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

Question

98\. Among the elements \(\mathrm{W}, \mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) having atomic numbers \(9,10,11\) and 12 respectively, the correct order of ionization energies is (a) \(\mathrm{W}>\mathrm{Y}>\mathrm{X}>\mathrm{Z}\) (b) \(\mathrm{X}>\mathrm{W}>\mathrm{Z}>\mathrm{Y}\) (c) \(X>Z>Y>W\) (d) \(\mathrm{Z}>\mathrm{Y}>\mathrm{X}>\mathrm{W}\) 99\. Which of the following orders are correct? (i) Thermal stability \(\mathrm{BeCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (ii) Basic nature \(\mathrm{LiOH}>\mathrm{NaOH}>\mathrm{KOH}>\mathrm{RbOH}>\mathrm{CsOH}\) (iii) Solubility in water \(\mathrm{BeSO}_{4}<\mathrm{MgSO}_{4}<\mathrm{CaSO}_{4}<\mathrm{SrSO}_{4}<\mathrm{BaSO}_{4}\) (iv) Melting point \(\mathrm{NaCl}>\mathrm{KCl}>\mathrm{RbCI}>\mathrm{CsCl}>\mathrm{LiCl}\) (a) (i), (iv) (b) (i), (ii), (iv) (c) (ii), (iii) (d) (i), (ii) (iii), (iv)

Step-by-Step Solution

Answer

For ionization energy: (c), and for orders: (a) with order (i) correct.

Step 1: Determine the Elements for Ionization Energy

The elements \( \mathrm{W}, \mathrm{X}, \mathrm{Y}, \mathrm{Z} \) have atomic numbers \( 9, 10, 11, 12 \) respectively. These correspond to Fluorine, Neon, Sodium, and Magnesium.

Step 2: Analyze Ionization Energy Order

Generally, ionization energy decreases down a group and increases across a period in the periodic table. For the given elements: Ne, a noble gas, has the highest ionization energy, followed by \( \mathrm{Mg} \), \( \mathrm{Na} \), and \( \mathrm{F} \). Therefore, the order is \( \mathrm{X} > \mathrm{Z} > \mathrm{Y} > \mathrm{W} \).

Step 3: Evaluate the Correct Option for Ionization Energy

Compare the determined order with the given options. Option (c), \( \mathrm{X} > \mathrm{Z} > \mathrm{Y} > \mathrm{W} \), perfectly matches our analysis.

Step 4: Assess the Thermal Stability Order

The stability of carbonates correlates with the increasing size of the cation, so \( \mathrm{BeCO}_{3} < \mathrm{MgCO}_{3} < \mathrm{CaCO}_{3} < \mathrm{SrCO}_{3} < \mathrm{BaCO}_{3} \) is correct (i).

Step 5: Assess the Basic Nature Order

The basic nature increases down the group for hydroxides, opposing the given order \( \mathrm{LiOH} > \mathrm{NaOH} > \mathrm{KOH} > \mathrm{RbOH} > \mathrm{CsOH} \), making it incorrect (ii).

Step 6: Assess the Solubility in Water Order

The solubility of sulfates generally decreases down the group, opposing the given order \( \mathrm{BeSO}_{4} < \mathrm{MgSO}_{4} < \mathrm{CaSO}_{4} < \mathrm{SrSO}_{4} < \mathrm{BaSO}_{4} \), making it incorrect (iii).

Step 7: Assess the Melting Point Order

The melting point generally decreases down the alkali metals in the chlorides, but the LiCl often exhibits exceptions. However, within the context of alkali metal chlorides, \( \mathrm{NaCl} > \mathrm{KCl} > \mathrm{RbCl} > \mathrm{CsCl} > \mathrm{LiCl} \), it is incorrect based on standard analysis (iv).

Step 8: Determine Correct Orders for the Second Question

Based on our analysis, only order (i) concerning thermal stability is correct.

Key Concepts

Ionization Energy

Ionization energy is a fundamental periodic trend that refers to the amount of energy required to remove the outermost electron from an isolated gaseous atom. When analyzing ionization energy within the periodic table, this energy generally increases as you move across a period from left to right. This is due to an increased effective nuclear charge which tightly holds onto the electrons. Conversely, it decreases as you move down a group because the valence shell becomes further from the nucleus with added electron shells, resulting in less nuclear attraction.
Understanding this concept explains why Neon has a higher ionization energy than Sodium and Magnesium. Neon is a noble gas with a complete outer shell, making its electrons harder to remove. Magnesium also has a higher ionization energy than Sodium, due to its more positively charged nucleus and complete outer shells in comparison. Consequently, the order is Ne > Mg > Na > F, aligning with the correct option of the exercise.

Thermal Stability of Carbonates

Thermal stability of carbonates refers to the ability of these compounds to resist decomposition when subjected to heat. As we traverse down Group 2 of the periodic table, the thermal stability of carbonates increases. This phenomenon is a result of increasing ionic size and subsequent lattice energy decrease, which stabilizes the resulting ions upon decomposition.
For instance, Beryllium carbonate decomposes easily upon heating compared to Barium carbonate, due to Beryllium's smaller ionic size which causes higher lattice energy and less stable carbonates. Hence, the effective order of thermal stability is BeCO₃ < MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃. This order is consistent with the correct answer identified in the exercise.

Basicity of Hydroxides

The basicity of hydroxides is an interesting periodic trend where the strength of a base increases as you go down a group in the periodic table. This trend is due to a decrease in electronegativity and an increase in the size of the metal cations, which makes it easier for the hydroxide ions to dissociate in water, thus enhancing their basic nature.
Contrary to what might be assumed, Lithium hydroxide (LiOH) is actually less basic compared to Cesium hydroxide (CsOH), with the latter's position lower in the periodic group contributing to its stronger basicity. Therefore, the presumed order of LiOH > NaOH > KOH > RbOH > CsOH was incorrect as basicity increases down the group.

Solubility of Sulfates

Solubility of sulfates in water follows a general trend in the periodic table where it decreases down the group. This trend is attributed to the increasing size of the cations and the resulting increase in lattice energy which makes it harder for the sulfates to dissolve in water.
However, Beryllium sulfate (BeSO₄) is an exception, being more soluble than Magnesium sulfate (MgSO₄) primarily due to its unique packing and bonding structure which offsets the expected trend. Understanding these nuances is crucial as evident in the incorrect order suggested in the exercise: BeSO₄ < MgSO₄ < CaSO₄ < SrSO₄ < BaSO₄. Instead, the normal trend should highlight decreasing solubility as one progresses down the group.