Question

What are the strongest attractive forces that must be overcome to (a) melt ice? (b) sublime bromine? (c) boil chloroform (CHCl \(_{3}\) )? (d) vaporize benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) ?

Short answer

Answer: a) For ice, the strongest attractive force is hydrogen bonding. b) For bromine, it is Van der Waals (London dispersion) forces. c) For chloroform, it is dipole-dipole interactions. d) For benzene, it is also Van der Waals (London dispersion) forces.

Step-by-step solution

Identify the molecule and its structure

Ice is a solid form of water (H2O), and its structure contains polar molecules held together by hydrogen bonding.

Determine the strongest attractive force that must be overcome

In the case of ice, the strongest attractive force that must be overcome is hydrogen bonding between the water molecules. #b) Sublime Bromine#

Identify the molecule and its structure

Bromine (Br2) is a non-polar diatomic molecule.

Determine the strongest attractive force that must be overcome

In the case of bromine, the strongest attractive force that must be overcome is the Van der Waals (London dispersion) forces between the non-polar bromine molecules. #c) Boil Chloroform (CHCl \(_{3}\) )#

Identify the molecule and its structure

Chloroform (CHCl\(_{3}\)) is a polar molecule with a central carbon atom bonded to one hydrogen and three chlorine atoms.

Determine the strongest attractive force that must be overcome

In the case of chloroform, the strongest attractive force that must be overcome is the dipole-dipole interaction between the polar chloroform molecules. #d) Vaporize Benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\)#

Identify the molecule and its structure

Benzene (C6H6) is an aromatic hydrocarbon with a planar hexagonal structure containing alternating single and double carbon bonds.

Determine the strongest attractive force that must be overcome

In the case of benzene, the strongest attractive force that must be overcome is the Van der Waals (London dispersion) forces between the non-polar benzene molecules.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative atoms such as oxygen, nitrogen, or fluorine. These bonds are significantly stronger than typical dipole-dipole interactions due to the great polarity of the H-X bond and the small size of the hydrogen atom.
In the case of water, the strong hydrogen bonds are responsible for many of its unique properties, such as its high boiling and melting points as compared to other molecules of similar size. In ice, the regular arrangement of water molecules held by hydrogen bonds forms a crystalline structure, which has to be disrupted to melt the ice.

• Hydrogen bonds are crucial in biological structures, like DNA, where they help stabilize the double helix.
• They also contribute to the three-dimensional shapes of proteins.

Understanding hydrogen bonding helps explain why water is liquid at room temperature and ice has a lower density than liquid water, allowing ice to float.

Van der Waals Forces

Van der Waals forces, also known as London dispersion forces, are weak intermolecular forces arising from momentary dipoles that occur due to fluctuations in the electron cloud of molecules. These forces are present in all molecular interactions, but they are particularly significant in non-polar molecules where no other stronger interactions (like hydrogen bonds or dipole-dipole interactions) exist.
For molecules such as bromine (Br extsubscript{2}) and benzene (C extsubscript{6}H extsubscript{6}), these forces are the primary factors that hold the molecules together in a liquid or solid state. Although individually weak, collectively they can have a significant impact, especially in large molecules or elements with larger electron clouds. Overcoming Van der Waals forces is essential for processes like sublimation of bromine or vaporization of benzene.

• These forces increase with larger, more polarizable electron clouds.
• They are the only intermolecular forces present between noble gas atoms.

Despite their weakness, Van der Waals forces are essential for understanding the properties of many substances.

Dipole-Dipole Interaction

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule. These interactions are stronger than Van der Waals forces but weaker than hydrogen bonds.
In substances like chloroform (CHCl extsubscript{3}), the difference in electronegativity between the carbon and chlorine leads to a polar molecule with a permanent dipole moment. Boiling chloroform involves overcoming these dipole-dipole interactions to transition the liquid into gas. These interactions are a key concept in understanding the physical properties, like boiling points, of polar substances.

• Essential for the mixing behavior of molecules, explaining solubility in mixed solutions.
• Play a critical role in the functionality of many chemical processes and reactions.

Dipole-dipole interactions allow us to predict how substances will behave in various conditions.

Sublimation

Sublimation is the process where a substance transitions directly from a solid to a gas phase without passing through the liquid state. This occurs under specific conditions of temperature and pressure and is driven by the requirement to overcome intermolecular forces.
Example substances that undergo sublimation include bromine, which sublimates by overcoming Van der Waals forces, and dry ice ( ext{CO extsubscript{2}}), which sublimates under standard atmospheric conditions. Sublimation is a useful process in certain industrial and laboratory settings.

• Used in freeze-drying processes to preserve perishable materials.
• Establishing vapor-liquid equilibrium for certain purification methods.

Understanding sublimation helps to explain various states of matter and how energy interactions involve phase transitions.