Photochemistry – Chemistry Optional PYQs (2000–2026)

Here are all 41 Photochemistry questions asked in UPSC Chemistry Optional Paper I from 2000 to 2026, with the latest questions 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.

41 questions

1. A flower is red in colour in daylight. What will be its colour in green light at night? Justify your answer. [2026/5m/1i]

2. Calculate the energy of 800 nm electromagnetic radiation per einstein in SI system. [2026/5m/3c(i)]

3. The photodecomposition of HI(g) takes place following three elementary steps: $HI\left(g\right)+h\nu \to H\left(g\right)+I\left(g\right)H\left(g\right)+HI\left(g\right)\stackrel{k_2}{\to }{H}_2\left(g\right)+I\left(g\right)I\left(g\right)+I\left(g\right)\stackrel{k_3}{\to }{I}_2\left(g\right)$ Find the expression of $-\frac{d\left[HI\left(g\right)\right]}{dt}$, quantum yield ($\varphi$) and kinetic order of the reaction. [2026/10m/3c(ii)]

4. A dye solution containing 1 g per 100 cc transmitted 60% of the blue light in a cell 1 cm thick. (i) What percentage of light would be absorbed by a solution containing 2 g per 100 cc in the same cell? (ii) What should be the cell thickness so that 90% of the light is absorbed by the original solution? [2025/5m/5b]

5. Consider the following photochemical reaction: $H_2\left(g\right)+B{r}_2\left(g\right)\stackrel{h\nu }{\to }2HBr\left(g\right)$ Give the mechanism of this reaction. Applying steady-state approximations to [Br] and [H], show that the rate of formation of HBr (g) varies with the square root of the intensity ($I_a$) of the absorbed radiation. What is the quantum yield for this reaction? Why is the value so low? [2025/20m/6b]

6. Indicating ‘Fluorescence’ and ‘Phosphorescence’ processes in Jablonski diagram, distinguish between both processes. [2024/10m/5a]

7. Calculate the number of moles of HCl (g) produced by the absorption of one Joule of radiant energy of wavelength 480 nm in the reaction $H_2\left(g\right)+C{l}_2\left(g\right)\to 2HCl\left(g\right)$ if the quantum yield of the photochemical reaction is $1.0\times {10}^6$. [Given: $N_A=6.022\times {10}^{23}mo{l}^{-1}$; $h=6.626\times {10}^{-34}$ Js; $c=2.998\times {10}^{8}m{s}^{-1}$]. [2024/10m/6a(i)]

8. On passing monochromatic light through a 0.04 molar solution kept in a cell of thickness 2 cm, the intensity of the transmitted light was reduced to 50%. Calculate the molar extinction coefficient. [2024/5m/6a(ii)]

9. Explain radiative and non-radiative processes by singlet and triplet electronic states of molecule. Also explain it through Jablonski diagram. [2023/10m/5b]

10. When an incident light of wavelength 300 nm is passed through a solution in a 1 cm cell, it transmits only 10% of the incident light. What percentage of light would be absorbed by the same solution if taken in a 0.5 cm cell? [2023/10m/8c]

11. Label each of the following processes with proper explanation in Jablonski diagram: (A) Allowed absorption (B) Fluorescence (C) Phosphorescence (D) Internal Conversion (IC) (E) Inter System Crossing (ISC) Discuss the mode of transition which is favourable for a photochemical reaction. [2022/10m/5b]

12. What is meant by steady-state approximation? How this approximation helps in deriving the kinetics of following photochemical reaction? $H_2\left(g\right)+C{l}_2\left(g\right)\to 2HCl\left(g\right)$ The quantum yield of this reaction is extremely large. Justify or criticize this statement. [2022/10m/8d]

13. State and derive Lambert-Beer law for absorption of light by solutions. [2021/10m/5b]

14. Radiation of wavelength 2500 Å was passed through a cell containing 10 ml of a solution which was 0.05 molar in oxalic acid and 0.01 molar in uranyl sulphate. After absorption of 80 joules of radiation energy, the concentration of oxalic acid was reduced to 0.04 molar. Calculate the quantum yield for the photochemical decomposition of oxalic acid at the given wavelength. (Given: $N=6.022\times {10}^{23}mo{l}^{-1}$, $h=6.626\times {10}^{-34}$ J s and $c=3\times {10}^{8}m{s}^{-1}$) [2021/20m/7c]

15. Discuss all the possible decay routes of an electronically excited molecule with the help of a neat Jablonski diagram. [2020/10m/5e]

16. The quantum yield for photo decomposition of HI(g) at wavelength 254 nm is 1.99. If a total of 3070 J of energy is absorbed then how many moles of HI(g) decomposed? [2020/10m/8b]

17. The quantum yield for the photo-chemical reaction between $H_2$ and $C{l}_2$ is ${10}^6$ whereas that for $H_2-B{r}_2$ reaction is almost unity. Account for the observation. [2019/15m/6a]

18. Distinguish fluorescence from phosphorescence. [2018/5m/6b(i)]

19. In the photobromination of cinnamic acid to dibromocinnamic acid using light of wavelength 4358 Å a light intensity of 14,000 ergs $se{c}^{-1}$ produced, and a decrease of 0.075 millimole of bromine during an exposure for 1105 sec. The solution absorbed 80.1% of the light passing through it. Calculate the quantum yield. [2018/5m/6b(ii)]

20. Define and explain Einstein’s law of photochemical equivalence. [2018/5m/8b(i)]

21. Discuss the photochemical reactions between hydrogen and chlorine. [2018/5m/8b(ii)]

22. What is ISC (Inter System Crossing) for an excited molecule in singlet state? Answer using Jablonski diagram. [2017/10m/5b]

23. Using a particular wavelength of light, a solution of a substance (molar mass $294gmo{l}^{-1}$) transmits 80% of incident light. Determine the concentration of the solution in $mol{L}^{-1}$. Given that optical path length of the spectrophotometer tube is 1.0 cm and absorptivity of the substance is $2.0L{g}^{-1}c{m}^{-1}$. [2017/10m/6b]

24. State and explain Einstein law of photochemical equivalence. What is quantum yield? Explain the causes of low quantum yield. [2016/15m/4a(i)]

25. In an experiment to measure the quantum efficiency of a photochemical reaction, the absorbing substance was exposed to 490 nm light from a 100 W source for 45 minutes. The intensity of the transmitted light was 40% of the intensity of the incident light. As a result of irradiation, 0.344 mol of the absorbing substance decomposed. Find the quantum efficiency. [2015/10m/5e]

26. Photodimerization of a monomer $M$ in the presence of a photoinitiator occurs by a chain polymerization process in three steps: Photoinitiation: Rate = $I_{abs}$; Propagation: Rate = $k_p\left[M\right]\left[M^{*}\right]$; Termination: Rate = $k_t\left[M^{*}\right]$ where $k_p$ and $k_t$ are the rate constants for the propagation and termination steps, involving the propagating chain radical $M^{*}$ and $I_{abs}$ is the intensity of absorbed light. Show that the rate of photopolymerization (i.e., the rate of loss of monomer) is proportional to the square root of the light intensity. [2015/15m/6a]

27. Distinguish between the following: (i) Photochemical and dark reactions (ii) Fluorescence and phosphorescence. [2014/10m/5e]

28. For the photochemical reaction $A\to B$, $1.5\times {10}^{-5}$ moles of B were formed on absorption of 6.2 J at 360 nm. Calculate the quantum efficiency. [2014/10m/6c]

29. Show the various photochemical processes and their approximate rate constants on a Jablonski diagram. [2014/10m/7d]

30. The molar enthalpy of formation of oxygen atoms is $246.5kJmo{l}^{-1}$. What is the longest wavelength photon which could dissociate $O_2$? [2013/10m/1e]

31. A solution of a dye has its maximum wavelength light absorption at 453 nm and maximum fluorescence emission at 508 nm. The quantum efficiency of fluorescence is 40%. What percentage of absorbed light is emitted as fluorescence? [2013/15m/7a]

32. A certain system absorbs $3.0\times {10}^{18}$ quanta of light per second. On irradiation for 10 minutes, $2.0\times {10}^{-3}$ moles of the reactants was found to have reacted. Calculate the quantum efficiency of the process. [2012/15m/7b]

33. A photochemical reaction with quantum efficiency of one shows no temperature dependence. Explain. [2010/10m/3a(i)]

34. Calculate the number of moles of HCl(g) produced by the absorption of one joule of radiant energy of wavelength 480 nm in the reaction $H_2\left(g\right)+C{l}_2\left(g\right)\to 2HCl\left(g\right)$ if the quantum yield of the photochemical reaction is $1.0\times {10}^6$. [2010/10m/3a(ii)]

35. (i) What are photochemical reactions? Explain quantum efficiency. (ii) The quantum efficiency of the reactions between hydrogen and chlorine is ${10}^5$ whereas that between hydrogen and bromine is 0.01. Explain the reason for this difference. [2009/20m/6c]

36. State and explain the law of photochemical equivalence. How are the discrepancies of this law explained? [2008/20m/6c]

37. Derive the kinetics of the photochemical reaction $H_2+B{r}_2\to 2HBr$ [2007/20m/5c]

38. Calculate the quantum yield for the photochemical reaction in which 4-heptanone on irradiation with 315 nm radiation, with power output of 50 W for 90 s under the condition of total absorption. produces 2.8 m.mol. of ethane. [2004/15m/3b]

39. The quantum efficiency for the destruction of a substance with 300 nm light is 0.30. How much time irradiation with 100 W source must continue to destroy one mole of the substance? [2001/20m/5a]

40. What role photosensitizers can play in the decay of photo-excited states? Explain with an example. [2000/20m/7a]

41. How is phosphorescence important in the decay of some photochemically excited molecules? Explain. [2000/20m/7b]

« Previous: Chemical Kinetics | Next: Surface Phenomena and Catalysis »

Back to: Chemistry Optional PYQs – Topic-wise

Last updated: 30 October 2026