Question

What gas is produced when magnesium is made to react with hydrochloric acid?
(A) Hydrogen
(B) Oxygen
(C) Carbon dioxide
(D) No gas is produced

Answer 1

The reaction is an example of a single displacement reaction. Single displacement reaction products can be easily predicted in accordance with the electrochemical series i.e. using the redox potentials of the different species involved.

Complete step by step solution:
In order to solve this question we first need to understand displacement reactions. Displacement reaction is a type of reaction in which an ion in a compound is displaced by an element (single displacement reaction) or an ion belonging to another compound (double displacement reaction).
Below is an example of a single displacement reaction:
$A-B\quad +\quad C\quad \longrightarrow \quad C-B\quad +\quad A$
Where A and C are different metal species and B is an anion or
A and C are different halogen species and B is a cation.
Below is an example of double displacement reaction:
$A-B\quad +\quad C-D\quad \longrightarrow \quad A-D\quad +\quad C-B\quad$
Where A and C are cations while B and D are anions.
In the above question, the reaction mentioned is a single displacement reaction which is basically an oxidation-reduction reaction wherein one species is oxidised and another is reduced. Not all chemical species can oxidise or reduce each other. Whether it will be oxidised or reduced by another species depends upon their redox potential which is a measure of the tendency of a species to accept or donate electrons to an electrode (generally the Standard hydrogen electrode) and thus be reduced or oxidised respectively. For determining which species will displace the other in single displacement reactions, we can use the electrochemical series in which the species are arranged in the order of decreasing oxidation potential:
“Li > Cs > Rb > K >Ba > Sr >Na > Ca > Mg > Be > Al > H( water) > Mn > Zn > Cr(+3)

Fe(+2) > Cd > Co > Ni > Sn > Pb > H(in acids) > Cu > Fe(+3) >Hg >Ag
Pd > Ir > Pt(+2) > Au”
As Mg metal has a greater oxidation potential than hydrogen, this implies that ${ Mg }^{ 2+ }$ ion has less reduction potential than ${ H }^{ + }$ ion and therefore ${ H }^{ + }$ ion has a greater tendency to get reduced and oxidised the Mg metal to ${ Mg }^{ 2+ }$.
Oxidation reaction: ${ Mg(s) }\longrightarrow { Mg }^{ 2+ }(aq)+2{ e }^{ - }$
Reduction reaction: ${ { 2H }^{ + }(aq)+2{ e }^{ - } }\longrightarrow { H }_{ 2 }(g)$

Hence the correct answer is (A) Hydrogen.

Note: The redox potentials are actually the reduction potentials of the species and not the oxidation potentials so if the redox potential of a species is given, it is actually its reduction potential. Also if the redox potential of a species is more negative than the other, then the former has a less tendency to get reduced and the latter has a more tendency to get reduced.