Why are the hydrogen bonds of a certain molecule weaker in solution (with water as a solvent) than in a vacuum?
I'm supposed to have three reasons, and so far all I can come up with is that the water lowers the activation energy required to form the bond and that the molecules' entropy increases in water. Am I missing something important?
The molecule can form H bonds with water, too.
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Your reasoning is correct, but there is one more important reason why hydrogen bonds of a certain molecule are weaker in solution (with water as a solvent) compared to a vacuum. Here are the three reasons:
1. Lowered activation energy: In a solution, water molecules can interact with the functional groups of the molecule, reducing the activation energy required to form hydrogen bonds. The presence of water molecules can stabilize the polar functional groups, making it easier for the hydrogen bond to form.
2. Increased entropy: When a molecule is in a solution, it experiences an increase in entropy due to the solvation process. The ordered arrangement of water molecules around the solute disrupts the organization of hydrogen bonds within the molecule. This increase in entropy makes it more favorable for the molecule to interact with water and weaken its internal hydrogen bonds.
3. Electrostatic screening: In a vacuum, there are no other molecules present to compete for the hydrogen bonding sites on the molecule. However, in a solution, water molecules can act as competitors for the hydrogen bonding sites. As a result, the electrostatic interactions between the water molecules and the hydrogen bonding sites on the molecule can partially screen the strength of the hydrogen bonds by weakening their attractive forces.
Overall, the combination of lowered activation energy, increased entropy, and electrostatic screening decreases the strength of hydrogen bonds in a certain molecule when it is in a solution with water as a solvent compared to a vacuum.
To understand why the hydrogen bonds of a certain molecule may be weaker in a solution with water as a solvent compared to a vacuum, there are several factors to consider. You have already mentioned two important factors, but let me elaborate on those and add a third reason:
1. Solvation Effect: Water has the ability to solvate (surround) molecules and ions by forming hydrogen bonds with them. When the molecule is in solution, the water molecules compete with the intramolecular hydrogen bonding interactions. The solvating water molecules can disrupt and weaken the hydrogen bonds within the molecule. Thus, the presence of water as a solvent weakens the hydrogen bonds.
2. Entropy Increase: When a molecule dissolves in water, it experiences an increase in entropy. In a vacuum, the molecule is localized and restricted in its movement. But in a solution, molecules move more freely due to thermal energy and the random motion of the surrounding water molecules. This increased molecular motion leads to greater disorder and higher entropy. Therefore, the entropic contribution favors the disruption of strong intramolecular hydrogen bonds.
3. Competitive Hydrogen Bonding: Water molecules are capable of forming hydrogen bonds not only with the solute molecule but also with other water molecules. This introduces competition between the solute-water hydrogen bonds and water-water hydrogen bonds. Water molecules tend to form stronger and more numerous hydrogen bonds with themselves than with other molecules. As a result, the hydrogen bonding between water molecules can further weaken the hydrogen bonds within the solute molecule.
By considering the solvation effect, increase in entropy, and competition with water-water hydrogen bonding, you have covered the three main reasons why the hydrogen bonds of a certain molecule may be weaker in solution with water as a solvent compared to a vacuum.