Manganese is extracted by heating manganese dioxide with aluminium powder. Which one of the following statements with regard to the reaction is correct?
- A.The reaction is exothermic.
- B.The reaction is endothermic.
- C.Manganese is produced as a solid.
- D.Manganese is more reactive than aluminium.
Correct answer
Explanation
The correct answer is A, The reaction is exothermic. Reducing manganese dioxide with aluminium, 3MnO₂ + 4Al → 3Mn + 2Al₂O₃, gives out a large amount of heat.
This method of reducing a metal oxide with aluminium powder is called aluminothermy, or the Goldschmidt thermite process. Aluminium has a very strong attraction for oxygen, and forming aluminium oxide releases so much energy that the metal produced is usually obtained in the molten state. The same principle is used in the thermite reaction of iron(III) oxide with aluminium, which gives molten iron for welding broken railway tracks. Aluminothermy is used for metals like manganese and chromium, whose oxides are hard to reduce with carbon.
B is wrong: the reaction releases heat; it does not absorb it.
C is wrong: the heat released is so great that the manganese generally comes out molten.
D is wrong: aluminium is more reactive than manganese, which is why it can take oxygen away from manganese dioxide.
Exam tip: Aluminothermy (thermite) — Al reduces MnO₂, Cr₂O₃ and Fe₂O₃; highly exothermic, used to weld rail tracks.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →