CSS Chemistry Past Paper 2025

Instructions specific to this paper

• First attempted Part-I (MCQS) on the separate OMR Answer Book which shall be taken back
• Overwriting/cutting of the options/answers will not be given credit.
• There is no negative marking. All MCQs must be attempted.
• Answers given anywhere else, other than OMR Answer Sheet, will not be considered.
• Attempt ONLY FOUR questions from PART-II. ALL questions carry EQUAL marks.

Questions

1. Q. 28 marks

   (a) Define adsorption and explain its types. Derive an expression for the Langmuir 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.

2. Q. 38 marks

   (a) What is laathanide contraction? Analyze its impact on the atomic radii and 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.

3. Q. 410 marks

   (a) Derive the Gibbs free energy equation from the first and second laws of 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.

4. Q. 510 marks

   (a) Why are Lewis acids and bases more versatile in their applications than 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 (05) such a reaction is independent of its initial concentration. (c) State and explain the Heisenberg Uncertainty Principle. How does it limit our ability (05) (20) to precisely determine the position and momentum of particles?

5. Q. 67 marks

   (a) Discuss the Freundlich adsorption isotherm. Derive the equation and highlight 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?

6. Q. 78 marks

   (a) Explain the relationship between entropy, enthalpy, and Gibbs free energy. Derive 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.

7. Q. 820 marks

   Define the following: (5 each) (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

Standard FPSC instructions

Printed on this and every CSS paper.

• Part-II is to be attempted on the separate Answer Book.
• All the parts (if any) of each question must be attempted at one place instead of at different places.
• Write Q. No. in the Answer Book in accordance with Q. No. in the question paper.
• No page/space be left blank between the answers. All the blank pages of the Answer Book must be crossed.
• Extra attempt of any question or any part of the attempted question will not be considered.