2.2.9 The properties of covalent substances

Given comparable molar masses, the relative strengths of intermolecular forces, are

$LDF<DD<HB$

Physical Properties of covalent structures

• Molecular substance's properties are mostly determined by their inter-molecular forces

• Covalent networks, with lattices, are dictated by their lattice features

Volatility

• Covalent network structures are solid at room temperature and pressure

  • Vaporizing takes a lot of energy because of strong covalent bonds holding structures togehter

    • $\therefore$ non-volatile and high melting and boiling points

• $\because$molecular substances' intermolecular forces need to be overcome to be vaporized ( converted to gas ), its easy and thus most molecular substances are volatile

  • However volatility varies depending on size and intermolecular forces, for small molecules like $N_2,CO_2,H_2O,$and hydrocarbons, they are gases or liquids at room temperatures.

  • Note larger molecules have lower voloatility and higher melting and boiling points --> Stronger LDFs

    • buckminsterfullerene ($C_{60}$) sublimes ( solid --> gas ) , at $500-600\degree C$

    • Component of candle wax, tetracosane ( $C_{24}H_{50}$), boils at $390\degree C$

Electrical conductivity

• to conduct electricity

  • Must contain a free-to-move charged particles

• Covalent structures are not generally conductive $\because$the electrons are locked up in localized bonds and don't contain ions

• OFC there are exceptions

  • graphite --> due to presence of delocalized electrons

  • Silicon --> is a semiconductor

  • Solar panels consists of photovoltaic cells that are generally made up of semi conductive silicon.

Solubility

• To dissolve, there must be a force of attraction between the solute and solvent

  • Network Structures mostly insoluble due to the such strong covalent bonds holding the atoms together

  • Molecular structures will dissolve if and only if the association/intermolecular forces between solute and solvent outweigh the intermolecular forces between the solute molecules

Like dissolves like, or:

• Non-polar solutes are likely to dissolve in non-polar solvents

• Polar solutes are likely to dissolve in polar solvents

• Dissolution is unlikely if solute is polar and solvent is non-polar.

TOK

• Although Like dissolves like is a good rule of thumb for most cases, it has no explanatory power. What are the advantages and disadvantages of having rules ? What are the alternatives? Do such rules have an "expiry date"? What rule of thumb do you use in other areas of knowledge.

• Iodine, $I_2$, dissolves easily in non-polar solvents like hexane

  • Insolube in water --> Cannot associate with water molecules

• Substances that can form hydrogen bonds are easier to dissolve in water

  • Can form hydrogen bonds with water

    • Ethanol & Water

      • THey are miscible --> Can form solution when mixed with any proportion

  • 

• Due to hydrocarbons being non-polar $\therefore$ do not form hydrogen bonds with water, thus an increasing carbon chain length means the solubility decreases as seen with the primary alcohols above.

• Fats and oils are non-polar and $\therefore$ do not dissolve in water ( hydrophobic --> water hating

examples:

• Vinegar ( ethanoic acid and polar ) + oil (non-polar ) = salad dressing

  • immiscible as they do not mix due to their differing polarities

• Soaps and detergent molecules often have a hydrophobic tail and a hydrophilic ( water-loving ) head.

  • Useful to remove stains, as they associate with both polar water, and non-polar fats and oils.

    • surface-active agents/surfactants

Global impacts of science

• Morpine and its derivatives known as opiates, are strong painkillers

  • High chance of misuse and addiction

• Effects on body depend on

1. abillity to cross blood-brain barrier

2. abillity to bind to the opiod receptors in brain ( for regulating pain, reward, and addictive behaviors )

• The blood-brain barrier is made up of cell membranes that coat blood vessels in brain that prevents polar molecules from entering brain

  • lilpophilic --> permable [(of a material or membrane) allowing liquids or gases to pass throughit] to lipid molecules

• Morpine is made up of one amino and two hydroxyl groups

  • Makes it polar enough to be soluble in water

  • Reduces its solubility in lipids to prevent ability to reach opioid receptors

• Diamorphine ( heroin ), replaces hydroxyl groups with ester groups --> reducing Polarity of molecule

  • Solube in lipids and crosses blood-brain barrier easily

  • Makes it 5x more potent analgesic ( relieving of pain ) than morpine

  • Side effects:

    • Tolerance

    • addiction

    • Central nervous system depression ( slowing of NS )