Which of the following compounds is the most stable?
A. AgCl
B. $CuCl _2$
C. $AlCl _3$
D. LiCl
Answer (2)
Stability of ionic compounds depends on lattice energy.
Higher charges and smaller ionic radii lead to greater lattice energies and thus greater stability.
A l 3 + has the highest charge and smallest size among the given cations.
Therefore, A lC l 3 is the most stable compound. A lC l 3
Explanation
Understanding the Problem We are given four compounds: AgCl, C u C l 2 , A lC l 3 , and LiCl. We need to determine which one is the most stable. The stability of an ionic compound is related to its lattice energy. Higher lattice energy generally means greater stability. Lattice energy is directly proportional to the charges of the ions and inversely proportional to the distance between them (related to ionic radii).
Analyzing the Charges Let's analyze the charges of the cations in each compound:
AgCl: A g + (Silver)
C u C l 2 : C u 2 + (Copper)
A lC l 3 : A l 3 + (Aluminum)
LiCl: L i + (Lithium) All compounds contain C l − (Chloride) as the anion.
Considering the Ionic Sizes Now, let's consider the sizes of the cations. Smaller ions generally lead to greater lattice energy because the distance between the ions is smaller. The ionic radii generally decrease across a period and increase down a group in the periodic table.
L i + is in Group 1 and is relatively small.
A g + is larger than L i + because it is further down the group and also has more electron shells.
C u 2 + and A l 3 + are smaller than A g + and L i + . A l 3 + is smaller than C u 2 + because it has a higher effective nuclear charge.
Comparing Lattice Energies Lattice energy is proportional to the product of the charges of the ions. Therefore, higher charges lead to greater lattice energy. Considering both charge and size:
AgCl: +1 charge, larger size
C u C l 2 : +2 charge, smaller size than AgCl, but larger than AlCl3 and LiCl. Since there are two chloride ions, we should consider the total negative charge is -2.
A lC l 3 : +3 charge, smallest size among the given cations. Since there are three chloride ions, we should consider the total negative charge is -3.
LiCl: +1 charge, small size A l 3 + has the highest charge and smallest size, which means A lC l 3 should have the highest lattice energy and be the most stable.
Conclusion Based on the analysis of charges and sizes, A lC l 3 is expected to be the most stable compound due to the high charge (+3) and relatively small size of the A l 3 + ion, leading to the highest lattice energy.
Examples
In material science, understanding the stability of compounds is crucial for designing new materials with specific properties. For example, aluminum oxide ( A l 2 O 3 ), which is closely related to A lC l 3 , is a very stable compound and is used as a protective coating due to its resistance to corrosion. The principles governing the stability of these compounds, such as charge and ionic size, are fundamental in predicting the behavior and applications of various materials.
The most stable compound among the options given is A lC l 3 , which has the highest charge (+3) and the smallest ionic radius, leading to the strongest lattice energy. This results in greater stability compared to the other compounds listed. Therefore, the answer is oxed{AlCl_3} .
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