Question

The black powder which gives greenish-yellow gas when heated with conc. hydrochloric acid is:
A. cuprous oxide
B. coal
C. lead oxide
D. manganese dioxide

Answer 1

Hint: Hydrochloric acid is an inorganic acid. The chemical formula is HCl. An inorganic compound is a compound that lacks carbon-hydrogen carbon bond, In which two or more chemical elements (other than carbon) combine with a definite proportion.

Complete step by step answer:
The color of manganese dioxide is a black color amorphous compound. When manganese dioxide when heated with concentrated hydrochloric acid and a greenish-yellow color gas is formed. This gas is chlorine gas.
The reaction is shown below,
 \[Mn{O_2} + 4HCl(conc)\xrightarrow{{Heat}}MnC{l_2} + 2{H_2}O + C{l_2}(g)\]

Hence option D is correct.

Additional information:
-The oxidation state means the total number of electrons that an atom can either gain or lose to form a chemical bond with another atom. D block elements are also known as transition elements. This means that the valence electrons of transition elements lie in their d orbitals.
-In case of. d-block elements due to the presence of electrons at d orbitals, which is closer to the outermost shell of the metal. They show a variable oxidation state. With increasing the number of electrons of the d orbitals (up to 5 electrons), the number of oxidation state increases.
-In the case of d electrons due to lower effective nuclear charge of attraction, the electrons can be removed to form different oxidation states.
For example, Manganese shows oxidation states like +2,+3,+4,+5,+6,+7. In these oxidation states, less common +3,+4,+5, are easily prepared.

Note:
 The greenish-yellow gas in which chlorine also acts as an oxidizing and bleaching agent. It can react with calcium hydroxide and forms white color bleaching powder. The overall reaction is shown below,
  \[\;\;Mn{O_2}\;{\text{ + }}conc.HCl\; \to C{l_{2}} + MnC{l_2} + {H_2}O\;{\text{ }}\xrightarrow{{2Ca(OH)}}Ca{(OCl)_{2}} + {H_2}O + CaC{l_2}\] .
Manganese dioxide can also be prepared by a disproportionation reaction of potassium manganate (green mass), the reaction is shown below.
  \[{K_2}Mn{O_4}\,\,\, + 4HCl\,\, \to \,\,2KMn{O_4}\,\, + \,\,Mn{O_{2}}\,\, + 2{H_2}O\,\, + 4KCl\] .