Why does the boiling point of a two-component mixture rise slowly throughout a simple distillation when the boiling point differences are not that large?
Thanks from Sheryl
The boiling point of a two-component mixture rises slowly throughout a simple distillation even when the boiling point differences are not that large due to a phenomenon known as boiling point elevation or boiling point rise.
Boiling point elevation occurs when a non-volatile solute is dissolved in a volatile solvent, such as water and ethanol mixture. In a simple distillation process, the liquid mixture is heated to vaporize the more volatile component (in this case, ethanol) and separate it from the less volatile component (water).
The reason for the slow increase in boiling point during distillation is that as the temperature increases, the vapor pressure of the liquid mixture also increases. Remember that boiling occurs when the vapor pressure of a liquid equals the atmospheric pressure. However, when a non-volatile solute is present, it interferes with the vaporization process.
The presence of the non-volatile solute alters the vapor pressure of the mixture, effectively increasing the boiling point. This change in vapor pressure occurs due to the solute-solvent interactions, which reduce the escape of solvent molecules from the liquid phase into the gas phase (vapor).
As a result, more heat energy is required to reach the higher vapor pressure needed to boil the mixture. This accounts for the slow rise in boiling point experienced during a distillation process, even when the boiling point differences between the components are not significant.
To determine the exact boiling point of a two-component mixture, you would normally use the phase diagram or boiling point composition curve specific to the particular mixture. These thermodynamic diagrams provide information about boiling points at different compositions and allow for more accurate predictions.