lesterine's answer to LockB's Secondary 4 A Maths Singapore question.
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C does not explain why the reaction soon stops. Infact, by logic, the reaction should continue as it is reactive. A is correct as calcium sulfate forms, preventing the reaction between the carbonate and the acid in acid rain. this causes the reaction to stop soon after. hope this helps :)
Date Posted:
3 years ago
LockB, your Chem MCQ is coming this Fri correct?
Anyway, think of it this way. You might even be familiar with this.
I obtain a small cut on my hand (analogous to the lumps of calcium carbonate in the question). My hand starts bleeding. But will the bleeding run (analogous to the reaction in the question) continuously without any end in sight?
Most of us will have a mechanism to start clotting the blood, forming a lump (analogous to the calcium sulfate) to prevent blood from further leaving the finger).
If we have a giant cut instead (analogous to the powdered calcium carbonate in the question), it will be a different story. There will be too much blood to lose before complete clotting can occur.
Anyway, think of it this way. You might even be familiar with this.
I obtain a small cut on my hand (analogous to the lumps of calcium carbonate in the question). My hand starts bleeding. But will the bleeding run (analogous to the reaction in the question) continuously without any end in sight?
Most of us will have a mechanism to start clotting the blood, forming a lump (analogous to the calcium sulfate) to prevent blood from further leaving the finger).
If we have a giant cut instead (analogous to the powdered calcium carbonate in the question), it will be a different story. There will be too much blood to lose before complete clotting can occur.
C is wrong because the reactivity is the same (both are limestone which is impure solid calcium carbonate)
An analogy would be a whole big tablet of Redoxon vitamin C vs. grinding this tablet into fine powder.
Both will dissolve in water quickly.
Now limestone is different.
A is correct because :
If you had powdered limestone (basically very fine and small bits/pieces/particles), the exposed surface are to volume ratio is very much larger than compared to lumps of limestone.
The powdered limestone would very quickly react with acid to form insoluble calcium sulfate.
For bigger lumps, only the outer surface reacts first, producing the insoluble calcium sulfate that coats on the surface of the lump, preventing further reaction with the acid.
It could be argued that the powdered limestone could have this coating issue as well, but since the particles are so much smaller and have much higher surface area to volume ratio,
the coating is much less significant. Most of the limestone powder would react away compared to the lumps.
We don't say lumps are less reactive just because the reaction stops fast.
If you had a less reactive chemical, the reaction would simply be slower from the beginning.
An analogy would be a whole big tablet of Redoxon vitamin C vs. grinding this tablet into fine powder.
Both will dissolve in water quickly.
Now limestone is different.
A is correct because :
If you had powdered limestone (basically very fine and small bits/pieces/particles), the exposed surface are to volume ratio is very much larger than compared to lumps of limestone.
The powdered limestone would very quickly react with acid to form insoluble calcium sulfate.
For bigger lumps, only the outer surface reacts first, producing the insoluble calcium sulfate that coats on the surface of the lump, preventing further reaction with the acid.
It could be argued that the powdered limestone could have this coating issue as well, but since the particles are so much smaller and have much higher surface area to volume ratio,
the coating is much less significant. Most of the limestone powder would react away compared to the lumps.
We don't say lumps are less reactive just because the reaction stops fast.
If you had a less reactive chemical, the reaction would simply be slower from the beginning.
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