Question

In which of the following exothermic reactions, the heat liberated per mole is the highest ?

• A. $CaO+H_{2}O \to Ca \left(OH\right)_{2}$
• B. $SrO+H_{2}O \to Sr \left(OH\right)_{2}$
• C. $BaO+H_{2}O \to Ba \left(OH\right)_{2}$
• D. $MgO +H_{2} O \to Mg \left(OH\right)_{2}$

Answer 1

The Correct Option is A

To determine which reaction liberates the most heat per mole, we need to compare the enthalpy changes of the given exothermic reactions. The heat liberated, or the enthalpy change, during a reaction is determined by the specific reaction enthalpies (∆H) which are often provided as standard tables or calculated experimentally. Reactions that involve the combination of metal oxides with water to form hydroxides are exothermic but can differ in the amount of heat they release.

Let's analyze the given reactions:

1. CaO + H_{2}O \to Ca(OH)_{2}
2. SrO + H_{2}O \to Sr(OH)_{2}
3. BaO + H_{2}O \to Ba(OH)_{2}
4. MgO + H_{2}O \to Mg(OH)_{2}

The enthalpy of hydration generally follows the trend of ionic size and charge of the participating ions:

• Calcium oxide (CaO) combines with water to form calcium hydroxide (Ca(OH)₂), releasing a substantial amount of energy due to the relatively smaller size and high charge density of calcium ions.
• Strontium oxide (SrO) releases slightly less energy due to the larger ionic radius compared to calcium.
• Barium oxide (BaO) releases even less energy due to an even larger ionic radius than strontium.
• Magnesium oxide (MgO) involves smaller ions than calcium, but its ionic bond is very strong, making its reaction with water less exothermic in practice compared to CaO.

From this analysis, the order of exothermicity typically decreases from calcium to barium. Hence, the reaction CaO + H_{2}O \to Ca(OH)_{2} liberates the most heat, because calcium's smaller cationic size and high charge density increase hydration energy upon forming hydroxide. Option 1 is correct.

Here, the values of the enthalpy change are critical if one were to discuss them (in kJ/mol) from data tables, but conceptually, this trend holds for such reactions.