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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