Week 10: Chapter 11 intermolecular forces: liquids and solids Flashcards
Intramolecular forces (covalent) influence molecular shapes and bond energies.
Intermolecular forces influence phase (state) properties:
generally influence physical properties rather than chemical properties.
exist between molecules but also between atoms and ions.
are generally much weaker that bonding forces within compounds.
rely on attraction between positively and negatively charged regions between molecules.
Intramolecular forces
Strengths of intermolecular forces vary depending on molecules and atoms in the molecules.
They are all much weaker than intramolecular forces – covalent, ionic or metallic bonds.
They govern materials physical states to maintain their equilibrium under a certain condition
Intermolecular forces - Dispersion forces
A temporary attractive force can be generated due to uneven distribution of electrons producing dipole induced dipole attraction.
As all molecules can induce dipole attractions, dispersion force exist in all molecules.
Dispersion forces and their magnitude
Magnitude of dispersion forces depends on how easily the electron cloud can be distorted – Polarisability.
Larger electron clouds distort more easily.
Boiling points increase with the total number of electrons due to higher dispersion forces.
Number of atoms within a molecule is also important in determining the strength of dispersion forces.
Permanent dipoles
Permanent dipoles appear when two atoms in a molecule have significantly different electronegativity. One atom attracts electrons more than another, becoming more negative whilst the other atom becomes more positive. A molecule with a permanent dipole moment is called a polar molecule.
Dipole-dipole interaction
Dipole-dipole interaction is a type of intermolecular force. Dipolar forces result from attraction between the positive region of one polar molecule and the negative region in another. Dipolar forces are stronger than dispersion forces.
Hydrogen bonds (Has video)
Similar to the dipole-dipole interaction, a hydrogen bond is an electrostatic attraction between a covalently bound hydrogen atom to a highly electronegative atom. As nitrogen (N), Oxygen (O) and Fluorine (F) are so electronegative, an intermolecular bond between hydrogen and one of these elements (N, O, F) in polar is the hydrogen bond.
3 types of intermolecular forces
Dispersion forces, Force strength = ~ 2 kJ·mol-1
Dipole-dipole interactions, Force strength = up to 20 kJ·mol-1
Hydrogen bonding, Force strength = up to 50 kJ·mol-1
There are four important properties in liquids that result from intermolecular forces. (Check slide for detail)
Viscosity
Surface tension
Capillary action
Vapour pressure
Phase and energy changes
The amount of heat required to affect a phase transition is dependent on the amount of the substance.
Vapour pressure has video if needed
Solids to liquids (e.g. ice melting)
Molar enthalpy of fusion (∆fusH) is the heat required to melt 1 mole of a substance at its normal melting point.
Liquids to gases (e.g. water to steam)
Molar enthalpy of vaporisation (∆vapH) is the heat required to vaporise 1 mole of a substance at its normal boiling point.
Solids to gases (e.g. ice to steam)
Molar enthalpy of sublimation (∆subH) is the heat required to vaporise 1 mole of a solid substance.
Phase changes has video
Phase changes of water at normal condition (1 atm)
The freezing and boiling point of water is at 0 °C and 100 °C respectively. Most of us can intuitively state this. Using this intuitive concept plus sublimation/deposition covered previously, phase changes can be represented as a simple 1-D diagram.
2 types of solids
Cry_______
Amo________
has video
Crystalline solid
Atoms or molecules are ordered in well-defined arrangements → infinite array of unit cells
Have flat surfaces or faces and defined angles between the faces
Have highly regular shape
E.g. sodium chloride, diamond, graphite, quartz, silicon
Amorphous solids
When a pure liquid or a melt is cooled slowly, it often solidified as a crystalline solid.
When solids form rapidly, their atoms, ions or molecules may become locked into positions other than those of a regular crystalline structure
These materials are said to be amorphous, meaning “without form”
E.g. Glass is an entire family of amorphous solids based on silica, SiO2.
