Water and ethanol have different boiling points primarily due to differences in their molecular structures and the types of intermolecular forces present in each substance.
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Hydrogen Bonding: Water (H₂O) has a stronger hydrogen bonding network than ethanol (C₂H₅OH). In water, each molecule can form up to four hydrogen bonds due to its two hydrogen atoms and two lone pairs of electrons on the oxygen atom. Ethanol can also form hydrogen bonds, but it has one less hydrogen bonding site compared to water because it has a larger hydrophobic hydrocarbon tail. This reduces the overall strength of hydrogen bonding in ethanol compared to water.
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Polarity: Both water and ethanol are polar molecules, but water's polarity is greater due to its bent shape and higher electronegativity of oxygen. This leads to stronger dipole-dipole interactions in water.
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Molecular Weight and Structure: Ethanol has a larger molecular structure and higher molecular weight than water, but this factor is less significant compared to the differences in hydrogen bonding. In fact, while larger molecules can have higher boiling points, the significant hydrogen bonding in water contributes to its higher boiling point.
As a result of these factors, water has a boiling point of 100°C at standard atmospheric pressure, while ethanol boils at around 78.37°C, making it clear that the strength and number of hydrogen bonds in water significantly influence its boiling point compared to ethanol.