CHMA10H3 Lecture Notes - Lecture 13: Vapor Pressure, Intermolecular Force, EnthalpyPremium
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CHMA10H3 - Introductory to Chemistry I: Structure and Bonding – Lecture 13: Properties
The Remarkable Properties of Water
Enthalpy Change of Vaporization:
•Vaporization (evaporation) is the process in which a substance in the liquid state
changes to the gaseous state.
•Energy needs to be supplied for such process in order to break intermolecular forces.
•The intramolecular forces (i.e. H-O bonds) are not broken and both liquid and vapour
are composed of H2O molecules.
•Energy changes under constant pressure conditions are called enthalpy changes.
•The symbol for enthalpy is H, and for enthalpy change is ΔH.
•The process opposite to vaporization is called condensation.
•During condensation, energy is removed from the system.
•The amount of energy required to evaporate 1 mol of a substance is called the molar
enthalpy change of vaporization
•One mole of water requires much more energy than the same amount of other common
substances with similar size molecules.
Equilibrium Vapour Pressure:
•During vaporization of water in a closed flask, as more and more molecules occupy the
vapour space, the frequency of those returning to the liquid becomes equal to the
frequency of molecules passing into the vapour. This is called a dynamic equilibrium
•The dynamic equilibrium of water is represented by a following formula: H2O(l) = H2)(g)
•The equilibrium vapour pressure of water is lower than other substances whose
molecules are similar size.
•The higher the equilibrium vapour pressure at a given temperature, the more volatile the
•The boiling point is the temperature at which the vapour pressure of the liquid is the
same as the pressure of the atmosphere acting down on the liquid surface.
•If the atmospheric pressure is equal to 1 atm, this temperature is called the normal
•The normal boiling point of water is much higher than other liquids with similar-sized
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