Problem 34
Question
Which of the following in each pair is likely to be more soluble in water: \((\mathbf{a})\) cyclohexane \(\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\) or glucose \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right),\) (b) propionic acid \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right)\) or sodium propionate \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COONa}\right)\) (c) HCl or ethyl chloride ( \(\left.\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\right)\) ? Explain in each case.
Step-by-Step Solution
Verified Answer
(a) Glucose, (b) Sodium propionate, (c) HCl are more soluble in water.
1Step 1: Compare Cyclohexane and Glucose
Cyclohexane (C₆H₁₂) is a non-polar molecule with no functional groups that can form hydrogen bonds with water. On the other hand, glucose (C₆H₁₂O₆) has multiple hydroxyl (OH) groups, which can form hydrogen bonds with water, making it highly soluble in water.
2Step 2: Compare Propionic Acid and Sodium Propionate
Propionic acid (CH₃CH₂COOH) can donate a proton to form hydronium ions in water, and it is somewhat polar due to its carboxylic acid group, allowing it to dissolve in water to some extent. Sodium propionate (CH₃CH₂COONa) is an ionic compound which fully dissociates in water, making it more soluble than propionic acid.
3Step 3: Compare HCl and Ethyl Chloride
HCl is a polar, strong acid that completely ionizes in water, producing hydronium and chloride ions. This property makes HCl very soluble in water. Ethyl chloride (CH₃CH₂Cl) is a less polar molecule and does not ionize in water, making it much less soluble compared to HCl.
Key Concepts
Polar and Non-Polar MoleculesHydrogen BondingIonic Compounds
Polar and Non-Polar Molecules
Understanding the difference between polar and non-polar molecules helps us predict whether a substance is likely to dissolve in water. **Non-polar molecules**, like cyclohexane (C₆H₁₂), lack regions of positive and negative charge. This is because they have symmetric arrangements of atoms and electrons, leading to an even distribution of electron density. As a result, they do not mix well with water, a polar substance.
**Polar molecules**, on the other hand, have an uneven distribution of charge across the molecule, creating areas of partial positive and negative charge. This occurs because the atoms in the molecule have different electronegativities, pulling electrons unevenly. Molecules like glucose (C₆H₁₂O₆) contain polar functional groups, such as hydroxyl (OH) groups, which interact favorably with water through hydrogen bonding. This makes them highly soluble in water.
**Key points to remember:**
**Polar molecules**, on the other hand, have an uneven distribution of charge across the molecule, creating areas of partial positive and negative charge. This occurs because the atoms in the molecule have different electronegativities, pulling electrons unevenly. Molecules like glucose (C₆H₁₂O₆) contain polar functional groups, such as hydroxyl (OH) groups, which interact favorably with water through hydrogen bonding. This makes them highly soluble in water.
**Key points to remember:**
- Polar molecules dissolve in polar solvents like water, while non-polar molecules do not.
- The presence of polar functional groups increases the solubility of a molecule in water.
Hydrogen Bonding
Hydrogen bonding significantly affects a molecule's solubility in water. This type of bond occurs when hydrogen, covalently bonded to a highly electronegative atom (like oxygen or nitrogen), experiences an attraction to a lone pair of electrons on another electronegative atom.
**Glucose exemplifies this concept perfectly.** With several hydroxyl groups, glucose can form multiple hydrogen bonds with water molecules. This interaction lowers the energy required for glucose molecules to mix with water, drastically increasing its solubility compared to non-polar molecules like cyclohexane.
Even though propionic acid (CH₃CH₂COOH) can form hydrogen bonds due to its carboxylic acid group, its solubility in water is comparatively limited. This limitation is because it does not ionize as completely as the ionic compounds, which dissociate entirely, enhancing solubility. Hydrophilic molecules that can establish hydrogen bonds will generally be more soluble in water, particularly when compared to their hydrophobic, non-hydrogen-bonding counterparts.
**Important points:**
**Glucose exemplifies this concept perfectly.** With several hydroxyl groups, glucose can form multiple hydrogen bonds with water molecules. This interaction lowers the energy required for glucose molecules to mix with water, drastically increasing its solubility compared to non-polar molecules like cyclohexane.
Even though propionic acid (CH₃CH₂COOH) can form hydrogen bonds due to its carboxylic acid group, its solubility in water is comparatively limited. This limitation is because it does not ionize as completely as the ionic compounds, which dissociate entirely, enhancing solubility. Hydrophilic molecules that can establish hydrogen bonds will generally be more soluble in water, particularly when compared to their hydrophobic, non-hydrogen-bonding counterparts.
**Important points:**
- Hydrogen bonding enhances solubility due to strong interactions with water molecules.
- The more hydrogen bonds a molecule can form, the more soluble it tends to be in water.
Ionic Compounds
Ionic compounds, such as sodium propionate (CH₃CH₂COONa), are highly soluble in water due to their ability to dissociate into ions. When ionic compounds dissolve, they separate into cations and anions. These ions are stabilized in solution by interaction with water molecules, which surround them due to their polar nature.
**Sodium propionate vs. propionic acid:** Sodium propionate completely dissociates into sodium ions and propionate ions in water, leading to a high level of solubility. In contrast, propionic acid, although polar, does not fully dissociate and thus has lower solubility. The strong ion-dipole interactions in ionic compounds are key to their solubility in polar solvents.
Similarly, HCl, when dissolved in water, ionizes into hydrogen and chloride ions. The full ionization is crucial for its strong solubility compared to molecules like ethyl chloride (CH₃CH₂Cl), which do not ionize. This ability to ionize and form ions that are surrounded and stabilized by water molecules makes ionic compounds particularly soluble.
**Key takeaways:**
**Sodium propionate vs. propionic acid:** Sodium propionate completely dissociates into sodium ions and propionate ions in water, leading to a high level of solubility. In contrast, propionic acid, although polar, does not fully dissociate and thus has lower solubility. The strong ion-dipole interactions in ionic compounds are key to their solubility in polar solvents.
Similarly, HCl, when dissolved in water, ionizes into hydrogen and chloride ions. The full ionization is crucial for its strong solubility compared to molecules like ethyl chloride (CH₃CH₂Cl), which do not ionize. This ability to ionize and form ions that are surrounded and stabilized by water molecules makes ionic compounds particularly soluble.
**Key takeaways:**
- Ionic compounds are highly soluble in water due to their ionization into cations and anions.
- These ions are stabilized by water molecules through ion-dipole interactions.
Other exercises in this chapter
Problem 31
(a) Would you expect stearic acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH},\) to be more soluble in water or in carbon tetrachloride? (
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Which of the following in each pair is likely to be more soluble in hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}:\) (a) \(\mathrm{CCl}_{4}\) or \(\mathrm{CaCl}_{2},
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Indicate whether each statement is true or false: (a) The higher the temperature, the more soluble most gases are in water. (b) The higher the temperature, the
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Indicate whether each statement is true or false: (a) If you compare the solubility of a gas in water at two different temperatures, you find the gas is more so
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