Surface Phenomena and Catalysis – Chemistry Optional PYQs (2000–2026)

Here are all 43 Surface Phenomena and Catalysis questions asked in UPSC Chemistry Optional Paper I from 2000 to 2026, arranged under the 2 topics of the official syllabus. Within each topic the latest questions come first.

Each question ends with a code such as [2024/15m/3b] – the year, the marks, the word limit (where UPSC set one) and the question number in the paper.

11.1] Adsorption and surface area

25 questions

1. Spontaneous adsorption of gas on solid surface favours low temperature. Explain, why. [2026/5m/1j]

2. At 0°C and 1 atm pressure, the volume of nitrogen ($N_2$) gas required to form a monolayer on a sample of charcoal is $155.5c{m}^{3}g{m}^{-1}$ of charcoal. Calculate the surface area per gram of charcoal. Area of cross-section of a $N_2$ molecule is $0.160{\left(nm\right)}^2$. [Given: $N_A=6.022\times {10}^{23}mo{l}^{-1}$; Molar volume of gas (at 0°C and 1 atm pressure), $V_m=22.414d{m}^{3}mo{l}^{-1}$]. [2025/5m/5c]

3. Mentioning the requisite assumptions, derive the equation of the Langmuir adsorption isotherm. Show that the Langmuir isotherm holds at low pressure but fails at high pressure. [2025/20m/6c]

4. How does water condense onto glass? [2024/5m/5b(i)]

5. How long will it take for about one-half of a monolayer to form on the surface if the surface is exposed to a pressure of $2.0\times {10}^{-11}$ torr? Assume that a monolayer formation needs an exposure of 1 Langmuir. Also give reason for using such low pressure in cleaning the surface. [2024/5m/5b(ii)]

6. At 0°C and 1 atm pressure, the volume of nitrogen gas required to cover a sample of an adsorbent is found to be $130c{m}^3{g}^{-1}$. Calculate the surface area per gram of adsorbent. Given that area occupied by a nitrogen molecule is $0.162{\left(nm\right)}^2$. [$N_A=6.022\times {10}^{23}mo{l}^{-1}$]. [2022/10m/7d]

7. Derive Langmuir adsorption isotherm. How does Langmuir adsorption isotherm help in elucidation of kinetics of a gaseous reaction on solid surface? [2022/10m/8e]

8. Derive an equation for Langmuir’s adsorption isotherm. Show that under limiting conditions of pressure, the system follows both first-order and zero-order of adsorption. [2021/20m/6c]

9. What would be the sign; negative, positive or zero; for ΔH, ΔS and ΔG for the process of spontaneous adsorption of gas on solid surface at constant temperature. Give reasons. [2020/10m/5d]

10. Explain Langmuir adsorption isotherm (i) when the gas pressure ‘P’ is very small and (ii) when the gas pressure ‘P’ is very high. [2018/10m/5b]

11. Write BET equation. Explain the terms involved in it. How is it useful for the determination of surface area? [2018/10m/7a(i)]

12. At constant pressure and temperature, adsorption of a gas on solid surface occurs spontaneously. For this natural process, what will be the signs of (i) $\Delta G$, (ii) $\Delta S$ and (iii) $\Delta H$? Give reason(s) in support of your answer. [2017/10m/5c]

13. Volume of nitrogen gas at 1 atm and 273 K required to cover 1 g of silica gel is $0.129d{m}^3$. Calculate the surface area of the gel if each nitrogen molecule occupies an area of $16.2\times {10}^{-10}{m}^2$. [2016/10m/8c]

14. The adsorption of a surfactant on colloidal $Zr{O}_2$ suspended in water followed a Langmuir isotherm with the results shown below: Given that 10 g of $Zr{O}_2$ colloid was used in the experiment and that the cross-sectional area of a single surfactant molecule is $0.25n{m}^2/molecule$. Calculate the surface area of the $Zr{O}_2$ in units of $m^2{g}^{-1}$.

[Figure: Langmuir plot: vertical axis 1/Moles adsorbed (mol⁻¹) with ticks 5000, 10000, 15000, 20000; horizontal axis 1/[Surfactant] (dm³ mol⁻¹) with ticks 0.1, 0.2, 0.3, 0.4. A straight line runs from the point (0.1, 5000) up to about (0.37, 20000), i.e. it passes through about (0.2, 10500) and (0.3, 16000); the line is not drawn below 0.1.] [2015/15m/6b]

15. Derive Langmuir’s Adsorption Isotherm and explain how it could be used for determination of the surface area of an adsorbent.

Some Useful Constants (printed at the head of Section A): ($R=8.3J{k}^{-1}mo{l}^{-1}$; $g=9.8m{s}^{-2}$; $1F=96,500C$; $N_A=6.0\times {10}^{23}mo{l}^{-1}$; $h=6.6\times {10}^{-34}J{s}^{-1}$; $c=3.0\times {10}^{8}m{s}^{-1}$) [2014/10m/8c]

16. How many types of absorption are there on solid adsorbents? Prove that, for chemisorption, the amount of the adsorbed phase m is equal to $\frac{AP}{1+KP}$ where P is equal to the pressure of the adsorbent, A and K are some constants of the system. [2013/10m/5e]

17. Derive the Langmuir adsorption isotherm for adsorption of a gas on a solid. Also, mention assumptions involved. [2012/10m/3a(i)]

18. Volume of nitrogen required to cover through monolayer 1 g of silica gel is $0.129d{m}^3$ at 1 atm and 273 K. Calculate the surface area of the gel if each nitrogen molecule occupies an area of $16.2\times {10}^{-10}{m}^2$. [2012/12m/5c]

19. Write BET isotherm equation and define the terms used therein. [2011/15m/5c]

20. Give the postulates on which Langmuir’s adsorption isotherm is based. Derive the expression for Langmuir’s adsorption isotherm. How does this equation account for the adsorption at (i) high pressure and (ii) low pressure? Show that when a diatomic gas adsorbs an atom on the surface of a solid, the Langmuir adsorption isotherm is modified to $\theta =\frac{{\left(Kp\right)}^{1/2}}{[1+{\left(Kp\right)}^{1/2}]}$ where, $\theta$ = fraction of surface covered, $K=\frac{k_{\mathrm{\text{adsorption}}}}{k_{\mathrm{\text{desorption}}}}$, $p$ = pressure of gas. [2010/20m/3b]

21. Derive an equation for Langmuir’s adsorption isotherm. Discuss the causes for deviations observed. [2009/20m/7a]

22. Derive expression for the BET adsorption isotherm. What is the application of this equation? [2007/20m/5d]

23. Derive Langmuir adsorption isotherm. How will you find out the isosteric enthalpy of adsorption? [2004/15m/3c]

24. Describe the techniques used for the measurement of surface area and types of acidic sites of a catalyst. [2002/20m/6c]

25. Write B.E.T. adsorption isotherm equation. How is the volume $v_m$ (the volume adsorbed when the solid surface is completely covered by monolayers of the adsorbed gas) determined? [2001/20m/5b]

11.2] Heterogeneous catalysis

18 questions

1. The kinetic order of the solid-surface catalyzed gas-phase decomposition reaction varies with the pressure of the gaseous reactant. Explain with plausible mechanism. [2026/15m/4c]

2. What is meant by heterogeneously catalyzed reactions? Mention the consecutive steps for a reaction to occur on a surface. Using Langmuir adsorption theory, account for the experimental observation that the decomposition of $P{H}_3$ on tungsten follows first order kinetics at low pressure and zeroth order kinetics at high pressure. [2024/15m/6b]

3. Derive a rate expression for a bimolecular surface reaction. Discuss the kinetics of such a reaction, when the gaseous reactants, say $A$ and $B$, are adsorbed in the following way: $A+B\to \mathrm{\text{Product}}$ (i) $A$ and $B$ are sparsely adsorbed. (ii) $A$ is relatively more strongly adsorbed than $B$. [2023/10m/7c]

4. Derive Langmuir adsorption isotherm. How does Langmuir adsorption isotherm help in elucidation of kinetics of a gaseous reaction on solid surface? [2022/10m/8e]

5. Show that a surface catalysed decomposition reaction follows zero order kinetics with respect to reactant when its pressure is high enough. [2020/15m/2d]

6. Unimolecular surface catalysed gas phase reactions follow first order kinetics at low pressures and zero order kinetics at high pressures. Why? [2019/10m/5e]

7. Explain the mechanism of reaction on heterogeneous catalysis. [2018/5m/7a(ii)]

8. What are the characteristics, essential for catalysis with specific reference to surface catalysis? [2017/10m/6a(i)]

9. Explain the adsorption theory of catalysis. Give one example and indicate the type of adsorption involved. What effect does a catalyst have on an equilibrium system? [2016/15m/3a(i)]

10. Explain the steps involved in the reaction $A+B\to C$ occurring under heterogeneous catalysis. [2012/10m/3a(ii)]

11. Distinguish between homogeneous and heterogeneous catalysis. [2011/15m/5d]

12. What are heterogeneous catalysts? Discuss the characteristics and mechanism of reaction on heterogeneous catalysts. [2008/20m/7a]

13. Explain the theories of heterogeneous catalysis. [2007/20m/6a]

14. Explain the difference between homogeneous and heterogeneous catalysis with a suitable example for each. [2006/15m/2c]

15. Explain homogenous and heterogeneous catalysis with a suitable example for each. Derive and expression for the rate of a surface catalyzed reaction. [2005/20m/8c]

16. Define heterogeneous catalysis. What are the various steps involved in it? Derive the rate equation for the simple surface catalysed reaction involving a single reactant and a single product. [2003/30m/4a]

17. Describe the techniques used for the measurement of surface area and types of acidic sites of a catalyst. [2002/20m/6c]

18. Explain the following facts from the standpoint of adsorption properties of reactant and product molecules (i) Decomposition of $N{H}_3$ on W is zero order (ii) Decomposition of $N_{2}O$ on Au is first order (iii) Recombination of H atoms on Au is second order. [2000/20m/8a]

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Last updated: 30 October 2026