Which of the listed reactions evolve a combustible gas...?

"A. magnesium hydroxide + hydrochloric acid".
"B. magnesium carbonate + hydrochloric acid".
"C. magnesium oxide + hydrochloric acid".
"D. magnesium metal + hydrochloric acid".

1 Answer
anor277
Apr 2, 2017

"Option B"

Explanation:

We can write out the individual stoichiometric equations for A, B, C, and D:

"A:"

Mg(OH)_2(s) + 2HCl(aq) rarr MgCl_2(aq) + 2H_2O(l)

"B:"

MgCO_3(s) + 2HCl(aq) rarr MgCl_2(aq) + H_2O(l) +CO_2(g)uarr

"C:"

MgO(s) + 2HCl(aq) rarr MgCl_2(aq) + H_2O(l)

"D:"

Mg(s) + 2HCl(aq) rarr MgCl_2(aq) + H_2(g)uarr

At 1st year university level I would expect you to know these reactions. Now ONLY "B" and "D" result in gas evolution. However, for "D", the gas evolved is dihydrogen, which is certainly combustible:

H_2(g) + 1/2O_2(g) rarr H_2O(g) + Delta

And hydrogen gas typically ignites with a pop. On the other hand, the gas evolved in "B" is non-combustible carbon dioxide, which WOULD extinguish a lit taper.

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