CHEM1312H - IMFs and Properties of Liquids

vaporization

-passage of molecules from the surface of a liquid into the gaseous or vapor state

-endothermic process

-some molecules on the surface of the liquid have sufficient kinetic energy to overcome the intermolecular forces + escape to the vapor phase

vaporization occurs more readily with

-increased temperature

-increased surface area

-decreased strength of IMFs

heat of vaporization (enthalpy of vaporization)

the energy required to vaporize 1 mol of liquid at 1 atm (units are kJ/mol)

Condensation

-molecules in the vapor phase with higher kinetic energy can collide with slower molecules and condensation occurs

-condensation occurs at the same time as vaporization but at a slower rate in an open system container

-exothermic process

vaporization and condensation

during a phase change, the amount of heat absorbed by a substance depends on the number of moles (n) and the enthalpy of vaporization or enthalpy of fusion (S→L)

amount of heat absorbed by a substance formula

-q = n x ΔH_vap

-if the phase change is exothermic, use the same equation ad add a negative sign to the ΔH_vap or ΔH_fus

how do the dynamics of molecules on the surface change in a closed container?

-water molecules evaporate at the beginning and the rate of vaporization is greater than condensation

-the molecules have nowhere to escape and as a result, they collide with one another and return to the container (condensation)

-this process keeps going until the rate of evaporation = the rate of condensation (dynamic equilibrium)

vapor pressure

-the pressure exerted by a vapor in dynamic equilibrium on the surface of its liquid or solid in a closed container system

-you can think of it as a measure of the tendency of molecules to escape from the surface of the liquid to the vapor phase

the vapor pressure of a particular liquid depends on…

IMFs and temperature

vapor pressure and IMFs

-weak intermolecular forces result in volatile substances and high vapor pressure (inverse relationship)

-strong intermolecular forces result in non-volatile substances with low vapor pressure

vapor pressure and temperature

-the higher the temperature, the higher the vapor pressure, therefore a larger number of particles have sufficient kinetic energy to overcome IMFs

-the lower the temperature, the lower the vapor pressure so there is a fewer number of particles in the gas phase

vapor pressure, volume, and surface area

-the vapor pressure of a liquid does not depend on volume or surface area of the container

-a liquid in dynamic equilibrium with its vapor is a balanced system that tends to return to equilibrium if disturbed

-if the piston is moved upwards (volume increases, pressure decreases), more liquid vaporizes until equilibrium is restored

-if the piston is moved downwards (volume decreases, pressure increases), more liquid condenses until equilibrium is restored

vapor pressure and boiling point

the relationship between temperature and vapor pressure can be expressed graphically (exponential)

boiling point

the temperature at which the vapor pressure of a liquid is equal to the external or atmospheric pressure

normal boiling point

the temperature at which its vapor pressure is 760 torr or 1 atm

clausius-clapeyron equation

allows for the determination of enthalpy of vaporization by measuring the vapor pressure of liquid as a function of temperature

clausius-clapeyron equation values

-ln - natural log

-P - vapor pressure

-R - universal gas constant (8.314 J/molK)

-ΔH_vap - enthalpy of vaporization (J/mol)

-T - kelvin

-C - constant characteristic of a liquid

two-point form of clausius-clapeyron equation def

we can use the two point form of the equation to calculate a variable such as pressure, temp, or ΔH_vap given other info in the question

things to keep in mind when using clausius-clapeyron equation

-the inverse of ln is “e” which is the natural base or exponential

-it does not matter which temp/pressure values you assign as P/T as long as you are consistent when solving the problem

-reciprocals might come in handy

-ln(P₁/P₂) = lnP₁-lnP₂