CAPF AC 2021 Q29: Science & Technology – Chemistry
- (a) The reaction is exothermic.
- (b) The reaction is endothermic.
- (c) Manganese is produced as a solid.
- (d) Manganese is more reactive than aluminium.
Answer: (a) The reaction is exothermic.
Explanation
Extracting manganese with aluminium
How to think: this is a thermit-type (aluminothermic) reaction.
A more reactive metal (aluminium) pulls oxygen away from a less reactive metal (manganese).
Such displacement reactions give out a lot of heat.
The reaction (NCERT): 3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat.
| Statement | True or false | Reason |
|---|---|---|
| The reaction is exothermic | True | Heat is written as a product |
| The reaction is endothermic | False | It releases heat, it does not absorb it |
| Manganese is produced as a solid | False | So much heat is given out that the metal forms in the molten state |
| Manganese is more reactive than aluminium | False | Aluminium is more reactive; that is why it displaces manganese |
Answer: (a) The reaction is exothermic.
Why each option is right or wrong
- (a) CORRECT. The reaction is highly exothermic; heat appears on the product side.
- (b) Incorrect. Heat is released, not absorbed, in this reaction.
- (c) Incorrect. The heat is so great that manganese is obtained in the molten (liquid) state.
- (d) Incorrect. Aluminium is more reactive; a less reactive metal cannot displace a more reactive one.
Key point
Aluminium reduces oxides of less reactive metals: 3MnO2 + 4Al → 3Mn(l) + 2Al2O3 + heat. Highly exothermic; the metal comes out molten. The same idea (Fe2O3 + Al) is the thermit reaction used to join rail tracks.
Remember
Thermit-type reactions are highly exothermic; metal comes out molten.
Common mistakes
- Marking solid manganese; NCERT stresses the metal is produced in the molten state.
- Thinking heating the mixture at the start makes the reaction endothermic.
- Reversing the reactivity and saying manganese is more reactive than aluminium.
Exam tip
Educated guess: (a) and (b) are opposites, so one of them is very likely the answer. A reaction used for welding rails (thermit) must give out heat.
Sources
- NCERT Class X Science — Metals and Non-metals (extracting metals in the middle of the activity series)
- NCERT Class XII Chemistry — General Principles and Processes of Isolation of Elements