F4CHEM[METAL REACTIVITY]

The calcium granule is coated with a layer of calcium oxide. When the calcium granule is put into water, only a small portion of surface area of the calcium metal can expose to the acid and react to give hydrogen gas. Most of the surface area of the granule is calcium oxide, which reacts with water to give calcium hydroxide but no hydrogen gas.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
CaO(s) + H2O(l) → Ca(OH)2(aq)

When the calcium oxide layer has completely reacted with water, all the surface of granule is calcium metal only. Thus the reaction between calcium and water goes faster.

2011-01-20 14:34:01 補充：
It should read ".... only a small portion of surface area of the calcium metal can expose to the acid and react to give hydrogen gas ...."

2011-01-20 14:39:52 補充：
In the real situation, the calcium granule is stored in a metal container for a long time. the granule is NOT treated to remove the calcium oxide layer before it is put into water. This is a real-life experiment, but NOT a hypothetic case.

2011-01-20 18:25:14 補充：
It should read ".... only a small portion of surface area of the calcium metal can expose to the water and react to give hydrogen gas ...."

2011-01-20 22:46:39 補充：
Due to the irregular shape of the calcium granule, the calcium oxide layer is not even in thickness. In my observation, when a calcium granule is put into water, there is no reaction at first, and then slow reaction, and finally fast reaction.

2011-01-20 22:46:46 補充：
It is believed that calcium starts reacting with water when some parts of the calcium oxide layer is "dissolved" in reaction, and the rate becomes fast when all the layer is "dissolved".

calcium oxide layer?
no, i don't think so. when the question says "it's calcium", then it's just calcium.
it's really a piece of calcium reacting with water, not a piece of calcium-that-exposed-to-air-for-some-time-before-reacting-with-water.

even if you're correct about oxide layer, the oxide layer would be even; probably NOT partly-covered and partly-exposed.

this observation is related to reaction rate.
rate of reaction not only depends on reactivity of metal, but also on many other effects.


my guess is, it's related to temperature.
the reaction between water and calcium is highly exothermic, raising the temperature of reaction mixture.

at first instance, the reaction is slow as calcium hydroxide formed is not very soluble.
after some time, temperature increases, and calcium hydroxide becomes more soluble, removing the covering-layer on calcium metal. also, temperature increase speeds up the reaction -- by increasing frequency of collision between calcium and water molecules.


and yes, water is not considered as acid. where does the acid come from?


2011-01-22 01:26:56 補充：
no, this question's probably NOT a real-situation experiment.
the question states "it's calcium", then we shouldn't assume anything on it.

unless the question says "in real life" or "a prolong-stored lump of calcium", making such assumptions is very dangerous.

2011-01-22 01:37:04 補充：
who "believes" that it's due to the oxide layer?
has it been proved, or is it just your hypothesis?

though not necessarily relaible, Wikipedia says "For example, where sodium and potassium react with water at room temperature, magnesium reacts only with steam and calcium with hot water:"

2011-01-22 01:39:24 補充：
http://en.wikipedia.org/wiki/Alkaline_earth_metal

"it is believed that" the initial rate of reaction is so slow that evolution of gas bubbles is not visible.

2011-01-22 01:39:41 補充：
as reaction proceeds, more and more heat is released, raising the surrounding temperature. reaction sppeds up when the water nearby is warm, and is fast when water is hot.

2011-01-22 01:41:10 補充：
--- since the reaction is exothermic.

when Q says "calcium", it's natural to assume it's really pure calcium, a well-prepared piece of calcium sample stored under paraffin oil.

2011-01-28 16:06:25 補充：
咦? 老爺子的討論怎麼不見了?

老實說, 這題目確實難辦, 尤其是要猜度設問者本身的想法.
如果問題(例如)先是比較鈉/鈣/鋅的反應活性, 然後再問"In practical, calcium reacts slower than sth. less reactive else", 那我會想是oxide layer;

如果問題先是問thiosulfate 的reaction rate affected by concentration, 然後再問calcium reaction rate, 那我就會想是temperature-dependent rate.

2011-01-28 16:11:14 補充：
揣摩上意, 揣摩設問者肚裡想法, 應該交給社會科學系/心理系/政治系去辦;
設立合理的假說, 然後以科學法度去驗證, 才是做科學工作的人該做的.
現在有兩個假說, 但又不能驗證, 叫人恨得心癢癢的...

the calcium metal can expose to the acid?(should be water) a little typing mistake made:P