Intermolecular Forces, Phase Changes, and Vapor Pressure: Study Notes for General Chemistry

Intermolecular Forces and Physical Properties

Types of Intermolecular Forces (IMFs)

Intermolecular forces are the attractive forces between molecules, influencing physical properties such as boiling point, melting point, and solubility.

• London Dispersion Forces (LDF): Present in all molecules, especially significant in nonpolar molecules. Caused by temporary dipoles due to electron movement.

• Dipole-Dipole Interactions: Occur between polar molecules due to permanent dipoles.

• Hydrogen Bonding: A strong type of dipole-dipole interaction occurring when H is bonded to N, O, or F.

• Ion-Dipole Forces: Occur between ions and polar molecules (important in solutions).

Table: Properties of Selected Compounds

The following table summarizes the polarity, hydrogen bonding ability, and main intermolecular forces for several compounds:

Formula	Polar, Nonpolar or Ion?	Can form hydrogen bonds?	Type of IM force present
NH3	Polar	Yes	Hydrogen bonding, dipole-dipole, LDF
HBr	Polar	No	Dipole-dipole, LDF
Cl2	Nonpolar	No	LDF
I2	Nonpolar	No	LDF
NO2	Polar	No	Dipole-dipole, LDF
CH4	Nonpolar	No	LDF
CCl4	Nonpolar	No	LDF
F2	Nonpolar	No	LDF
CH3OH	Polar	Yes	Hydrogen bonding, dipole-dipole, LDF
CH3CH2OH	Polar	Yes	Hydrogen bonding, dipole-dipole, LDF

Additional info: Table entries inferred based on chemical knowledge.

Boiling Point Comparisons

The boiling point of a substance is influenced by the strength of its intermolecular forces. Stronger IMFs result in higher boiling points.

• CO2 vs. CCl4: Both are nonpolar, but CCl4 has a higher molar mass and stronger LDF, so it has a higher boiling point.

• H2O vs. H2S: H2O forms hydrogen bonds, which are much stronger than the dipole-dipole and LDF in H2S, so H2O has a higher boiling point.

• CH3OH vs. CH3CH2OH: Both can hydrogen bond, but CH3CH2OH has a higher molar mass and more surface area, leading to stronger LDF and a higher boiling point.

Vapor Pressure and Clausius-Clapeyron Equation

Vapor Pressure Calculations

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. It increases with temperature.

• Clausius-Clapeyron Equation:

$\ln\left(\frac{P_2}{P_1}\right) = -\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_2} - \frac{1}{T_1}\right)$

• $P_1$, $P_2$ = vapor pressures at temperatures $T_1$, $T_2$ (in Kelvin)

• $\Delta H_{vap}$ = enthalpy of vaporization (J/mol)

• $R$ = gas constant ($8.314$ J/mol·K)

Example: Given the vapor pressure of ethyl ether at 18.0°C (401 torr) and $\Delta H_{vap} = 26.0$ kJ/mol, calculate the vapor pressure at 32.0°C.

Boiling Point and Vapor Pressure

• The normal boiling point is the temperature at which the vapor pressure equals 1 atm (760 torr).

• To find vapor pressure at a different temperature, use the Clausius-Clapeyron equation.

Phase Changes and Heating Curves

Phase Change Calculations

When a substance changes phase (solid, liquid, gas), energy is absorbed or released. The total energy required depends on the specific heat capacities and enthalpy of phase changes.

• Specific Heat ($c$): The amount of heat required to raise the temperature of 1 gram of a substance by 1 K.

• Enthalpy of Fusion ($\Delta H_{fus}$): Energy required to melt 1 mole of a solid at its melting point.

• Enthalpy of Vaporization ($\Delta H_{vap}$): Energy required to vaporize 1 mole of a liquid at its boiling point.

Example: To calculate the total energy required to heat a substance from below its melting point to above its boiling point, sum the energy for each step (heating solid, melting, heating liquid, vaporizing, heating gas):

• q1 = $m c_{solid} \Delta T$ (solid phase)

• q2 = $n \Delta H_{fus}$ (melting)

• q3 = $m c_{liquid} \Delta T$ (liquid phase)

• q4 = $n \Delta H_{vap}$ (vaporization)

• q5 = $m c_{gas} \Delta T$ (gas phase)

Additional info: The sign of $\Delta H$ is positive for endothermic (energy absorbed) and negative for exothermic (energy released) processes.

Phase Diagrams

Understanding Phase Diagrams

A phase diagram shows the state of a substance (solid, liquid, gas) at various temperatures and pressures.

• Regions: Each region represents a different phase (A: solid, B: liquid, C: gas).

• Triple Point: The unique set of conditions where all three phases coexist.

• Critical Point: The end point of the liquid-gas boundary; above this, the substance is a supercritical fluid.

Example: At 100 atm and 50°C, use the diagram to determine the phase. If a substance is moved from -50°C and 6 atm to 20 atm and 0°C, track the path on the diagram to identify phase changes.

Experimental Determination of Enthalpy of Vaporization

Using Vapor Pressure Data

The enthalpy of vaporization can be determined from vapor pressure data at different temperatures using the Clausius-Clapeyron equation. By plotting $\ln P$ vs. $1/T$, the slope equals $-\Delta H_{vap}/R$.

Temp K	Pressure (kPa)
300.2	6.586
313.4	14.31
324.2	25.57
348.2	76.51
354.8	100.46

Procedure:

• Convert temperatures to $1/T$ (in K-1).

• Take the natural logarithm of each pressure value.

• Plot $\ln P$ vs. $1/T$ and determine the slope.

• Calculate $\Delta H_{vap}$ from the slope: $\text{slope} = -\Delta H_{vap}/R$.

Additional info: This method is commonly used in laboratory settings to experimentally determine enthalpy of vaporization.