Arrange the following substances not listed in Table 8.2 according to their expected lattice energies, listing them from lowest lattice energy to the highest: MgS, KI, GaN, LiBr.
The Correct Answer and Explanation is :
To arrange the given substances (MgS, KI, GaN, LiBr) according to their expected lattice energies, we need to consider the factors that influence lattice energy. Lattice energy is the energy released when ions in the gas phase come together to form an ionic solid. The higher the charges of the ions and the smaller their radii, the higher the lattice energy.
Lattice Energy Influencing Factors:
- Ion Charge: Higher charges lead to a stronger electrostatic attraction between ions, which results in a higher lattice energy.
- Ion Size: Smaller ions have a stronger electrostatic attraction because the ions are closer together, increasing the lattice energy.
Analysis of Each Substance:
- MgS (Magnesium Sulfide):
- Magnesium (Mg²⁺) has a charge of +2 and sulfur (S²⁻) has a charge of -2.
- The high charge on both ions (2+ and 2-) leads to a strong electrostatic attraction.
- Mg²⁺ is relatively small, which also increases the lattice energy.
- KI (Potassium Iodide):
- Potassium (K⁺) has a charge of +1 and iodide (I⁻) has a charge of -1.
- The charges are lower compared to MgS, and potassium and iodide ions are relatively large, meaning weaker electrostatic forces and a lower lattice energy.
- GaN (Gallium Nitride):
- Gallium (Ga³⁺) has a charge of +3 and nitrogen (N³⁻) has a charge of -3.
- The high charges of both ions (3+ and 3-) will lead to a very high lattice energy due to strong electrostatic attraction. Even though the ions are larger than in MgS, the high charges more than compensate for this.
- LiBr (Lithium Bromide):
- Lithium (Li⁺) has a charge of +1 and bromide (Br⁻) has a charge of -1.
- The charges are the same as KI, but lithium ions are smaller than potassium ions, meaning the electrostatic forces are stronger than in KI. This leads to a higher lattice energy than KI but still lower than MgS and GaN.
Lattice Energy Ranking (from lowest to highest):
- KI (Lowest lattice energy due to low charges and large ions)
- LiBr (Slightly higher lattice energy due to the smaller size of Li⁺ compared to K⁺)
- MgS (Higher lattice energy due to high charges on both ions, but with moderate ion size)
- GaN (Highest lattice energy due to the extremely high charges on Ga³⁺ and N³⁻)
Final Answer:
KI < LiBr < MgS < GaN