Irwin Talesnick, Kingston ON
Science is a verb.
Irwin contributed many answers in a well-received series called “Questions and answers”
More questions & answers, Reprinted from Chem 13 News May 1972, page 13
Question: In a recent experiment on reactivities of metals in the lab, I
found that calcium metal reacted well with water as one might expect,
while magnesium ribbon showed little or no reaction at room
temperature. If the water was replaced with dilute H2SO4 the
magnesium reacted quite well. Again in order with what is expected.
However calcium plus H2SO4 reacted only mildly in comparison to the
magnesium plus acid or calcium plus water. This is not what I would
expect since I felt the dilute H2SO4 should cause calcium to react even
better than in water; in fact, better than the magnesium.
If I added a soluble salt to water to provide sulphate ions, again the
calcium reaction slowed down.
Could you tell me if there is some irregular effect of sulphate here? By
the way, this irregularity in reactivity did not show up in using
hydrochloric acid.
The reaction of sulfuric acid with calcium metal produces a
coating of calcium sulfate (CaSO4) on the metal. Since the
calcium sulfate is insoluble in water, the coating acts as a
protective layer thus preventing further attack on the metal by
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16 Chem 13 News/October 2008
the acid. In the case of the reaction of sulfuric acid with
magnesium, the magnesium sulfate is soluble in the aqueous
medium, and hence the reaction proceeds until all of the
magnesium is used up, assuming that excess sulfuric acid was
present. When hydrochloric acid was used the reaction
proceeded rapidly, as expected, since calcium chloride is
soluble in the aqueous medium.
The addition of the sulfate salt to the water acted in the same
way as the sulfuric acid since the calcium ions from solution
were precipitated by the excess sulfate ion, and again provided
a protective layer on the calcium metal.
An additional answer might be that both the calcium and
magnesium are initially covered with a protective layer of
calcium oxide and magnesium oxide, respectively. The oxides
of calcium and magnesium are both soluble in water as well as
in hydrochloric acid, but not in sulfuric acid. Any reaction of the
calcium oxide with the aqueous sulfuric acid would, again, result
in the formation of an additional calcium sulfate protective
layer. ∎