Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The alkaline earth metal nitrate that does not crystallise with water molecules, is: (a) \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{Sr}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{Ba}\left(\mathrm{NO}_{1}\right)_{2}\)

Step-by-Step Solution

Answer

The correct answer is (d) \(\mathrm{Ba(NO_3)_2}\).

Step 1: Identifying Alkaline Earth Metals

The alkaline earth metals belong to Group 2 of the periodic table. These elements are Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), and Barium (Ba). Among the options given, the relevant alkaline earth metals are represented within their nitrates: \(\mathrm{Mg}\), \(\mathrm{Ca}\), \(\mathrm{Sr}\), and \(\mathrm{Ba}\).

Step 2: Understanding Crystallization with Water

Metal nitrates often crystallize with water to form hydrates. However, the tendency to form hydrates decreases as we move down the group in the periodic table due to decrease in hydration energy. Hydration energy decreases because the size of the metal increases, leading to less attraction between the metal ions and water molecules.

Step 3: Comparing Hydration Energies

As stated in the previous step, the hydration energy decreases as you go down the group. Therefore, Barium (Ba) has the lowest hydration energy among the given options. This implies that \(\mathrm{Ba(NO_3)_2}\) would be least likely to crystallize with water.

Step 4: Conclusion

Given the properties of the alkaline earth metals and their nitrates, the nitrate salt least likely to crystallize with water is \(\mathrm{Ba(NO_3)_2}\) due to its low hydration energy as Barium is the heaviest element in the alkaline earth group provided in the options.

Key Concepts

Hydration Energy

Hydration energy is an important concept in chemistry that refers to the energy change associated with the dissolution of an ion in water. In simple terms, it can be described as the amount of energy released when water molecules surround and interact with ions from a compound. This energy is crucial because it affects the solubility and crystallization of ionic compounds.

Alkaline earth metals, which are part of Group 2 in the periodic table, exhibit varying hydration energies. Magnesium (Mg), as a smaller ion, has a higher hydration energy compared to Barium (Ba), which is larger. This higher hydration energy means Mg ions form stronger attractions with water molecules. As we descend the group, with elements like Calcium (Ca), Strontium (Sr), and Barium (Ba), the ions become larger. Larger ions mean weaker attractions to water molecules, thus decreasing hydration energies.

Hence, the ability of a compound to crystallize with water is directly linked to hydration energy, with larger ions having lesser tendencies to form hydrates.

Crystallization

Crystallization is the process by which a solid forms, where the atoms or molecules are highly structured in a specific orderly pattern. It often occurs naturally as a cooling or evaporation process that brings molecules closer together.

In the case of metal nitrates, which often form hydrates, the process of crystallization involves both the metal ions and water molecules. Water molecules integrate into the crystal lattice, stabilizing the structure through interactions like hydrogen bonding. However, not all metal nitrates crystallize with water, which is influenced by the metal's hydration energy.

In alkaline earth nitrates, such as those of Magnesium (Mg), Calcium (Ca), Strontium (Sr), and Barium (Ba), the likelihood of forming hydrates decreases as you move down the group in the periodic table. This is because the hydration energy decreases as well, which reduces the metal ion's ability to attract and integrate water molecules into its crystal structure.

Periodic Table

The periodic table is a crucial tool in understanding the properties of elements and predicting their chemical behavior. It organizes elements based on increasing atomic number and groups elements with similar properties together, such as the alkaline earth metals in Group 2.

The arrangement in the periodic table helps chemists understand trends, such as atomic size and hydration energy. As you move down the group in the periodic table from Magnesium (Mg) to Barium (Ba), both the atomic and ionic sizes increase. This increase in size is accompanied by a decrease in hydration energy, which affects properties like solubility and crystallization behavior.

By using the periodic table, one can predict that Barium (Ba), being at the bottom of the alkaline earth metals group, will have the least hydration energy among Group 2 elements. This insight helps in determining that Barium nitrate (\(\mathrm{Ba(NO_3)_2}\)) will be the least likely to crystallize with water, as explained in the exercise.