Zusammenfassung der Ressource
6.3 Intermolecular Forces
- Covalent bonding is strong and holds the atoms in a amolecule together. INTERMOLECULAR
forces are weak interactions between dipoles of different molecules. Intermolecular forces fall
into three main categories...
- ... Induced dipole-dipole interactions / London Forces
- 1-10 kJmol-1
- ... Permanent dipole-dipole interactions
- 3-25 kJmol-1
- ... Hydrogen Bonding
- 10-40kJmol-1
- Intermolecular forces are responsible for physical
properties such as melting/boiling points, where as
covalent bonds determine the identity and chemical
reactions of molecules
- Induced dipole-dipole interactions (London Forces)
- Weak intermolecular forces
- exist between ALL molecules
- They act between induced dipoles in different molecules
- What is an induced dipole?
- Electrons are mobile and so their movement produces a changing dipole in a molecule
- At any instant, an instantaneous dipole will exist, but its position is constantly shifting
- The instantaneous dipole INDUCES a dipole on a neighbouring molecule
- The induced dipole induces further dipoles on neighbourin gmolecules, which then attract one another
- Induced dipole-dipoles are temporary
- The strength of induced dipole-dipole interactions
- The more electrons in each molecule, the larger the instantaneous and induced dipole, this
means the induced dipole-dipole interactions are greater and the attractive forced between the
molecules are stronger.
- Larger numbers of electrons means larger induced dipoles, so, more energy is needed to overcome the intermolecular forces, increasing the BOILING POINT
- Permanent Dipole-Dipole Interactions
- act between the permanent dipole in different polar molecules
- Comparing Hydrogen Chloride and Fluorine
- Molecule
- F - F
- none
- yes
- no
- 9 x 2 = 18
- -220
- H-Cl
- H s+ -- Cl s-
- yes
- yes
- 1+17 = 18
- -85
- - Fluorine molecules are non-polar and only have London Forces between molecules
- - Hydrogen chloride molecules are polar and have London forces AND permanent dipole-dipole interactions between molecules
- - Extra energy is needed to break the additional permanent dipole-dipole interactions between hydrogen chloride molecules
- - Therefore the boiling point of Hydrogen Chloride is higher than Fluorine
- Dipole
- London
Forces
- Permanent
Dipole-dipole
interactions
- Number
of
Electrons
- Boiling point/
degrees C
- Simple Molecular Substances
- A simple molecular substance is made up of simple molecules - small
units containing a definite number of atoms with a definite
molecular formula, such as neon, Ne, hydrogen, H2, water H2O and
carbon dioxide CO2
- In the solid state = form regular structure called a simple molecular lattice
- molecules held in place by weak intermolecular forces
- atoms within each
molecule are bonded
together strongly by
covalent bonds
- Properties of simple molecular substances
- Low melting and boiling point
- At room temperature, simple molecular substances may exist as
solids, liquids or gases. All SMS can be solidified into simple
molecular lattices be reducing the temperature
- In a simple molecular
lattice, the weak
intermolecular forces can
be broken even by the
energy present at low
temperatures
- SMS have LOW melting and boiling points
- When a SM lattice is broken apart during melting
- only the weak intermolecular forces break
- the covalent bonds are strong and DO NOT break
- Solubility
- Covalent substances with simple molecular structures fall into two categories, polar and non-polar
- Solubility of non-polar simple molecular substances
- When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent
- The interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves
- Therefore, non-polar SMS tend to be soluble in non-polar solvents
- When a SMS is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules
- The intermolecular bonding within the polar solvent is too strong to be broken
- Therefore SMS tend to be insoluble in polar solvents
- Solubility of polar SMS
- Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other
- Process is similar to dissolving an ionic compound
- Solubility depends on the strength of the dipole and can be hard to predict. Some compounds such as
ethanol, C2H5OH, contain both polar (the O-H) and non-polar (the C chain) parts in their structure and can
dissolve in both polar and non-polar solvents
- Electrical Conductivity
- No mobile charged particles in SMS
- With no charged particles that can
move, there is nothing to complete
and electrical circuit
- Therefore SMS are non-conductors of electricity