CHM1046 Module 1

Molecular Comparison of States of Matter

• Physical States of Water:

  • Gas (Steam):

    • Temperature: $100^\text{o}\text{C}$

    • Density at $1\text{ atm}$: $5.90 \times 10^{-4}\text{ g/cm}^3$

    • Molar Volume: $30.5\text{ L}$

    • Molecular arrangement: Widely spaced molecules moving rapidly and independently in space.

  • Liquid (Water):

    • Temperature: $20^\text{o}\text{C}$

    • Density at $1\text{ atm}$: $0.998\text{ g/cm}^3$

    • Molar Volume: $18.0\text{ mL}$

    • Molecular arrangement: Closely spaced molecules that remain in contact but are free to slide past one another.

  • Solid (Ice):

    • Temperature: $0^\text{o}\text{C}$

    • Density at $1\text{ atm}$: $0.917\text{ g/cm}^3$

    • Molar Volume: $19.6\text{ mL}$

    • Molecular arrangement: Closely spaced molecules locked into a fixed three-dimensional framework.

• Fundamental Properties of the States of Matter:

  • Gas:

    • Density: Low

    • Shape: Indefinite (takes the shape of its container)

    • Volume: Indefinite (expands to fill its container)

    • Intermolecular Forces Strength (Relative to Thermal Energy): Weak

    • Compressibility: Highly compressible because molecules are separated by large distances.

  • Liquid:

    • Density: High

    • Shape: Indefinite (assumes the shape of its container)

    • Volume: Definite (resists volume changes)

    • Intermolecular Forces Strength (Relative to Thermal Energy): Moderate

    • Compressibility: Not easily compressible because molecules are already in close contact.

  • Solid:

    • Density: High

    • Shape: Definite (rigid shape)

    • Volume: Definite (resists volume changes)

    • Intermolecular Forces Strength (Relative to Thermal Energy): Strong

    • Compressibility: Not easily compressible due to tight particle packing.

• Structural Classification of Solids:

  • Crystalline Solids:

    • Definition: Solids whose constituent particles (atoms, ions, or molecules) are arranged in an orderly, repeating, three-dimensional geometric pattern exhibiting long-range order.

    • Examples: Sodium chloride (table salt) and diamond.

  • Amorphous Solids:

    • Definition: Solids whose constituent particles do not possess a regular repeating geometric pattern over a long range.

    • Examples: Plastics and glass.

Fundamental Nature and Types of Intermolecular Forces

• Origin of Intermolecular Forces:

  • Intermolecular forces arise from electrostatic interactions between full charges, partial charges, or temporary induced charges on neighboring molecules, atoms, or ions.

  • Intermolecular Forces vs. Intramolecular Bonding:

    • Intramolecular bonding (covalent, ionic, metallic bonds) holds atoms together within a molecule or compound and is governed directly by Coulomb's Law ($E = \frac{k q_1 q_2}{r}$).

    • Intermolecular forces occur between separate molecules and are generally significantly weaker than intramolecular chemical bonds because charges are typically smaller and distances ($r$) are larger.

• Dispersion Forces (London Dispersion Forces):

  • Definition & Mechanism:

    • Present in all atoms and molecules.

    • Caused by fluctuations in the electron distribution within atoms or nonpolar molecules, creating an instantaneous (temporary) dipole.

    • An instantaneous dipole on one atom induces a corresponding dipole on a neighboring atom, resulting in an attractive force.

*   **Effect of Molecular Size / Molar Mass**:
    *   Larger electron clouds are more polarizable (easier to distort), leading to stronger dispersion forces and higher boiling points.

*   **Effect of Molecular Shape / Surface Area**:
    *   Elongated molecules have a larger surface area of interaction, resulting in stronger dispersion forces than spherical molecules of equal molar mass.

*   Trend in Straight-Chain Alkanes

• Dipole-Dipole Forces:

  • Definition:

    • Attractive forces present exclusively between polar molecules possessing permanent dipole moments.

    • The positive end ($\text{δ}+$) of one polar molecule is electrostatically attracted to the negative end ($\text{δ}-$) of a neighboring polar molecule.

*   **Effect of Dipole Moment on Boiling Point** (for molecules of similar molar mass):

• Hydrogen Bonding:

  • Definition & Mechanism:

    • An exceptionally strong type of dipole-dipole force occurring when a hydrogen atom is directly bonded to a highly electronegative atom ($\text{O}$, $\text{N}$, or $\text{F}$).

    • The electronegative atom strongly pulls electron density away from hydrogen. Because hydrogen has no inner-shell shielding electrons, its nucleus becomes de-shielded, leaving an exposed proton.

    • This highly concentrated center of positive charge ($\text{δ}+$) strongly attracts electron pairs on electronegative atoms ($\text{O}$, $\text{N}$, or $\text{F}$) of neighboring molecules.

*   **Anomalous Boiling Points in Group 6A Hydrides vs. Group 4A Hydrides**

• Ion-Dipole Forces:

  • Definition:

    • Occurs when an ionic compound is mixed with a polar compound.

    • The electrostatic attraction between a full ionic charge (cation or anion) and the partial charge ($\text{δ}+$ or $\text{δ}-$) of a polar molecule.

    • Responsible for the hydration and dissolution of ionic solids in water: positive ions ($\text{Na}^+$) attract the oxygen end ($\text{δ}-$) of $\text{H}_2\text{O}$, while negative ions ($\text{Cl}^-$) attract the hydrogen end ($\text{δ}+$) of $\text{H}_2\text{O}$.

• Summary Ranking of Intermolecular Forces Strength:

  • Dispersion Force: Present in all molecules/atoms; weakest relative strength.

  • Dipole-Dipole Force: Present in polar molecules; moderate relative strength.

  • Hydrogen Bonding: Present in molecules containing $\text{H}$ bonded to $\text{F}$, $\text{O}$, or $\text{N}$; strong relative strength.

  • Ion-Dipole Force: Present in mixtures of ionic compounds and polar compounds; strongest relative intermolecular force.

Intermolecular Forces in Action: Liquid Properties

• Surface Tension:

  • Definition: The energy required to increase the surface area of a liquid by a given amount, or the quantitative measure of the elastic force at the surface of a liquid.

  • Molecular Mechanism:

    • Interior molecules experience intermolecular attractions in all directions, yielding no net force.

    • Surface molecules experience attractions only downward and sideways into the liquid, yielding a net inward force pulling surface molecules toward the interior.

    • To maximize interior interactions, liquids minimize their surface area, assuming spherical droplets.

  • Specific Value: Surface tension of water is $0.073\text{ J/m}^2$ at room temperature.

• Viscosity:

  • Definition: The resistance of a liquid to flow.

  • Units: $1\text{ poise} = 1\text{ P} = 1\text{ g}\text{ cm}^{-1}\text{s}^{-1}$. Commonly expressed in centipoise ($\text{cP}$).

  • Dependence on Temperature:

    • Viscosity decreases as temperature increases because higher thermal energy overcomes intermolecular forces.

    • Viscosity of Water at Various Temperatures:

      • $20^\text{o}\text{C}$: $1.002\text{ cP}$

      • $40^\text{o}\text{C}$: $0.653\text{ cP}$

      • $60^\text{o}\text{C}$: $0.467\text{ cP}$

      • $80^\text{o}\text{C}$: $0.355\text{ cP}$

      • $100^\text{o}\text{C}$: $0.282\text{ cP}$

  • Dependence on Molecular Structure / Molar Mass:

    • Viscosity increases with higher molar mass and longer molecular chains due to increased dispersion forces and molecular entanglement.

    • Viscosity of $n$-Hydrocarbons at $20^\text{o}\text{C}$:

      • $n\text{-Pentane}$ ($\text{C}_5\text{H}_{12}$): Molar mass = $72.15\text{ g/mol}$, Viscosity = $0.240\text{ cP}$

      • $n\text{-Hexane}$ ($\text{C}_6\text{H}_{14}$): Molar mass = $86.17\text{ g/mol}$, Viscosity = $0.326\text{ cP}$

      • $n\text{-Heptane}$ ($\text{C}_7\text{H}_{16}$): Molar mass = $100.2\text{ g/mol}$, Viscosity = $0.409\text{ cP}$

      • $n\text{-Octane}$ ($\text{C}_8\text{H}_{18}$): Molar mass = $114.2\text{ g/mol}$, Viscosity = $0.542\text{ cP}$

      • $n\text{-Nonane}$ ($\text{C}_9\text{H}_{20}$): Molar mass = $128.3\text{ g/mol}$, Viscosity = $0.711\text{ cP}$

• Capillary Action:

  • Definition: The ability of a liquid to flow against gravity up a narrow tube.

  • Interplay of Forces:

    • Cohesive Forces: Intermolecular attractions between liquid molecules themselves.

    • Adhesive Forces: Attractions between liquid molecules and the surface of the tube.

  • Meniscus Shapes in Glass Tubes:

    • Water: Forms a concave meniscus because adhesive forces between water and glass (silicate surfaces) are stronger than cohesive forces among water molecules.

    • Mercury: Forms a convex meniscus because cohesive metallic bonding forces among mercury atoms are stronger than adhesive forces between mercury and glass.

Vaporization, Vapor Pressure, and Thermodynamics

• Process of Vaporization (Evaporation):

  • Molecules in a liquid are in constant kinetic motion.

  • Thermal energy determines the kinetic energy distribution of molecules (described by Maxwell-Boltzmann distributions).

  • Only molecules at the liquid surface with kinetic energy exceeding the minimum threshold needed to overcome intermolecular attractions can escape into the gas phase.

  • Temperature Effect: Higher temperature shifts the kinetic energy distribution to higher values, increasing the fraction of surface molecules capable of evaporating.

• Volatility:

  • Volatile Liquids: Liquids that evaporate easily due to weak intermolecular forces (e.g., acetone, diethyl ether).

  • Nonvolatile Liquids: Liquids that do not evaporate easily due to strong intermolecular forces (e.g., motor oil, glycerol).

• Heat of Vaporization ($ΔH_\text{vap}$):

  • Definition: The enthalpy change required to vaporize one mole of liquid to gas at constant temperature and pressure.

  • Vaporization is endothermic ($ΔH_\text{vap} > 0$); condensation is exothermic ($ΔH_\text{cond} = -ΔH_\text{vap}$).

  • Comparison of Heats of Vaporization:

    • Water ($\text{H}_2\text{O}$): Normal boiling point = $100^\text{o}\text{C}$, $ΔH_\text{vap}$ at boiling point = $40.7\text{ kJ/mol}$, $ΔH_\text{vap}$ at $25^\text{o}\text{C}$ = $44.0\text{ kJ/mol}$

    • Rubbing alcohol / Isopropyl alcohol ($\text{C}_3\text{H}_8\text{O}$): Normal boiling point = $82.3^\text{o}\text{C}$, $ΔH_\text{vap}$ at boiling point = $39.9\text{ kJ/mol}$, $ΔH_\text{vap}$ at $25^\text{o}\text{C}$ = $45.4\text{ kJ/mol}$

    • Acetone ($\text{C}_3\text{H}_6\text{O}$): Normal boiling point = $56.1^\text{o}\text{C}$, $ΔH_\text{vap}$ at boiling point = $29.1\text{ kJ/mol}$, $ΔH_\text{vap}$ at $25^\text{o}\text{C}$ = $31.0\text{ kJ/mol}$

    • Diethyl ether ($\text{C}_4\text{H}_{10}\text{O}$): Normal boiling point = $34.6^\text{o}\text{C}$, $ΔH_\text{vap}$ at boiling point = $26.5\text{ kJ/mol}$, $ΔH_\text{vap}$ at $25^\text{o}\text{C}$ = $27.1\text{ kJ/mol}$

• Dynamic Equilibrium & Vapor Pressure:

  • In a closed container, vaporization occurs until the concentration of gas-phase molecules reaches a level where the rate of condensation equals the rate of evaporation.

  • Dynamic Equilibrium Equation: $\text{Rate of Evaporation} = \text{Rate of Condensation}$

  • Vapor Pressure: The pressure exerted by a gas in dynamic equilibrium with its liquid phase.

  • Disturbance of Dynamic Equilibrium (Le Chatelier's Principle):

    • Volume Increase: Pressure drops, rate of vaporization exceeds condensation rate until equilibrium pressure is restored.

    • Volume Decrease: Pressure rises, rate of condensation exceeds vaporization rate until equilibrium pressure is restored.

• Boiling Point and Vapor Pressure Curves:

  • Boiling Point: Temperature at which the liquid's vapor pressure equals the external pressure.

  • Normal Boiling Point: Boiling point under an external pressure of $1\text{ atm}$ ($760\text{ torr}$ or $760\text{ mmHg}$).

    • Diethyl ether normal boiling point: $34.6^\text{o}\text{C}$

    • Ethyl alcohol (ethanol) normal boiling point: $78.3^\text{o}\text{C}$

    • Water normal boiling point: $100^\text{o}\text{C}$

• Clausius-Clapeyron Relationship:

  • The non-linear temperature dependence of vapor pressure is expressed linearly by the Clausius-Clapeyron equation:         $α(P_\text{vap}) = -\frac{ΔH_\text{vap}}{R}\times \frac{1}{T} + α(β)$         where $R = 8.314\text{ J}\text{ mol}^{-1}\text{K}^{-1}$, $T$ is absolute temperature in Kelvin, and $β$ is a chemical constant.

  • Plot Analysis:

    • Plotting $α(P_\text{vap})$ on the y-axis vs. $\frac{1}{T}$ on the x-axis yields a straight line with slope $m = -\frac{ΔH_\text{vap}}{R}$.

    • Calculated Example from Experimental Data:

      • Slope = $-3478\text{ K}$

      • $ΔH_\text{vap} = -(\text{slope}) \times R = -(-3478\text{ K}) \times 8.314\text{ J}\text{ mol}^{-1}\text{K}^{-1} = 28.92 \times 10^3\text{ J/mol} = 28.92\text{ kJ/mol}$

*   **Two-Point Form of the Clausius-Clapeyron Equation**: