In the chapter of Thermodynamics in Class 11 Chemistry, there are several important questions that often come up in exams. Here are some of them along with brief answers:
1. What is the first law of thermodynamics?
Answer: The first law of thermodynamics is the law of conservation of energy. It states that energy can neither be created nor destroyed, but it can be converted from one form to another. Mathematically, it is expressed as:
\Delta U = Q - W
= change in internal energy,
= heat absorbed by the system,
= work done by the system.
2. What is enthalpy (H)? How is it related to internal energy (U)?
Answer: Enthalpy (H) is a thermodynamic quantity that represents the heat content of a system at constant pressure. It is defined as:
H = U + PV
= enthalpy,
= internal energy,
= pressure,
= volume. The change in enthalpy () gives the heat absorbed or released at constant pressure:
\Delta H = \Delta U + P\Delta V
3. What is the second law of thermodynamics?
Answer: The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Entropy always tends to increase in a spontaneous process. It also implies that energy conversions are not 100% efficient, and some energy is always lost as heat. The mathematical form is:
\Delta S \geq 0
4. What is entropy (S)?
Answer: Entropy is a measure of the randomness or disorder of a system. It is a state function, and its change () is calculated by:
\Delta S = \int \frac{dQ}{T}
= infinitesimal heat absorbed or released,
= temperature.
For a reversible process:
\Delta S = \frac{Q_{rev}}{T}
5. What is Gibbs free energy (G)?
Answer: Gibbs free energy is a thermodynamic quantity that helps predict the spontaneity of a process at constant pressure and temperature. It is defined as:
G = H - TS
= Gibbs free energy,
= enthalpy,
= temperature,
= entropy.
A process is spontaneous if , non-spontaneous if , and at equilibrium if .
6. Explain the relationship between work and heat in an isothermal process.
Answer: In an isothermal process, the temperature remains constant (
), and the internal energy of an ideal gas does not change (\Delta U =