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MCQEasyJEE 2023Trends in Atomic Radius & Ionisation Energy

JEE Chemistry 2023 Question with Solution

Ion having the highest hydration enthalpy among the given alkaline earth metal ions is:

  • A

    Be2+\mathrm{Be}^{2+}

  • B

    Ba2+\mathrm{Ba}^{2+}

  • C

    Ca2+\mathrm{Ca}^{2+}

  • D

    Sr2+\mathrm{Sr}^{2+}

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The ions are Be2+\mathrm{Be}^{2+}, Ba2+\mathrm{Ba}^{2+}, Ca2+\mathrm{Ca}^{2+} and Sr2+\mathrm{Sr}^{2+}.

Find: Which ion has the highest hydration enthalpy.

Hydration enthalpy is greater for ions with higher charge density. For alkaline earth metal ions, the charge is the same for all given ions, so the deciding factor is ionic size.

Smaller ions interact more strongly with water molecules, so hydration enthalpy decreases as ionic size increases down the group.

Therefore, the order of hydration enthalpy is:

Be2+>Mg2+>Ca2+>Sr2+>Ba2+\mathrm{Be}^{2+} > \mathrm{Mg}^{2+} > \mathrm{Ca}^{2+} > \mathrm{Sr}^{2+} > \mathrm{Ba}^{2+}

Among the given options, Be2+\mathrm{Be}^{2+} has the highest hydration enthalpy. Therefore, the correct option is A.

Charge Density Analysis

Given: All the given ions are alkaline earth metal ions with charge +2+2.

Find: The ion with maximum hydration enthalpy.

Hydration enthalpy depends on charge density, and charge density increases when charge is high and size is small.

This relation can be expressed qualitatively as:

Charge density∝ChargeSize\text{Charge density} \propto \frac{\text{Charge}}{\text{Size}}

Since all the ions have the same charge, hydration enthalpy is inversely related to size:

Hydration enthalpy∝1Size\text{Hydration enthalpy} \propto \frac{1}{\text{Size}}

Down the group from beryllium to barium, ionic size increases. Hence hydration enthalpy decreases down the group.

So the order is:

Be2+>Mg2+>Ca2+>Sr2+>Ba2+\mathrm{Be}^{2+} > \mathrm{Mg}^{2+} > \mathrm{Ca}^{2+} > \mathrm{Sr}^{2+} > \mathrm{Ba}^{2+}

Thus, Be2+\mathrm{Be}^{2+} has the highest hydration enthalpy, so the correct option is A.

Common mistakes

  • Comparing only the positive charge and ignoring ionic size. All the given ions have charge +2+2, so size becomes the deciding factor. Use charge density, not charge alone.

  • Assuming hydration enthalpy increases down the group. This is wrong because ionic size increases down the group, which lowers interaction with water. Remember that smaller ions have higher hydration enthalpy.

  • Confusing hydration enthalpy with ease of losing electrons or metallic character. These are different trends. For hydration enthalpy, focus on ion size and charge density.

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