E. (Reactants) - ?B.E. (Products)
A process which can take place by itself is called spontaneous process. A process which can neither take place by itself or by initiation is called non Spontaneous.
(i) Tendency for minimum energy state. (ii) Tendency for maximum randomness.
It is measure of randomness or disorder of 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.
In any spontaneous process, the entropy of the universe always increases. A spontaneous process cannot be reversed.
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.
?G = ?H – T. ? S. (i) For endothermic process may be non spontaneous at law temp.
(ii) For exothermic process may be non spontaneous at high temp. and spontaneous at law temp.
?rGo = ??fGo (p) - ? ?fGo (r)
?G = ?Go + RTlnQ & ?Go = –2.303RT logk.
?rSo = ?? So (p) - So (r)
Thermodynamics is the science that deals with the study of different forms of energy and their quantitative relationships.
The First Law of Thermodynamics is the law of conservation of energy, stating that energy can neither be created nor destroyed, only converted from one form to another. Mathematically, ?U = q + w.
Enthalpy (H) is a thermodynamic property defined as H = U + PV. It represents the total heat content of a system. The change in enthalpy (?H) is equal to the heat absorbed or released at constant pressure.
Entropy (S) is a measure of the randomness or disorder of a system. Entropy generally increases in spontaneous processes.
The Second Law of Thermodynamics states that in any spontaneous process, the entropy of the universe always increases.
Gibbs free energy (G) is a thermodynamic potential that can be used to calculate the maximum reversible work that may be performed by a thermodynamic system at constant temperature and pressure. It is defined as G = H – TS.
A process is spontaneous if the change in Gibbs free energy (?G) is negative. If ?G is positive, the process is non-spontaneous. If ?G is zero, the system is at equilibrium.
Copyright @ ncerthelp.com A free educational website for CBSE, ICSE and UP board.
