CSS 2025 Chemistry Combined Papers Past Paper

Paper details

Examination

CSS

Year

2025

Subject

Chemistry

Paper

Combined Papers

Subject type

Optional

Mode

Subjective

Questions recorded from this paper

These 12 question wordings are transcribed from CSS Vista’s structured Chemistry past-paper record for CSS 2025.

1. Q7 · Thermodynamics(a) Explain the relationship between entropy, enthalpy, and Gibbs free energy. Derive (08) an expression for equilibrium using these thermodynamic quantities. (b) Derive the first law of thermodynamics and explain its connection with internal (07) energy and enthalpy. (c) Define the concept of activation energy and illustrate its significance in reaction (05) (20) kinetics using the Arrhenius equation.
2. Q4 · adsorption isotherms(a) Derive the Gibbs free energy equation from the first and second laws of (10) thermodynamics and discuss its importance in predicting the spontaneity of chemical reactions. (b) Explain the principle of fractional distillation. Compare it with simple distillation (05) using a diagram and examples of their applications. (c) Adsorption isotherms fail at high pressure. Why? Explain with reference to the (05) (20) limitations of the Freundlich isotherm.
3. Q6 · Langmuir adsorption isotherm and Freundlich Adsorption isotherm(a) Discuss the Freundlich adsorption isotherm. Derive the equation and highlight (07) its limitations. (b) Using a diagram, explain the principle and working of a galvanic cell. Discuss (07) its components with the example of a Zn-Cu system. (c) Why does an exothermic reaction sometimes become non-spontaneous at high (06) (20) temperatures?
4. Q8 · Gaussian distributionDefine the following: (5 each) (20) (a) Difference between Homogenous and Heterogenous Catalysis (b) Define Molecular Orbital Theory (MOT) (c) Difference between ANOVA and Gaussian Distribution (d) Define Chromatography and write key differences between Paper Chromatography and Gas Chromatography ` Roll Number
5. Q2 · Acids and Bases(a) Define adsorption and explain its types. Derive an expression for the Langmuir (08) adsorption isotherm and discuss factors influencing adsorption on solids. (b) Why are transition metal complexes often colored? Explain using crystal field (07) theory with an example. (c) Compare and contrast the Lewis concept and the Bronsted-Lowry theory of acids and (05) (20) bases. Include relevant examples and limitations.
6. Q5 · Acids and Bases(a) Why are Lewis’s acids and bases more versatile in their applications than (10) Bronsted-Lowry acids and bases? Provide two examples to support your answer. (b) Derive the integrated rate law for a first-order reaction and explain why the half-life of such a reaction is independent of its initial concentration. (05) (c) State and explain the Heisenberg Uncertainty Principle. How does it limit our ability to precisely determine the position and momentum of particles? (05) (20)
7. Q3 · lanthanide contraction(a) What is lanthanide contraction? Analyze its impact on the atomic radii and (08) chemical properties of lanthanides and post-lanthanides. (b) Analyze the quantum numbers associated with atomic orbitals and describe their (07) importance in explaining electron configurations. (c) Describe the catalytic activity of transition metals with reference to their variable (05) (20) oxidation states. Support your explanation with one example.
8. Q2 · Dipole moment(a) Elaborate with examples that how resonance, hyperconjugation and inductive effect, can (10) contribute towards the stability of carbocations. (a) How ‘dipole moment’ can be measured? Indicate, among the following molecules which does (10) not have dipole moment and why? i. CCl4 ii. BF3 iii. CH4 iv. NH3
9. Q3 · alkenes and alkynes(a) How does the reaction mechanism work for the electrophilic addition of bromine to ethene? (10) Explain the intermediate (s). (b) Describe the IUPAC rules for naming alkenes and alkynes with appropriate examples. (10)
10. Q4 · Benzene structure(a) Categorize benzene substituents, into activating and deactivating groups. Explain their (12) influence on ortho, para and meta orientation of incoming groups in electrophilic substitution reactions. (b) How did Kekulé propose the benzene structure, and what evidence supported his model? (8)
11. Q5 · phenols(a) Compare the reactivity of acid halides, anhydrides, esters, and amides toward nucleophilic (10) substitution. What kind of products do you expect when these react with Grignard reagent (RMgX)? (b) Explain the reaction mechanism of Williamson synthesis to prepare ethers. (5) (c) Discuss the acidic nature of phenols. Why picric acid (C6H3N3O7) is considered strong acid? (5)
12. Q6 · Elimination reactions, Zaitsev rule and Hofmann rule(a) Given below is the structure of (1R, 2S, 5R) menthol. Draw the structures of all possible (8) stereoisomers of menthol. (b) How does Hofmann’s rule differ from Zaitsev’s rule? Explain each with suitable example. (6) (c) Describe the main components of a UV/Visible spectrophotometer and their functions. (6)

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