UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III
Q1(e). Heating rate, particle size and heat of reaction in TGA
Question
What happens in the thermogravimetric analysis for the following conditions? (i) Higher furnace heating rate (ii) Larger particle size of sample (iii) For sample undergoing endothermic reaction
Answer in brief
(i) and (ii): the mass-loss step moves to higher temperature and becomes broader. (iii): the sample cools itself and lags behind the furnace, so the recorded decomposition temperature is too high.
Key points
- Higher heating rate: Ti and Tf shift to higher values and the step spreads over a wider range; consecutive steps overlap (poorer resolution).
- Larger particles: less surface area, slower heat transfer and slower escape of evolved gas; decomposition starts later and ends at a higher temperature.
- Endothermic reaction: the sample absorbs heat and cools itself, so its temperature lags behind the furnace (programmed) temperature.
- The recorded decomposition temperature is higher than the true one and the step broadens; an exothermic reaction does the opposite.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversthermogravimetric analysisTGAheating rateparticle sizethermal lagTG curve
All parts of Question 1
- 1(a) Sample mass for a sampling standard deviation of ±5 %5 marks
- 1(b) van Deemter equation and its significance5 marks
- 1(c) Non-absorbing analyte by UV-vis spectrophotometry5 marks
- 1(d) Flame absorption profiles of magnesium and silver5 marks
- 1(e) Heating rate, particle size and heat of reaction in TGA5 marks
- 1(f) Quadrupole mass analyser in ICP–MS5 marks
- 1(g) Gravimetric estimation of nickel in steel5 marks
- 1(h) Lattice spacing from Bragg’s law5 marks