Why the vapor pressure of water and ethanol is 760 mm of Hg at 100 degree Celsius and 78.5 degree Celsius respectively ?
Ethanol is easier to boil than water. The ethanol molecules are less attracted to one another.
The vapor pressure of a liquid is the pressure at which its molecules escape from the liquid phase and enter the gas phase. It is dependent on the temperature and the intermolecular forces present in the liquid.
At a given temperature, the liquid molecules that have enough kinetic energy will be able to overcome the intermolecular forces and escape into the gas phase. As the temperature increases, more molecules gain enough energy to escape, resulting in a higher vapor pressure.
Water and ethanol have different intermolecular forces, leading to different vapor pressures at the same temperature. Water molecules are held together by hydrogen bonding, which is a strong intermolecular force. This requires more energy for the water molecules to escape, thus resulting in a lower vapor pressure compared to ethanol.
Ethanol, on the other hand, has weaker intermolecular forces due to the presence of hydrogen bonding but not as extensively as in water. Therefore, ethanol molecules require less energy to overcome the intermolecular forces and escape into the gas phase, resulting in a higher vapor pressure compared to water.
At the boiling point, the vapor pressure of a liquid becomes equal to the atmospheric pressure, which is typically taken as 760 mmHg. This is because the boiling point is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure, allowing the liquid to boil.
In the case of water, at 100 degrees Celsius, the vapor pressure becomes equal to 760 mmHg, causing it to boil. Likewise, ethanol reaches its boiling point at 78.5 degrees Celsius when its vapor pressure equals 760 mmHg.
In summary, the difference in intermolecular forces between water and ethanol leads to different vapor pressures at the same temperature, with water having a lower vapor pressure at 100 degrees Celsius (760 mmHg) and ethanol having a higher vapor pressure at 78.5 degrees Celsius (760 mmHg).