Problem 95 - Chemistry: The Central Science in SI Units, Global Edition

Question

You have samples of four compounds: diethyl ether, butane, 1 -chlorobutane, and ethylene glycol. You measure the boiling points of the compounds as \(-0.5^{\circ} \mathrm{C}, 35^{\circ} \mathrm{C}\), \(77^{\circ} \mathrm{C}\), and \(195^{\circ} \mathrm{C}\), but then lose the labels for each sample. Make the following predictions: (a) Which compound boils at \(-0.5^{\circ} \mathrm{C} ?(\mathbf{b})\) Which boils at \(35^{\circ} \mathrm{C} ?(\mathbf{c})\) Which boils at \(77^{\circ} \mathrm{C} ?(\mathbf{d})\) Which boils at \(195^{\circ} \mathrm{C} ?\)

Step-by-Step Solution

Answer

(a) Butane boils at -0.5°C. (b) Diethyl ether boils at 35°C. (c) 1-Chlorobutane boils at 77°C. (d) Ethylene glycol boils at 195°C.

Step 1: Examine the properties of the compounds

First, let's take a look at the properties of each compound and the boiling points we have: Compound 1: Diethyl ether (C4H10O) - It is an ether with weak dipole-dipole interactions due to the presence of an oxygen atom. Compound 2: Butane (C4H10) - It is an alkane with only London dispersion forces. Compound 3: 1-Chlorobutane (C4H9Cl) - It is an alkyl halide with a dipole-dipole interaction due to the presence of the chlorine atom, and London dispersion forces. Compound 4: Ethylene glycol (C2H6O2) - It is an alcohol with two hydroxyl groups, which can form strong hydrogen bonds. Boiling points: -0.5°C, 35°C, 77°C, and 195°C.

Step 2: Determine the order of boiling points based on intermolecular forces

The strength of the intermolecular forces in each of the compounds will determine its boiling point. We can determine which compound has the lowest boiling point (and therefore which one is likely to be the one boiling at -0.5°C) by identifying the compound with the weakest intermolecular forces. Order based on boiling points: 1. Lowest boiling point: Compound with the weakest intermolecular forces. 2. Second lowest boiling point: Compound with weak intermolecular forces, but stronger than the first one. 3. Second highest boiling point: Compound with stronger intermolecular forces. 4. Highest boiling point: Compound with the strongest intermolecular forces.

Step 3: Match the boiling points to the compounds

Now let's match the compounds with the boiling points: 1. Compound with the weakest intermolecular forces: Butane (only London dispersion forces), therefore, boiling point -0.5°C. 2. Second weakest intermolecular forces: Diethyl ether (weak dipole-dipole interactions), therefore, boiling point 35°C. 3. Next strongest intermolecular forces: 1-Chlorobutane (stronger dipole-dipole interaction due to the presence of chlorine atom), therefore boiling point 77°C. 4. Strongest intermolecular forces: Ethylene glycol (hydrogen bonding due to the presence of hydroxyl groups), therefore, boiling point 195°C.

Step 4: Conclusion

Now we know the boiling points of each compound: (a) The compound that boils at -0.5°C is Butane. (b) The compound that boils at 35°C is Diethyl ether. (c) The compound that boils at 77°C is 1-Chlorobutane. (d) The compound that boils at 195°C is Ethylene glycol.

Key Concepts

Boiling Point

The boiling point of a substance is a key physical property that indicates the temperature at which it changes from a liquid to a gas. This temperature can reveal much about the intermolecular forces present in the substance. The stronger the intermolecular forces, the more heat (or higher temperature) is needed to overcome those forces and cause the liquid to boil.

For instance, consider four compounds—diethyl ether, butane, 1-chlorobutane, and ethylene glycol. The boiling points of these compounds are vastly different due to varying strengths of intermolecular forces. Stronger forces correspond to higher boiling points, as more energy is required to separate the molecules.

In simpler terms:

• Weak forces mean lower boiling points.
• Strong forces mean higher boiling points.

Understanding boiling points helps us predict and explain the behavior of substances under different conditions.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. This type of bonding significantly influences boiling points due to its strength and impact on molecular structure.

In ethylene glycol, the presence of two hydroxyl groups (OH) allows for extensive hydrogen bonding. This results in a much higher boiling point of 195°C compared to other substances with weaker intermolecular interactions.

Key points about hydrogen bonding:

• Occurs only with hydrogen bonded to F, O, or N.
• Creates strong attractions between molecules.
• Leads to considerably higher boiling points.

The strength of hydrogen bonds showcases how they can dramatically affect physical properties, such as the boiling point of a liquid.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force and are present in all molecules, whether polar or nonpolar. However, they are the only type of intermolecular force present in nonpolar molecules like butane.

These forces arise due to temporary shifts in electron density within widely spaced atoms, causing a momentary dipole. Butane, which primarily experiences these weak forces, boils at a low temperature of early 0°C.

Essential aspects of London dispersion forces include:

• Weak and temporary.
• Present in all molecules.
• Stronger in larger or heavier atoms/molecules.

Although weak individually, when summed across large molecules, they can have a noticeable impact on the boiling point, albeit usually lower than substances with stronger bond types.