An Overview of Ncert Books Class 11 Chemistry Chapter 6 Free Download
FAQs on Ncert Books Class 11 Chemistry Chapter 6 Free Download
1. What types of questions are considered important from CBSE Class 11 Chemistry Chapter 6, Thermodynamics, for the 2025-26 exams?
For the Class 11 exams, important questions from Thermodynamics cover a range of formats. Students should focus on:
- Definitions (1-Mark): Questions asking to define key terms like system (open, closed, isolated), state functions, internal energy, and entropy.
- Numericals (2-3 Marks): Problems based on calculating work done, internal energy change (ΔU), enthalpy change (ΔH) using Hess's Law, and Gibbs free energy (ΔG).
- Conceptual Questions (3-Marks): Questions that test the understanding of concepts like spontaneity, the First Law of Thermodynamics, and the relationship between ΔH and ΔU.
- Derivations & HOTS (5-Marks): Longer questions may involve deriving the relationship between Cₚ and Cᵥ or explaining the criteria for spontaneity in detail, often combining multiple concepts.
2. Which topics in Thermodynamics carry the most weightage in exams?
Based on board trends and the CBSE syllabus, the most crucial topics for scoring well in Thermodynamics are Hess's Law of Constant Heat Summation, calculations involving standard enthalpies of formation (ΔfH°) and combustion (ΔcH°), the relationship between enthalpy and internal energy (ΔH = ΔU + Δn₉RT), and the concept of Gibbs Free Energy (ΔG) as the primary criterion for spontaneity. Questions from these areas are frequently asked as they test both theoretical knowledge and numerical application.
3. What are some expected numerical problems from Thermodynamics in the Class 11 exam?
Students can expect numerical problems focused on a few key areas:
- Calculating the enthalpy of a reaction using Hess's Law or given data on standard enthalpies of formation.
- Calculating work done during the expansion or compression of a gas under isothermal and reversible conditions (w = -2.303 nRT log(V₂/V₁)).
- Using the equation ΔG° = ΔH° - TΔS° to determine the spontaneity of a reaction at a given temperature.
- Calculating the change in internal energy (ΔU) from the given enthalpy change (ΔH) for reactions involving gases.
Practising these types is essential for the exam.
4. State Hess's Law of Constant Heat Summation. Why is it an important concept for exams?
Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or in several steps. This law is extremely important from an exam perspective because it allows us to calculate the enthalpy changes for reactions that cannot be measured directly in a laboratory, such as very slow or very explosive reactions. It is a direct consequence of enthalpy being a state function, a concept frequently tested in exams.
5. How does the First Law of Thermodynamics lead to the concept of enthalpy (ΔH)? Why is ΔH often more useful than internal energy change (ΔU) in chemistry?
The First Law of Thermodynamics, ΔU = q + w, relates internal energy change to heat (q) and work (w). Most chemical reactions in a lab occur in open vessels at constant atmospheric pressure. Under these conditions, the work done is pressure-volume work (-PΔV), and the heat exchanged is qₚ. By substituting, we get ΔU = qₚ - PΔV, or qₚ = ΔU + PΔV. This heat at constant pressure is defined as enthalpy change (ΔH). Therefore, ΔH is more useful because it directly measures the heat absorbed or released in most real-world chemical reactions, making it a more practical quantity than ΔU.
6. Why is Gibbs free energy (ΔG) considered the ultimate criterion for the spontaneity of a process?
Gibbs free energy is the ultimate criterion because it synthesises the two key factors that determine spontaneity into a single function: the enthalpy change (ΔH), which represents the tendency to achieve a lower energy state, and the entropy change (ΔS), which represents the tendency towards maximum disorder. The equation ΔG = ΔH - TΔS shows that for a process to be spontaneous, the overall ΔG must be negative. It provides a complete and reliable prediction by balancing the effects of both energy and disorder at a constant temperature and pressure.
7. How should a student differentiate between intensive and extensive properties in an exam answer for full marks?
For full marks, an answer must clearly define both terms and provide contrasting examples. An extensive property is one that depends on the amount of matter in the system (e.g., mass, volume, enthalpy). A intensive property is one that is independent of the amount of matter (e.g., temperature, pressure, density, molar enthalpy). A key point to add is that the ratio of two extensive properties results in an intensive property (e.g., mass/volume = density). Mentioning this shows a deeper understanding of the concept.
8. What are the limitations of using bond enthalpy data to calculate reaction enthalpy that should be mentioned in an exam?
While useful for estimations, calculations using bond enthalpies have key limitations that are important to mention for a comprehensive answer. Firstly, bond enthalpies are average values derived from various compounds, not the exact value for the specific molecule in the reaction. Secondly, these calculations are strictly valid only for reactions occurring in the gaseous phase, as intermolecular forces in liquids and solids are not accounted for. Therefore, the calculated reaction enthalpy is an approximation and not an exact experimental value.
