Question

For each of the following pairs of substances, predict which will have the higher melting point, and indicate why: (a) HF, \(\mathrm{HCl} ;\) (b) C (graphite), \(\mathrm{CH}_{4}\); (c) \(\mathrm{KCl}, \mathrm{Cl}_{2}\); (d) \(\mathrm{LiF}, \mathrm{MgF}_{2}\).

Short answer

(a) HF has a higher melting point than HCl due to stronger hydrogen bonds. (b) Graphite has a higher melting point than CH4 due to its strong covalent network structure. (c) KCl has a higher melting point than Cl2 because of the stronger ionic bonds. (d) MgF2 has a higher melting point than LiF due to stronger electrostatic forces between the ions.

Step-by-step solution

(a) Comparing HF and HCl for melting point

In these two compounds, molecule type is the same: both are hydrogen halides. The difference lies in the size and electronegativity of the halogen atom. HF has a smaller and more electronegative halogen atom (F) with a stronger hydrogen bond than HCl. Therefore, HF will require more energy to break the bonds, and this results in a higher melting point for HF compared to HCl.

(b) Comparing C (graphite) and CH4 for melting point

Graphite is an allotrope of carbon with a covalent network structure and strong bonds between carbon atoms. CH4, on the other hand, is a molecular compound with molecules held together by weak London dispersion forces. As a result, significantly less energy is needed to break the bonds in CH4 compared to the bonds in graphite. Therefore, graphite has a higher melting point than CH4 due to its strong covalent network structure.

(c) Comparing KCl and Cl2 for melting point

KCl is an ionic compound made up of K+ and Cl- ions, held together by strong electrostatic forces between the positively charged potassium ions and the negatively charged chloride ions. Cl2 is a molecular compound made up of Cl atoms that are held together by weak London dispersion forces. Due to the stronger ionic bonds in KCl, it will have a higher melting point than Cl2.

(d) Comparing LiF and MgF2 for melting point

Both LiF and MgF2 are ionic compounds, but they differ in their lattice structure and the charges on the ions. LiF consists of Li+ and F- ions, while MgF2 consists of Mg2+ and F- ions. The electrostatic forces are stronger between Mg2+ and F- ions in MgF2 due to their higher charges compared to the Li+ and F- ions in LiF. As a result, MgF2 will have a higher melting point than LiF because more energy is required to break the stronger ionic bonds in MgF2.

Key concepts

Hydrogen Bonding

Understanding melting points requires a grasp of the types of bonds and forces between particles in a substance. Hydrogen bonding is a special kind of dipole-dipole attraction that occurs specifically between a hydrogen atom, which is covalently bonded to a highly electronegative element like nitrogen, oxygen, or fluorine, and another electronegative atom. This intermolecular force is remarkably stronger than other dipole-dipole interactions and vastly stronger than London dispersion forces.

For example, in the compound hydrofluoric acid (HF), hydrogen bonds form between the hydrogen atom of one molecule and the fluorine atom of another molecule. These hydrogen bonds are so strong that they significantly increase the melting point of a substance. Compared to hydrogen chloride (HCl), which does not exhibit such strong hydrogen bonding due to chlorine's lower electronegativity, HF will have a higher melting point, as observed in the exercise solution.

Covalent Network Solid

A covalent network solid is a type of chemical compound where atoms are bonded covalently in a continuous network that spans the entire material. These covalent bonds are typically much stronger than intermolecular forces like hydrogen bonding or London dispersion forces. Therefore, covalent network solids often have high melting and boiling points due to the substantial energy required to break the network of covalent bonds.

One of the classic examples of a covalent network solid is graphite, an allotrope of carbon. In graphite, each carbon atom is bonded to three other carbon atoms, creating a strong, stable lattice of covalent bonds. This structure is vastly different from that of methane (CH₄), where carbon atoms are bonded to hydrogen atoms, and molecules are held together by relatively weak forces. Thus, graphite's melting point is significantly higher than that of methane, as explained in the solution.

Ionic Bonding

In contrast to covalent bonding, ionic bonding involves the electrostatic attraction between positively and negatively charged ions. This type of bond forms when an atom loses electrons to become a positively charged cation, while another atom gains those electrons to become a negatively charged anion. Ionic bonds are generally stronger than most intermolecular forces, leading to higher melting points for ionic compounds.

As demonstrated in the solution, potassium chloride (KCl) is a substance with ionic bonds between potassium cations (K⁺) and chloride anions (Cl^-), resulting in a high melting point. Molecular compounds like chlorine gas (Cl₂), held together by weaker forces, tend to melt at much lower temperatures. This significant difference in bond strength is why KCl melts at a much higher temperature than Cl₂. This example clearly illustrates the impact of ionic bonding on a substance's melting point.

Lattice Structure

The lattice structure of a compound is the arrangement of ions, atoms, or molecules in a regular, repeating pattern. It is closely related to the type and strength of bonding, and it significantly influences the melting point. A more complex or denser lattice structure generally means stronger bonds and, consequently, higher melting points.

In the case of ionic compounds like lithium fluoride (LiF) and magnesium fluoride (MgF₂), the ions are arranged in a predictable pattern that maximizes the attraction between oppositely charged ions. The lattice structure of MgF₂ involves Mg²⁺ ions and two F^- ions, creating a very strong electrostatic attraction due to the double positive charge on magnesium, requiring more energy to disrupt this lattice than the singly charged Li⁺ ions in LiF. Thus, MgF₂ exhibits a higher melting point, as stronger electrostatic forces within a more complex lattice structure mean more energy is needed to melt the compound.