Question

CH₄ can be prepared by the reaction of H₂O with

(A) Mg₂ C₃ 

 (B) CaC₂ 

(C) Be₂C 

D) Al₄C₃

Answer 1

Correct option (C) Be₂C (D) Al₄ C₃

Explanation:

Be₂C → 2Be²⁺ + C^4-, Al₄C₃ →4Al³⁺ + C^4-

Be₂C + 4H₂O → CH₄ + 2Be(OH)₂

Al₄C₃ + 12H₂O →  3CH₂ + 4Al(OH)₃

Carbides of Be and Al contain methinide ion (C^4- ) and thus give CH₄ gas (due to the diagonal relationship of Be and Al)

Whereas the carbides of Mg, i.e., Mg₂ C₃ , contain propynide ion (C_34- ) and hence give propyne gas.

Mg₂C₃ + 4H₂O → CH₃ - C ≡ CH + 2Mg(OH)₂

The carbides of rest of the second group, i.e., Ca, Sr, and Ba(CaC₂ , SrC₂ and BaC₂ ) give ethyne (acetylene) gas because they contain acetylide or ethynide ion(C₂ ^2-)

CaC₂ → Ca²⁺ + C₂^2-