Thermodynamics Class 11 Notes - Chapter 6

Thermodynamics deals with the change of energy in physical and chemical processes. It allows us to study these changes quantitatively as well as to make effective predictions. Therefore, we divide the universe as surrounding and system. Physical or chemical processes lead to progression or immersion of heat part of it may be converted to work. According to the first law of thermodynamics, these quantities are related as ∆U = q + w where q is heat and w is work. Change in internal energy is a state function and depends on only the initial and final states. On the other hand, w and q are not state functions. The change in temperature due to the transfer of heat one system to another system can be measured.

  • Heat evolved or absorbed is expressed as q = C∆T.
  • In case of expansion of gas, work is measured as w = –pex∆V

Enthalpy Change

Enthalpy change is defined as the change in energy of a system at constant pressure. It is denoted as ΔH. Enthalpy is a state function. Enthalpy change, ∆H = ∆U + ∆ngRT, can be calculated directly from the change in heat at constant pressure, ∆H = qp

Hess’s law

Hess’s law states that the standard reaction enthalpy for overall reaction taking place in several steps will be equal to the sum of standard enthalpies of intermediate reaction steps by assuming that each step occurs at the same temperature.

Few Important Questions

  1. Given: Aluminium quantity 60.0 g, a temperature increases from 35°C to 55°C, molar heat capacity 24 J mol–1 K–1 Calculate the number of kJ of heat.
  2. Given: Equilibrium constant of a reaction is 10, T = 300 K, R = 8.314 JK–1mol–1 Calculate the value of ∆G0?
  3. ∆U = 0 for an isolated system, calculate ∆S

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n- Octane in the presence of Cr2O3/Al2O3  at  873K changes to