Question

Explain the following observations: (a) The surface tension of \(\mathrm{CHBr}_{3}\) is greater than that of \(\mathrm{CHCl}_{3}\). (b) As temperature increases, oil flows faster through a narrow tube. (c) Raindrops that collect on a waxed automobile hood take on a nearly spherical shape. (d) Oil droplets that collect on a waxed automobile hood take on a flat shape.

Short answer

The surface tension of CHBr3 is greater than that of CHCl3 due to stronger London dispersion forces between the CHBr3 molecules, resulting from the larger electron cloud of bromine. When the temperature increases, the viscosity of oil decreases, allowing it to flow faster through a narrow tube. Raindrops take on a nearly spherical shape on a waxed automobile hood due to the surface tension of water, while oil droplets form a flat shape because of their weaker surface tension and lower surface energy.

Step-by-step solution

Observation (a): The surface tension of CHBr3 is greater than that of CHCl3.

The surface tension of a liquid is a result of the cohesive forces between its molecules. In this case, both CHBr3 and CHCl3 molecules have a central carbon atom bonded to the halogen atoms. The difference between these two liquids lies in the halogen atoms: CHBr3 has bromine atoms, while CHCl3 contains chlorine atoms. Bromine is more massive and has a larger atomic radius, which means that each CHBr3 molecule has a larger electron cloud. As a result, the London dispersion forces (a type of van der Waals forces) between the neighboring CHBr3 molecules are stronger than those between the CHCl3 molecules, leading to a higher surface tension for CHBr3.

Observation (b): As temperature increases, oil flows faster through a narrow tube.

As the temperature of a fluid increases, its viscosity decreases. Viscosity is the measure of a fluid's resistance to flow and is related to the internal friction between the layers of the fluid. In the case of oil, the increase in temperature will cause the molecules to move faster and have more energy, which reduces the attractive forces between them. This reduction in forces will lower the viscosity, allowing the oil to flow more easily and faster through the narrow tube.

Observation (c): Raindrops that collect on a waxed automobile hood take on a nearly spherical shape.

The spherical shape observed in raindrops is due to the surface tension of the water. Surface tension is a force that tends to minimize the surface area of a liquid, and for a given volume, the smallest surface area is that of a sphere. When raindrops land on a waxed automobile hood, the hydrophobic nature of the wax prevents the water from spreading out and forces the water molecules to stay close together. As a result, the cohesive forces within the water overcome the adhesive forces between the water and the waxed surface, causing the raindrops to form nearly spherical shapes to minimize the surface area.

Observation (d): Oil droplets that collect on a waxed automobile hood take on a flat shape.

Unlike water, oil is a non-polar liquid, which means it will have weaker surface tension. Additionally, the oil has a lower surface energy than the water, meaning it tends to adhere to surfaces more easily. As a result, the oil droplets that collect on a waxed automobile will interact with the wax differently than water droplets. The low surface tension of oil and its affinity to the waxed surface allows it to spread out and form a flatter shape when collecting on the waxed automobile hood.

Key concepts

Cohesive Forces

Cohesive forces are the attractive forces that hold the molecules of a substance together. Imagine these forces as invisible bonds that keep liquids from spilling apart. These are especially significant in liquids, where molecules tend to stick together.
Typically, cohesive forces arise because of molecular interactions like hydrogen bonding, van der Waals forces, and more strong dipole-dipole attractions. In the context of surface tension, these forces prevent the liquid from splattering and create surfaces that resist penetration.
For instance, when comparing halogenated compounds such as \(\mathrm{CHBr}_{3}\) and \(\mathrm{CHCl}_{3}\), the heavier bromine atoms in \(\mathrm{CHBr}_{3}\) result in stronger London dispersion forces. This makes water molecules in \(\mathrm{CHBr}_{3}\) hold on to each other more tightly than in \(\mathrm{CHCl}_{3}\), giving it a higher surface tension.

• Surface tension is stronger in liquids with higher cohesive forces.
• The stronger the molecular interactions, the higher the surface tension.

Viscosity

Viscosity refers to a liquid's resistance to flow. Visualize honey drizzling from a spoon, slowly creeping into a cup. That's high viscosity at play.
Factors that impact viscosity include temperature and the nature of the liquid molecules themselves. As temperature increases, molecules move more energetically and can slide past one another more easily.
In oil, for instance, as you heat it up, it flows faster through a narrow tube. This happens because increased temperature reduces the viscosity of the oil, making it more fluid and less resistant to flow.

• Temperature and viscosity have an inverse relationship.
• Viscosity reflects a liquid's internal friction.

Hydrophobic Interactions

Hydrophobic interactions occur when non-polar substances, like oils or grease, repel water. Consider a water droplet beading up on a waxed car, which doesn't like getting wet.
This behavior emerges because water, a polar substance, tends to minimize its exposure to non-polar surfaces. The waxed hood on the car is hydrophobic, meaning that it's water-repellent.
Because of this, cohesive forces within raindrops are stronger than the adhesive forces between them and the wax. This causes raindrops to remain nearly spherical as they rest on the hood.

• Hydrophobic surfaces repel water, leading to spherical water droplets.
• Water's polarity plays a crucial role in its behavior on hydrophobic surfaces.

Non-Polar Liquids

Non-polar liquids, like oil, behave differently than polar liquids like water. In the presence of a hydrophobic surface, their behavior contrasts starkly.
Due to their weaker cohesive forces, non-polar liquids like oil have lower surface tension. This allows them to spread out more easily. Consider an oily stain on a wax surface, it's more flat than spherical. This is because the low cohesive forces allow non-polar liquids to spread and adhere to surfaces.
On a waxed car hood, oil droplets flatten out because the adhesion between oil and wax overpowers any cohesion within the oil, leading to its spread.

• Non-polar liquids have less surface tension.
• Their interaction with surfaces is dominated by adhesion over cohesion.