Problem 80 - Chemistry The Central Science In Si Units

Question

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\) are both liquids at room temperature and room pressure, and have about the same molecular weight. (a) One of these liquids is much more viscous than the other. Which one do you predict is more viscous? (b) One of these liquids has a much lower normal boiling point \(\left(36.1^{\circ} \mathrm{C}\right)\) compared to the other one \(\left(198^{\circ} \mathrm{C}\right) .\) Which liquid has the lower normal boiling point? (c) One of these liquids is the major component in antifreeze in automobile engines. Which liquid would you expect to be used as antifreeze? (d) One of these liquids is used as a "blowing agent" in the manufacture of polystyrene foam because it is so volatile. Which liquid would you expect to be used as a blowing agent?

Step-by-Step Solution

Answer

(a) Ethylene glycol is more viscous. (b) Pentane has the lower boiling point. (c) Ethylene glycol is used in antifreeze. (d) Pentane is used as a blowing agent.

Step 1: Understanding Viscosity

Viscosity is a measure of a liquid's resistance to flow. Substances with stronger intermolecular forces are generally more viscous. Ethylene glycol has -OH groups that can form strong hydrogen bonds, making it more viscous compared to pentane, which only has weak van der Waals forces between its molecules.

Step 2: Comparing Boiling Points

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. Stronger intermolecular forces result in higher boiling points. Ethylene glycol, which can form hydrogen bonds, will have a higher boiling point (198°C) compared to pentane, which lacks these strong interactions and has a lower boiling point (36.1°C).

Step 3: Identifying the Antifreeze Component

Antifreeze solutions need to have a high boiling point and low freezing point, along with low volatility. Ethylene glycol, with its high boiling point and ability to lower freezing points, is commonly used as the main component in antifreeze, unlike the more volatile pentane.

Step 4: Selecting the Blowing Agent

Blowing agents in manufacturing require high volatility to facilitate the expansion of materials like polystyrene. Pentane, with its low boiling point of 36.1°C and relatively high volatility, is used as a blowing agent in polystyrene foam production.

Key Concepts

Viscosity

Viscosity is like a liquid's internal friction; it determines how "thick" or "runny" a liquid is. When you think about substances like syrup and water, syrup is clearly more viscous because it flows much slower compared to water.
The reason behind this difference in flow lies within the molecular structure of these liquids. Substances with stronger intermolecular forces, like hydrogen bonds, tend to be more viscous.

• Hydrogen Bonding: Ethylene glycol contains hydroxyl groups (-OH), capable of forming strong hydrogen bonds.
• Van der Waals Forces: Pentane has these much weaker interactions, resulting in less resistance to flow.

Because ethylene glycol can form hydrogen bonds, it is more viscous than pentane. This makes it thicker and less likely to pour freely.

Boiling Point

The boiling point of a liquid is simply the temperature at which it begins to transition from a liquid to a gas. It's like when you boil water; heat increases the temperature until it starts to turn into steam at 100°C at sea level.
Liquids with strong intermolecular forces need more energy (higher temperatures) to break these bonds and enter the gaseous state.

• Higher Boiling Point: Ethylene glycol has a boiling point of 198°C due to its ability to form hydrogen bonds.
• Lower Boiling Point: Pentane boils at just 36.1°C because it only engages in weaker van der Waals interactions.

This means pentane reaches the boiling state much quicker with less heat, while ethylene glycol needs a much higher temperature due to its stronger hydrogen bonds.

Antifreeze

Antifreeze is crucial in cars to prevent engines from freezing in cold weather and overheating during the summer. It must have properties that allow it to withstand both extremes of temperature.
Key characteristics of antifreeze include:

• High Boiling Point: Prevents the engine from overheating by remaining liquid even at high temperatures.
• Low Freezing Point: Minimizes the risk of damage by staying liquid in cold climates.
• Low Volatility: Reduces the rate of vaporization to avoid frequent loss.

Given these requirements, ethylene glycol is the preferred antifreeze component because it boasts a high boiling point and low freezing point, making it effective in keeping the engine temperature stable across a wide temperature range.