Lead forms two compounds with chlorine, PbCl2 and PbCl4. The melting point of PbCl2 is 501oC, and that of PbCl4 is -15 oC. Interpret these melting points in terms of the types of solids.
The Correct Answer and Explanation is :
The significant difference in melting points between PbCl2 and PbCl4 can be explained by their different structures and types of bonding, which results in different types of solids.
PbCl2, with a high melting point of 501°C, is an ionic solid. In this structure, lead (Pb) is in a +2 oxidation state and forms ionic bonds with chloride ions (Cl-). This compound crystallizes in a lattice structure where each lead ion is surrounded by chloride ions in a way that maximizes electrostatic attraction between the oppositely charged ions. Ionic solids are characterized by strong bonding forces between the ions, which require considerable energy to overcome. This is the reason for the high melting point of PbCl2; a lot of energy is needed to disrupt these strong ionic bonds and melt the solid.
On the other hand, PbCl4, which has a melting point of -15°C, indicating it is a liquid at room temperature, is a covalent compound. In this molecule, lead is in a +4 oxidation state and shares electrons with chlorine atoms to form covalent bonds. PbCl4 exists as molecular crystals where molecules are held together by weaker van der Waals forces rather than ionic bonds. These intermolecular forces are much easier to overcome compared to the ionic bonds in PbCl2, which explains why PbCl4 melts at such a low temperature.
Furthermore, the low melting point of PbCl4 also reflects its instability and tendency to decompose into PbCl2 and Cl2 under normal conditions. This instability is another reason why the forces holding the compound together in the solid state are relatively weak.
These properties illustrate how the type of bonding and the structure of the compound affect their physical properties, such as melting points. PbCl2, being an ionic solid, has a much higher melting point due to strong ionic bonds, while PbCl4, with covalent bonding and weaker intermolecular forces, melts at a much lower temperature.
Now, let’s generate an image illustrating the crystal structures of PbCl2 and PbCl4 to visualize these explanations better.
Here’s the illustration depicting the crystal structures of PbCl2 and PbCl4. You can see how the types of bonding influence the arrangement and strength of the interactions within each compound.