Question

Which of the following reactions will not produce hydrogen gas?
(i) Fe+dil.HCl
(ii) Zn+dil.HCl
(iii) Cu+dil.HCl
(iv) Ag+dil.HCl

(a) (i),(iii),(iv)
(b) (iii),(iv)
(c) (i),(iv)
(d) (ii),(iv)

Answer 1

Hint: Some of the metals, when reacted with acids, there are chances that they can displace hydrogen gas from the acid. Only some of the metals have this ability. If the metal is above hydrogen in the electrochemical series, then, it can displace hydrogen from acid and those below hydrogen in the series cannot.

Complete step by step solution:
Those metals which are above hydrogen in electrochemical series have the ability to displace hydrogen from an acid, when the metal is reacted with the acid. In the above options we can see that Fe and Zn lie above hydrogen in an electrochemical series that is their reduction potential( it is the tendency of species to get reduced by gaining an electron) is lower than that of hydrogen. Thus they can produce hydrogen when reacted with dilute HCl. The reactions are as follows:
\[Fe+2HCl(dil)\to FeC{{l}_{2}}+{{H}_{2}}\uparrow \]
\[Zn+HCl(dil)\to ZnC{{l}_{2}}+{{H}_{2}}\uparrow \]
In case of Cu and Ag, they come below hydrogen in the electrochemical series that is their reduction potential ( it is the tendency of species to get reduced by gaining an electron) is higher than that of hydrogen. They do not have the ability to displace hydrogen from an acid. They do not react with dilute HCl.

Therefore, the correct answer is option (b).

Additional Information:
Arrangement of various redox equilibria in order of their standard electrode potentials is called electrochemical series. The most negative redox potential values are placed at the top and the most positive are in the bottom of an electrochemical series. Each redox potential shows whether the position of the equilibrium lies above or below hydrogen equilibrium. The difference in the positions of equilibrium causes the number of electrons which build up on the metal electrode and the platinum of the standard hydrogen electron to be different. This will produce potential differences.

Note: When the reduction potential of the metal is lower than that of hydrogen, it can displace hydrogen from an acid. If the reduction potential is higher than that of hydrogen, it cannot produce hydrogen when reacted with acid.