Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of hydration energy of alkaline earth metal ion is (a) \(\mathrm{Mg}^{2+}>\mathrm{Be}^{2+}>\mathrm{Ba}^{2+}>\mathrm{Ca}^{2+}>\mathrm{Sr}^{2+}\) (b) \(\mathrm{Be}^{2+}>\mathrm{Mg}^{2+}>\mathrm{Ca}^{2+}>\mathrm{Sr}^{2+}>\mathrm{Ba}^{2+}\) (c) \(\mathrm{Ba}^{2+}>\mathrm{Be}^{2+}>\mathrm{Ca}^{2+}>\mathrm{Mg}^{2+}<\mathrm{Sr}^{2+}\) (d) \(\mathrm{Be}^{2+}>\mathrm{Ca}^{2+}>\mathrm{Sr}^{2+}>\mathrm{Ba}^{2+}>\mathrm{Mg}^{2+}\)

Step-by-Step Solution

Answer

Option (b) is correct: Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺.

Step 1: Understand the Concept of Hydration Energy

Hydration energy is the energy released when ions are surrounded by water molecules in a solution. It depends on the charge and size of the ion. Generally, smaller ions with higher charges have more negative (higher) hydration energies, as they attract water molecules more strongly.

Step 2: Analyze Charges and Sizes of Ions

Alkaline earth metals form ions with a +2 charge. However, their ionic sizes differ. In order from smallest to largest radius, they are: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺.

Step 3: Determine the Order of Hydration Energy

Since smaller ions have generally higher (more negative) hydration energies, the correct order from largest to smallest hydration energy should follow the order of decreasing ionic size: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺.

Step 4: Identify the Correct Option

Compare the determined order Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ with the provided options. The correct order matches option (b).

Key Concepts

Alkaline Earth Metals

Alkaline earth metals are a group of elements found in Group 2 of the periodic table. They are: Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), and Barium (Ba). These metals are characterized by having two electrons in their outermost shell. This makes them prone to losing these electrons to achieve a stable electron configuration similar to noble gases. When alkaline earth metals lose their two outermost electrons, they form ions with a +2 charge. This characteristic charge influences how they interact with other elements and compounds, particularly water. The ionic forms of alkaline earth metals play a significant role in various chemical reactions, including those involving hydration energy.

Ionic Size

Ionic size refers to the size of an ion, or how large an ion's electron cloud is. When comparing ionic sizes among ions with the same charge, such as alkaline earth metal ions, the size generally increases down the group in the periodic table. This is because each successive element in the group has an additional electron shell. The order from smallest to largest ionic radii for the alkaline earth metal ions is: Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. Smaller ions tend to have greater hydration energy because they can attract water molecules more closely.

Charge of Ions

The charge of an ion arises when an atom either loses or gains electrons. For alkaline earth metals, they typically lose their outer two electrons, forming ions with a +2 charge. This +2 charge is crucial because it means these ions have a strong ability to attract negatively charged entities, like the oxygen in water molecules. The stronger the attraction between the ion and the surrounding water, the greater the hydration energy will be. Understanding the role of ionic charge helps in predicting how these ions might interact in different chemical contexts.

Ion-Water Interaction

Ion-water interaction refers to how ions, such as those from alkaline earth metals, interact with water molecules. When an ion is immersed in water, it attracts water molecules. Water is a polar molecule with a partial negative charge on the oxygen and a partial positive charge on the hydrogens. This polarity allows the positive ion to attract the negative end, (oxygen), of the water molecule. The smaller the ion and the higher its charge, the stronger the ion-water interaction. Such strong interactions release more energy during hydration, known as hydration energy, which helps determine the solubility and chemical behavior of the ions in solution.