Chemistry S & P Block Elements MDCAT 2012
PMDC Verified Question 99 of 104
Alkaline earth metal hydroxides decompose on heating. Which of the following reactions is a correct representation of this decomposition?
A
\( \text{M(OH)}_{2(s)} \longrightarrow \text{MO}_{(s)} + \text{H}_2\text{O}_{(l)} \)
B
\( \text{2MOH}_{(s)} \longrightarrow \text{2MO}_{(s)} + \text{H}_{2(g)} \)
C
\( \text{MOH}_{(s)} \longrightarrow \text{M}_2\text{O}_{(s)} + \text{H}_2\text{O}_{(l)} \)
D
\( \text{4MOH}_{(s)} \longrightarrow \text{4M}_{(s)} + \text{H}_2\text{O}_{(l)} + \text{O}_{2(s)} \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: \( \text{M(OH)}_{2(s)} \longrightarrow \text{MO}_{(s)} + \text{H}_2\text{O}_{(l)} \)
Concept:

Alkaline earth metals (Group II-A) have a valency of +2. Their hydroxides have the general formula \( \text{M(OH)}_2 \).

Solution:

  • When subjected to thermal decomposition, Group II-A hydroxides break down to yield their corresponding solid metal oxide and liquid/gaseous water.
  • The balanced general chemical equation is:

    $$ \text{M(OH)}_{2(s)} \longrightarrow \text{MO}_{(s)} + \text{H}_2\text{O}_{(l)} $$

  • For example, Magnesium hydroxide decomposes as: \( \text{Mg(OH)}_{2(s)} \longrightarrow \text{MgO}_{(s)} + \text{H}_2\text{O}_{(l)} \).


Why other options are incorrect:

  • Options B, C, & D: These options incorrectly utilize the formula \( \text{MOH} \), which is the general formula for Group I-A (Alkali metal) hydroxides, not Alkaline earth metals. Furthermore, the products listed in B and D are chemically incorrect for this standard thermal decomposition.

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