Question

Explain in terms of forces between structural units why (a) \(\mathrm{Br}_{2}\) has a lower melting point than \(\mathrm{NaBr}\). (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) has a higher boiling point than butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\). (c) \(\mathrm{H}_{2} \mathrm{O}\) has a higher boiling point than \(\mathrm{H}_{2} \mathrm{Te}\). (d) Acetic acid \(\mathrm{CH}_{3}-\mathrm{C}-\mathrm{OH}\) has a lower boiling point

Short answer

Question: Explain the differences in melting and boiling points of various compounds in terms of forces between their structural units. Answer: The differences in melting and boiling points of compounds can be attributed to the varying strengths of the intermolecular forces acting between their structural units. Compounds with stronger forces, like ionic bonding and hydrogen bonding, have higher melting/boiling points, while those with weaker forces, such as van der Waals forces, exhibit lower melting/boiling points. For example, NaBr with ionic bonding has a higher melting point than Br₂ with van der Waals forces, and H₂O with hydrogen bonding has a higher boiling point than H₂Te with van der Waals forces.

Step-by-step solution

Identify the Type of Forces

First, we need to identify the type of forces acting between the structural units in each compound. These forces can be ionic, covalent, or van der Waals, and they directly impact the melting/boiling points. (a) Br₂: Intermolecular forces: van der Waals forces NaBr: Intermolecular forces: ionic bonding (b) C₂H₅OH: Intermolecular forces: hydrogen bonding and van der Waals forces C₄H₁₀: Intermolecular forces: van der Waals forces (c) H₂O: Intermolecular forces: hydrogen bonding H₂Te: Intermolecular forces: van der Waals forces (d) Acetic acid (CH₃C(O)OH): Intermolecular forces: hydrogen bonding

Compare the Strengths of Intermolecular Forces

Next, we need to compare the relative strengths of the intermolecular forces in each compound. The stronger the forces, the higher the melting/boiling points. Ionic forces > hydrogen bonding > van der Waals forces (a) Br₂ has weaker van der Waals forces compared to the ionic bonding in NaBr, which leads to its lower melting point. (b) C₂H₅OH has stronger hydrogen bonding compared to the van der Waals forces in C₄H₁₀, which leads to its higher boiling point. (c) H₂O has stronger hydrogen bonding compared to the van der Waals forces in H₂Te, which leads to its higher boiling point. (d) Although Acetic acid has hydrogen bonding, it has more molecular flexibility due to larger molecular structure than water, which leads to its lower boiling point.

Key concepts

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular interaction, often present in molecules that do not possess charged atoms or ions. These forces arise due to temporary fluctuations in the electron distribution within molecules, resulting in temporary dipoles that attract each other.
Van der Waals forces are generally weaker than both ionic and hydrogen bonds.

• They are essential in gases and lower molecular weight organic compounds.
• The presence of these forces contributes to many physical properties of compounds, like boiling points and melting points.

Since Van der Waals forces are weaker, compounds that rely on them tend to have lower melting and boiling points in comparison to those with stronger intermolecular forces.

Hydrogen Bonding

Hydrogen bonding is a stronger type of polar interaction and occurs in molecules where hydrogen is covalently bonded to highly electronegative atoms, such as nitrogen, oxygen, or fluorine. This type of bonding is much stronger than van der Waals interactions.
In hydrogen bonding:

• The presence of hydrogen bonds affects the boiling and melting points of the compounds significantly.
• Such bonds require a considerable amount of energy to break, which leads to higher boiling points as seen in water (H₂O).

For instance, ethanol (C₂H₅OH) exhibits hydrogen bonding, which is why it has a higher boiling point than butane (C₄H₁₀) which only experiences van der Waals forces. This shows how hydrogen bonding can drastically influence the physical properties of a material.

Ionic Bonding

Ionic bonding involves the complete transfer of electrons from one atom to another, resulting in the formation of ions. These ions are held together by strong electrostatic forces of attraction between positively and negatively charged ions.
Compounds with ionic bonds, like sodium bromide (NaBr), possess a crystal lattice structure. This structure contributes to:

• Very high melting points compared to molecular compounds exhibiting van der Waals forces.
• Ionic bonds are generally the strongest types of bond forces. For this reason, NaBr has a higher melting point than Br₂, which depends solely on van der Waals forces.

These strong interactions require significant energy to overcome, making ionic compounds more thermally stable.

Melting Points

Melting points are determined by the strength and type of intermolecular forces in a substance. They represent the temperature at which a solid becomes a liquid.
Several factors affect melting points:

• Substances with strong ionic or hydrogen bonds generally exhibit higher melting points.
• In contrast, substances that rely on weak van der Waals interactions tend to melt at lower temperatures.

For example, NaBr has a much higher melting point than Br₂ because it consists of ionic bonds, compared to the weaker van der Waals forces in Br₂. Understanding the link between melting points and intermolecular forces allows for better prediction and comprehension of a substance's physical properties.

Boiling Points

Boiling points indicate the temperature at which a liquid turns into vapor. Stronger intermolecular forces result in higher boiling points because more energy is needed to transition the liquid into a gaseous state.
Influences on boiling points include:

• Hydrogen bonding significantly raises boiling points, as seen in water (H₂O) having a higher boiling point than hydrogen telluride (H₂Te).
• The presence of van der Waals forces usually leads to lower boiling points, like in the case of butane (C₄H₁₀) compared to ethanol (C₂H₅OH).

Hydrogen bonds involve strong interactions, which mean that compounds like water require more heat to overcome these bonds and transition into gas compared to compounds that only exhibit van der Waals forces.