Textbook Question
Using this graph of CS2 data, determine (a) the approximate vapor pressure of CS2 at 30°C,
Ch.11 - Liquids and Intermolecular Forces
Chapter 11, Problem 8
Ethylene glycol (HOCH2CH2OH) and pentane (C5H12) are both liquids at room temperature and room pressure, and have about the same molecular weight. (b) One of these liquids has a much lower normal boiling point (36.1 °C) compared to the other one (198 °C). 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?
Verified step by step guidance1
Step 1: Understand the concept of boiling point. The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. Substances with stronger intermolecular forces have higher boiling points.
Step 2: Compare the intermolecular forces of ethylene glycol and pentane. Ethylene glycol has hydrogen bonding due to the presence of -OH groups, while pentane has only London dispersion forces.
Step 3: Determine which liquid has the lower boiling point. Since pentane has weaker intermolecular forces (London dispersion forces) compared to ethylene glycol (hydrogen bonding), pentane will have a lower boiling point.
Step 4: Identify the liquid used as antifreeze. Antifreeze requires a high boiling point and the ability to form hydrogen bonds to mix well with water. Ethylene glycol, with its hydrogen bonding, is used as antifreeze.
Step 5: Identify the liquid used as a blowing agent. A blowing agent should be volatile, meaning it has a low boiling point. Pentane, with its lower boiling point, is used as a blowing agent in polystyrene foam manufacture.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Intermolecular Forces
Intermolecular forces are the attractive forces between molecules that influence their physical properties, including boiling points. Ethylene glycol, with its hydroxyl (-OH) groups, can form hydrogen bonds, leading to stronger intermolecular attractions compared to pentane, which primarily exhibits weaker van der Waals forces. This difference in intermolecular forces is a key factor in determining the boiling points of these substances.
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Intermolecular vs Intramolecular Forces
Boiling Point
The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure, allowing it to transition from liquid to gas. A higher boiling point indicates stronger intermolecular forces that require more energy to overcome. In this case, ethylene glycol has a significantly higher boiling point than pentane due to its ability to form hydrogen bonds, making it less volatile.
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Applications of Liquids
Different liquids have specific applications based on their chemical properties. Ethylene glycol is commonly used as antifreeze in automobile engines due to its low freezing point and ability to prevent engine freezing. Conversely, pentane, being more volatile, is often used as a blowing agent in the production of polystyrene foam, where rapid vaporization is advantageous for creating foam structures.
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Related Practice
Textbook Question
The molecules
have the same molecular formula (C3H8O) but different chemical structures. (b) Which molecule do you expect to have a larger dipole moment? [Sections 11.2 and 11.5]
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Textbook Question
The phase diagram of a hypothetical substance is
(b) What is the physical state of the substance under the following conditions? (i) T = 150 K, P = 0.2 atm; (ii) T = 100 K, P = 0.8 atm; (iii) T = 300K, P = 1.0atm. [Section 11.6]
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Textbook Question
At three different temperatures, T1, T2, and T3, the molecules in a liquid crystal align in these ways:
(a) At which temperature or temperatures is the substance in a liquid crystalline state? At those temperatures, which type of liquid crystalline phase is depicted?
Textbook Question
At three different temperatures, T1, T2, and T3, the molecules in a liquid crystal align in these ways:
(b) Which is the highest of these three temperatures? [Section 11.7]