Covalent Bonding Study Notes

PRINCIPLES OF CHEMISTRY

COVALENT BONDING

EXTENSION WORK

  • Melting and Boiling Points:
      - Not always increase with molecular mass ($M$).
      - Rule applicable only to very similar substances such as:     - Halogens     - Alkanes (explained in Chapter 23)
      - Counterexamples:
        - Water:
          - $M = 18$,       - Boiling point = $100^ ext{°}C$
        - Ethane:
          - $M = 30$,       - Boiling point = $-89^ ext{°}C$
        - Ammonia ($NH_3$):
          - $M = 17$,       - Boiling point = $-33^ ext{°}C$
        - Phosphine ($PH_3$):
          - $M = 34$,       - Boiling point = $-88^ ext{°}C$

KEY POINT

  • Definition of Covalent Molecular Compounds:
      - All covalent compounds with a simple molecular structure.

  • Intermolecular Forces and Energy:
      - Breaking intermolecular forces when melting or boiling substances.
      - Boiling points increase down groups: more energy needed to break stronger intermolecular forces as $M$ increases.
      - Example:
        - Series of boiling points for hydrocarbons: $CH_4$, $C_2H_6$, $C_3H_8$ increases with relative molecular mass.
      - Further discussion in Chapter 23.

SOME OTHER PHYSICAL PROPERTIES OF COVALENT COMPOUNDS

  • Electrical Conductivity:
      - Covalent molecular compounds do not conduct electricity due to absence of overall electrical charge.
      - No ions present; electrons held tightly in covalent bonds cannot move between molecules.

  • Solubility in Water:
      - Generally insoluble; exceptions include:
        - Ethanol ($C_2H_5OH$)
        - $NH_3$ and $HCl$ that react with water during dissolution.

  • Solubility in Organic Solvents:
      - Covalent molecular substances are often soluble in organic solvents.

GIANT COVALENT STRUCTURES
DIAMOND
KEY POINT

  • Tetrahedral Arrangement:
      - Defined as a triangular-based pyramid.
      - Central atom with four corners occupied by attached atoms.
      - Visual representation in Figure 8.26, showing five atoms indicating tetrahedral arrangement.

  • Understanding of Diagram Representation:
      - Some carbon atoms appear to form only one or two bonds in the diagram.
      - Actual structure extends in three dimensions; every atom attaches to four other atoms.
      - Each line represents a covalent bond.

  • Properties of Diamond:
      - Pure carbon; each carbon atom forms four covalent bonds.
      - Strong bonding in tetrahedral arrangement.
      - Structure is continuous in three dimensions, not a defined molecule due to variable atom counts.

  • Melting and Boiling Points of Diamond:
      - Diamond has a high melting and boiling point due to strong carbon-carbon covalent bonds throughout the crystal.
      - Significant energy required to break these bonds.
      - Contrast with simple molecular structures like $CH_4$; melting/boiling requires less energy to overcome weaker intermolecular forces rather than strong covalent bonds.

PRINCIPLES OF CHEMISTRY
COVALENT BONDING

EXTENSION WORK

  • Melting and Boiling Points:
      - The relationship between molecular mass (MM) and the melting and boiling points of substances is not straightforward; it does not always increase with an increase in molecular mass.
      - This rule is applicable mainly to very similar substances such as:
        - Halogens, which typically exist as diatomic molecules, and
        - Alkanes, which are saturated hydrocarbons (fully bonded with hydrogen, explained in Chapter 23).
      - Counterexamples:
        - Water:
          - M=18M = 18,
          - Boiling point = 100ext°C100^ ext{°}C, which is unusually high for its molecular mass due to strong hydrogen bonding.
        - Ethane:
          - M=30M = 30,
          - Boiling point = −89ext°C-89^ ext{°}C, exhibiting much lower boiling point relative to its mass, showing weaker van der Waals forces in simple molecular compounds.
        - Ammonia (NH3NH_3):
          - M=17M = 17,
          - Boiling point = −33ext°C-33^ ext{°}C, slightly higher than ethane due to hydrogen bonding, but still much lower than water.
        - Phosphine (PH3PH_3):
          - M=34M = 34,
          - Boiling point = −88ext°C-88^ ext{°}C, demonstrating a trend similar to ethane, indicative of weak intermolecular forces at play.

KEY POINT

  • Definition of Covalent Molecular Compounds:
      - Covalent molecular compounds are characterized by molecules consisting of two or more nonmetal atoms bonded together through covalent bonds, creating a simple molecular structure with discrete units.

  • Intermolecular Forces and Energy:
      - The process of melting or boiling a substance involves breaking intermolecular forces rather than breaking covalent bonds. This is crucial because covalent bonds within the molecules remain intact during phase changes.
      - Boiling points generally increase down groups in the periodic table, as heavier molecules have a stronger van der Waals attraction, requiring more energy to overcome these forces.
      - Example of boiling points for hydrocarbons as relative molecular mass increases:
        - CH4CH_4 (Methane): −161ext°C-161^ ext{°}C,
        - C2H6C_2H_6 (Ethane): −89ext°C-89^ ext{°}C,
        - C3H8C_3H_8 (Propane): −42ext°C-42^ ext{°}C — highlighting the trend of increasing boiling points as molecular size increases.
      - Further discussion on this topic can be found in Chapter 23.

SOME OTHER PHYSICAL PROPERTIES OF COVALENT COMPOUNDS

  • Electrical Conductivity:
      - Covalent molecular compounds generally do not conduct electricity. This is due to the absence of overall electrical charge in their structure that is characterized by localized bonding electrons transferred in covalent bonds, as there are no free-moving electrons or ions, which are required for electrical conduction.

  • Solubility in Water:
      - Most covalent molecular compounds are insoluble in water due to their nonpolar characteristics; however, notable exceptions include:     - Ethanol (C2H5OHC_2H_5OH), which can form hydrogen bonds with water, providing solubility.
        - Ammonia (NH3NH_3) and hydrochloric acid (HClHCl) that chemically react with water during dissolution, thus displaying solubility.

  • Solubility in Organic Solvents:
      - Many covalent molecular substances show a tendency to be soluble in organic solvents (such as acetone and ether) due to similar intermolecular forces being present between the solvent molecules and solute molecules, aiding solubility through interactions.

GIANT COVALENT STRUCTURES

DIAMOND

KEY POINT

  • Tetrahedral Arrangement:
      - Diamond's structure is characterized by a tetrahedral arrangement, meaning each carbon atom bonds to four other carbon atoms arranged in a three-dimensional space, forming a complex network known as a giant covalent structure.
      - This arrangement can be visualized as a triangular-based pyramid, essential for the stability of the structure.

  • Understanding of Diagram Representation:
      - In graphical representations, some carbon atoms might appear to form only one or two bonds. However, the reality is every carbon atom is connected to four other atoms, maintaining a continuous three-dimensional lattice structure, which is much stronger than any discrete molecule. Each line in the diagram indicates the presence of a covalent bond between the atoms.

  • Properties of Diamond:
      - Composed solely of carbon, with each carbon atom forming four covalent bonds to other carbon atoms in a rigid tetrahedral shape.
      - This strong bonding arrangement accounts for the exceptional hardness and durability of diamond, making it a valuable material in both jewelry and industrial applications.

  • Melting and Boiling Points of Diamond:
      - Diamond possesses a very high melting and boiling point due to the presence of strong carbon-carbon covalent bonds that extend throughout the entire crystal structure. Breaking these strong bonds requires considerable energy, explaining its high thermal stability, contrasting with simple covalent molecules, such as CH4CH_4, where lower energy is required to overcome weaker intermolecular forces to transition from a solid to a liquid state, allowing them to melt or boil at relatively low temperatures.