Question

Types of Crystalline Solids Which solid in each pair has the higher melting point and why? \(\begin{array}{ll}{\text { a. } \operatorname{Fe}(s) \text { or } \mathrm{CCl}_{4}(s)} & {\text { b. } \mathrm{KCl}(s) \text { or } \mathrm{HCl}(s)} \\ {\text { c. } \mathrm{Ti}(s) \text { or } \mathrm{Ne}(s)} & {\text { d. } \mathrm{H}_{2} \mathrm{O}(s) \text { or } \mathrm{H}_{2} \mathrm{S}(s)}\end{array}\)

Short answer

Fe(s) has a higher melting point than CCl4(s) due to strong metallic bonds. KCl(s) has a higher melting point than HCl(s) because of its ionic bonds. Ti(s) has a higher melting point than Ne(s) as it forms strong metallic bonds. H2O(s) has a higher melting point than H2S(s) due to hydrogen bonding.

Step-by-step solution

Identify Types of Crystalline Solids

Determine the type of bonding in each solid to understand their properties. Metals like Fe and Ti form metallic bonds, CCl4 forms covalent bonds, ionic compounds like KCl form ionic bonds, and molecules like HCl, H2O, and H2S form molecular bonds. Noble gases like Ne exist as atoms held together by weak van der Waals forces.

Compare Melting Points of Fe(s) and CCl4(s)

Iron (Fe) is a metal that forms metallic bonds, which are strong due to the delocalized sea of electrons throughout the lattice. Carbon tetrachloride (CCl4) is a molecular solid with covalent bonds between atoms but weaker London dispersion forces between molecules. Metals typically have higher melting points than molecular solids; therefore, Fe has the higher melting point.

Compare Melting Points of KCl(s) and HCl(s)

Potassium chloride (KCl) is an ionic solid, while hydrogen chloride (HCl) is a molecular solid with relatively weak intermolecular forces. Ionic bonds are stronger than intermolecular forces; hence, KCl has the higher melting point.

Compare Melting Points of Ti(s) and Ne(s)

Titanium (Ti) is a transition metal with strong metallic bonds, whereas neon (Ne) is a noble gas with single atoms held together by weak van der Waals forces. Consequently, Ti has the higher melting point.

Compare Melting Points of H2O(s) and H2S(s)

Water (H2O) forms hydrogen bonds, which are stronger than the dipole-dipole and London dispersion forces present in hydrogen sulfide (H2S). Therefore, H2O has the higher melting point.

Key concepts

Types of Bonding

Bonding types determine the internal structure and thereby the physical properties of a substance, including melting points.
In general, there are three primary bonding types found in crystalline solids: metallic, ionic, and covalent (molecular).

Metallic bonds consist of positive ions surrounded by a sea of delocalized electrons. This structure confers strength and conductivity and generally leads to high melting points. Ionic bonds, found in salts, involve the electrostatic attraction between oppositely charged ions and typically also result in high melting points. Molecular bonds, on the other hand, occur between nonmetal atoms that share electrons. The molecules in these solids are held together by weaker intermolecular forces, resulting in lower melting points.

Metallic Bonds

Metallic bonds are at the heart of metals' distinctive properties.
In metallic bonding, atoms release some electrons to create a shared pool which holds the atoms together in a lattice. The electrons can move freely, contributing to metals’ excellent electrical and thermal conductivity. The strength of metallic bonds provides metals with high melting and boiling points - a property easily observed when comparing a metal like iron to a molecular solid such as carbon tetrachloride.

Ionic Bonds

Ionic bonds are typically found in crystalline salts, like sodium chloride (table salt) or potassium chloride.
These bonds form when one atom donates an electron to another, resulting in a positively charged cation and a negatively charged anion. The electrostatic attraction between these oppositely charged ions creates a rigid and strong lattice structure which usually translates into a high melting point for the ionic solid.

Molecular Bonds

Molecular bonds entail the sharing of electrons between atoms to fill their outer electron shells.
This kind of bonding forms molecules with definite shapes and a set number of atoms, like the carbon tetrachloride molecule (CCl4). In solids, these molecules are held together by intermolecular forces rather than by strong ionic or metallic bonds, which often leads to lower melting points.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion between neighboring particles, like atoms or molecules, that are less strong than intramolecular forces.
They include London dispersion forces, dipole-dipole interactions, and hydrogen bonds. These are notably weaker than the bonds within molecules (covalent) or between ions (ionic), which accounts for substances with molecular bonding having lower melting points compared to ionic or metallic solids.