how does H-bonding affect freezing point depression? I did a lab and calculated my exp molecular weight for benzoic acid and i got a value that is double the known value. My prof said it is due to H-bonding...i don't understand what effect it has on the MW...
Hydrogen bonding refers to a specific type of intermolecular force that occurs when a hydrogen atom bonded to an electronegative atom (such as oxygen, nitrogen, or fluorine) interacts with another electronegative atom. In the case of benzoic acid, the carboxylic acid group (-COOH) can form hydrogen bonds.
Freezing point depression is a phenomenon that occurs when a solute is dissolved in a solvent, causing the freezing point of the solvent to decrease. This is due to the disruption of the solvent's crystal lattice structure by the solute particles.
In the case of benzoic acid, the presence of hydrogen bonding can significantly affect the freezing point depression. When benzoic acid molecules dissolve in a solvent, such as water, the hydrogen bonding between the carboxylic acid groups in benzoic acid and the water molecules can disrupt the formation of ice crystals.
The formation of hydrogen bonds between benzoic acid and water molecules leads to a higher effective molecular weight for benzoic acid. This is because the hydrogen bonds contribute to the overall mass of the benzoic acid-water system. As a result, when calculating the experimental molecular weight of benzoic acid in your lab, the value obtained may be higher than the known value if hydrogen bonding is not taken into account.
To determine the molecular weight of benzoic acid accurately, it is important to consider the effect of hydrogen bonding. You can account for this by taking the hydrogen bonding into consideration during the calculations or by adjusting the known molecular weight based on the extent of hydrogen bonding in the system.
In summary, the presence of hydrogen bonding between benzoic acid and solvent molecules, such as water, can affect the freezing point depression and lead to an increased apparent molecular weight when not appropriately accounted for in experimental calculations.