Question

Which of the silanes are formed by the reactions of $Mg_2Si + H_2SO_4 \longrightarrow$
A. $Si_4H_{10}$
B. $Si_3H_8$
C. $Si_5H_{10}$
D. $Si_6H_{14}$
E. $Si_7H_9$
Choose the correct answer from the options given below :

• A. B, C, E only
• B. A, D only
• C. B, D, E only
• D. A, B, D only

Hint:

Silanes follow the general formula $Si_nH_{2n+2}$, exactly like alkanes. Any formula that deviates from this (like $Si_5H_{10}$ or $Si_7H_9$) in introductory inorganic chemistry is usually incorrect or represents a much rarer cyclic variant not formed via simple $Mg_2Si$ hydrolysis.

Answer 1

The Correct Option is D

Step 1: Concept:
The question asks to identify the specific silane compounds produced when magnesium silicide ($Mg_2Si$) reacts with a dilute acid like sulfuric acid ($H_2SO_4$).

Step 2: Key Formula or Approach:
The reaction of magnesium silicide with dilute aqueous acids is a classic synthesis route for silanes (silicon hydrides). This reaction does not yield a single product, but rather a complex mixture of gaseous and liquid silanes.
These synthesized silanes are analogous to alkanes and follow the general formula $Si_nH_{2n+2}$ for saturated, non-cyclic chains.

Step 3: Step-by-step Explanation:

• The reaction primarily yields monosilane ($SiH_4$) and disilane ($Si_2H_6$). However, higher order silanes up to hexasilane ($Si_6H_{14}$) are also formed in the resulting mixture and can be separated by fractional distillation.

• We must evaluate the provided chemical formulas against the general rule for linear/branched silanes: $Si_nH_{2n+2}$.

• A. $Si_4H_{10$:} Here $n=4$. The formula requires $2(4) + 2 = 10$ hydrogens. This is Tetrasilane, a valid saturated straight-chain or branched silane formed in this reaction.

• B. $Si_3H_8$: Here $n=3$. The formula requires $2(3) + 2 = 8$ hydrogens. This is Trisilane, a valid product.

• C. $Si_5H_{10$:} Here $n=5$. For a straight chain, it should be $H_{12}$. $Si_5H_{10}$ corresponds to a cyclopentasilane (a cyclic structure). While cyclic silanes exist, they are not the primary, standard products expected from the acid hydrolysis of $Mg_2Si$ taught in standard inorganic courses.

• D. $Si_6H_{14$:} Here $n=6$. The formula requires $2(6) + 2 = 14$ hydrogens. This is Hexasilane, the highest commonly noted stable liquid silane formed in this mixture.

• E. $Si_7H_9$: Here $n=7$. This formula is completely invalid for any standard stable silane network. It lacks enough hydrogen atoms to satisfy silicon's tetravalency without an implausible number of double bonds or rings (which silicon resists forming).

• Therefore, only the saturated, straight-chain formulas A, B, and D represent the correct products.

Step 4: Final Answer:
The correctly identified products are A, B, and D, which corresponds to option (D).