Ncert Books Class 11 Chemistry Chapter 5 Free Download
FAQs on Ncert Books Class 11 Chemistry Chapter 5 Free Download
1. Which topics from Chapter 5, States of Matter, are most important for the Class 11 Chemistry exam 2025-26?
For the CBSE Class 11 Chemistry exam 2025-26, the most frequently asked and high-weightage topics from Chapter 5, States of Matter, are:
- Gas Laws: Boyle's Law, Charles's Law, and Avogadro's Law, including their mathematical expressions and graphical representations.
- Ideal Gas Equation (PV=nRT): Derivation and numerical problems based on it are very common for 3-mark questions.
- Dalton's Law of Partial Pressures: Conceptual and numerical questions, especially those involving the collection of gas over water.
- Kinetic Molecular Theory of Gases: The postulates themselves are often asked.
- Deviation from Ideal Behaviour: The reasons for deviation and the van der Waals equation are key Higher Order Thinking Skills (HOTS) topics.
2. State and explain Boyle's Law. How can a student secure full marks for its graphical representation in an exam?
Boyle's Law states that for a fixed amount of gas at a constant temperature, the pressure (P) is inversely proportional to its volume (V). Mathematically, P ∝ 1/V or PV = k (where k is a constant). To secure full marks for a graphical representation, you must include two plots:
- A plot of Pressure (P) vs. Volume (V) which results in a rectangular hyperbola.
- A plot of Pressure (P) vs. 1/Volume (1/V) which is a straight line passing through the origin.
Ensure both graphs have clearly labelled axes and mention that the temperature is constant.
3. What is a typical 3-mark numerical problem based on the Ideal Gas Equation from this chapter?
A common 3-mark question involves using the Ideal Gas Equation, PV = nRT, to find one unknown variable when others are given. For example: "Calculate the temperature of 4.0 moles of a gas occupying 5 dm³ at 3.32 bar. (Given R = 0.083 bar dm³ K⁻¹ mol⁻¹)." Marks are typically distributed for writing the correct formula, substituting the values correctly, and calculating the final answer with the correct units.
4. Why do real gases deviate from ideal gas behaviour, especially under conditions of high pressure and low temperature?
This is a high-order thinking (HOTS) question. Real gases deviate from ideal behaviour because the Kinetic Molecular Theory of Gases makes two faulty assumptions that are not valid under all conditions:
- Faulty Assumption 1 (No Intermolecular Forces): It assumes there are no forces of attraction between gas molecules. This is incorrect, as real gases experience forces like van der Waals forces. These forces become significant at low temperatures when molecules move slower and can attract each other.
- Faulty Assumption 2 (Negligible Molecular Volume): It assumes the volume of gas molecules is negligible compared to the container's volume. This fails at high pressures when molecules are compressed, and their individual volume becomes a significant fraction of the total volume.
5. What is the physical significance of the van der Waals constants 'a' and 'b' in an exam context?
In the van der Waals equation, (P + an²/V²)(V - nb) = nRT, the constants 'a' and 'b' are introduced to correct the ideal gas law for real gases. Their significance is:
- The constant 'a' accounts for the magnitude of intermolecular forces of attraction. A higher value of 'a' signifies stronger attraction between the gas molecules.
- The constant 'b' corrects for the finite volume of gas molecules themselves. It represents the effective or 'excluded volume' per mole of the gas.
6. Explain Dalton's Law of Partial Pressures. How is it applied to find the pressure of a dry gas collected over water?
Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures that each gas would exert if it occupied the same volume alone at the same temperature. In exam questions involving gas collected over water, the collected gas is moist. To find the pressure of the dry gas, you must subtract the water vapour pressure (aqueous tension) from the total measured pressure: P_dry gas = P_total - Aqueous Tension.
7. What is Critical Temperature (Tc)? Why is this concept important for the liquefaction of gases?
The Critical Temperature (Tc) of a substance is the highest temperature at which it can be liquefied, regardless of how much pressure is applied. Above this temperature, the substance exists only as a gas. This concept is crucial because it sets the upper temperature limit for liquefying a gas. For instance, carbon dioxide (Tc ≈ 304 K) can be liquefied at room temperature (around 298 K) by applying sufficient pressure, but gases like nitrogen and oxygen, with very low critical temperatures, must be cooled significantly below room temperature before they can be liquefied.
8. Differentiate between surface tension and viscosity, as expected for a Class 11 exam answer.
This is a frequently asked question. For full marks, a clear differentiation is needed:
- Surface Tension: It is the energy required to increase the surface area of a liquid by one unit. It arises from the unbalanced cohesive intermolecular forces at the liquid's surface, causing the liquid to minimise its surface area.
- Viscosity: It is the measure of a fluid's resistance to flow. It represents the internal friction between adjacent layers of the liquid as they slide past one another.
A key point to add is that for most liquids, both surface tension and viscosity decrease as the temperature increases.
