Problem 34 - Chemistry: The Central Science in SI Units, Global Edition

Question

Based on their composition and structure, list \(\mathrm{CH}_{3} \mathrm{COOH},\) \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) in order of (a) increasing intermolecular forces, \((\mathbf{b})\) increasing viscosity, \((\mathbf{c})\) increasing surface tension.

Step-by-Step Solution

Answer

Based on their composition and structure, the order of the molecules CH3COOH, CH3COOCH3, and CH3CH2OH is: a) increasing intermolecular forces: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid) b) increasing viscosity: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid) c) increasing surface tension: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid)

Step 1: Identify Intermolecular Forces

First, let's identify the types of intermolecular forces present in the given molecules 1. CH3COOH (Acetic Acid) has hydrogen bonding (due to the presence of OH group), dipole-dipole forces (due to polar C=O bond), and London dispersion forces. 2. CH3COOCH3 (Methyl Acetate) has dipole-dipole forces (due to polar C=O bond) and London dispersion forces 3. CH3CH2OH (Ethanol) has hydrogen bonding (due to the presence of OH group), dipole-dipole forces (due to polar C-O bond), and London dispersion forces.

Step 2: Compare the Strength of Intermolecular Forces

Now, let's compare the strengths of the intermolecular forces present in these molecules. Generally, hydrogen bonding is the strongest intermolecular force followed by dipole-dipole forces and London dispersion forces. Between CH3COOH and CH3CH2OH, since both have hydrogen bonding, their strengths depend on the extent of hydrogen bonding. Acetic acid (CH3COOH) has a stronger hydrogen bonding because of its more electronegative oxygen atom in the C=O bond. Ethanol (CH3CH2OH) has weaker hydrogen bonding in comparison. Methyl acetate (CH3COOCH3) does not have hydrogen bonding and has the weakest intermolecular forces among these three molecules.

Step 3: List Molecules in Increasing Order of Intermolecular forces

Using the information above, we can list the molecules in increasing order of intermolecular forces: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid)

Step 4: List Molecules in Increasing Order of Viscosity

Viscosity depends on the strength of intermolecular forces present in the molecules. Stronger intermolecular forces lead to higher viscosity. Thus, the order of increasing viscosity is: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid)

Step 5: List Molecules in Increasing Order of Surface Tension

Surface tension is also determined by the strength of intermolecular forces. Molecules with stronger intermolecular forces will have higher surface tension. Therefore, the order of increasing surface tension is: CH3COOCH3 (Methyl Acetate) < CH3CH2OH (Ethanol) < CH3COOH (Acetic Acid) To summarize, based on their composition and structure, the order of the molecules CH3COOH, CH3COOCH3, and CH3CH2OH is: a) increasing intermolecular forces： CH3COOCH3 < CH3CH2OH < CH3COOH b) increasing viscosity： CH3COOCH3 < CH3CH2OH < CH3COOH c) increasing surface tension： CH3COOCH3 < CH3CH2OH < CH3COOH

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a type of strong intermolecular attraction. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, like oxygen, nitrogen, or fluorine. This results in the hydrogen acquiring a partial positive charge. In turn, this hydrogen attracts nearby electronegative atoms, creating a hydrogen bond.

• For instance, in acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)), the hydrogen of the hydroxyl group \(OH\) is involved in forming hydrogen bonds with other oxygen atoms. This significantly increases the molecule's intermolecular forces.
• Similarly, ethanol (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)) also forms hydrogen bonds, but they are weaker compared to acetic acid.
• On the contrary, methyl acetate (\(\mathrm{CH}_3\mathrm{COOCH}_3\)) lacks this feature as it has no hydrogen atoms bonded to high electronegative atoms, showing only weaker dipole-dipole and London dispersion forces.

Hydrogen bonds are vital as they greatly increase the boiling points, viscosity, and surface tension of substances. Hence, compounds capable of hydrogen bonding are often observed to have more enhanced physical properties.

Viscosity

Viscosity refers to a fluid's resistance to flow. It is directly influenced by the nature and strength of intermolecular forces within the fluid.

Substances possessing stronger intermolecular forces tend to have higher viscosity as these forces hinder the ease of movement of molecules past one another. This is why liquids like molasses or honey flow much more sluggishly compared to water.

Let's relate this to the given molecules:

• Methyl acetate (\(\mathrm{CH}_3\mathrm{COOCH}_3\)), having the weakest intermolecular forces, shows the lowest resistance to flow, thus, its viscosity is the least among the three.
• Ethanol (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)), showcasing hydrogen bonding, has a moderate viscosity.
• Acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)), which exhibits robust hydrogen bonding, displays the highest viscosity of the three, resisting flow more than ethanol and methyl acetate.

Understanding viscosity is essential in applications like cooking, oil refinement, and even in biological systems, where the flow of molecules and substances is critical.

Surface Tension

Surface tension is the tendency of a liquid's surface to shrink to the smallest possible area. The phenomenon results from intermolecular forces differing at the interface between liquid and air. At the surface, molecules are only attracted inwards by other liquid molecules, creating a 'tight film'.

A classic example of surface tension is a water droplet forming a bead on a leaf. It is also why small insects can "walk" on water without sinking.

Surface tension is affected by the strength of the intermolecular forces:

• Methyl acetate (\(\mathrm{CH}_3\mathrm{COOCH}_3\)), with the weakest intermolecular forces, will have the lowest surface tension among the three molecules.
• Ethanol (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)) will have a moderate surface tension due to its capacity to form hydrogen bonds.
• Acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)), exhibiting significant hydrogen bonding, has the highest surface tension, aligning with its stronger intermolecular forces.

Understanding surface tension aids in fields such as materials science, biology, and even improving consumer products like detergents and coatings.