Dipole-dipole forces, dispersion forces and hydrogen bonds are intermolecular forces (forces between molecules) whereas covalent and ionic bonds are intermolecular forces (forces between the atoms that make up the molecule).
INTERMOLECULAR FORCES
*Dispersion forces are the weakest intermolecular force.
- They are caused by dipoles which are momentarily created as a result of an uneven distribution of electrons (they aren't orbiting evenly) in a neighbouring molecule. Due to the creation of the dipole the electrons of that molecule are attracted to the nucleus of the neighboring molecule.
- This is the only intermolecular force that can occur for non-polar molecules.
- It is safe to assume for the HSC syllabus that dispersion forces occur in all molecules
*Dipole-dipole attractions are stronger than dispersion forces
- They occur between molecules which have permanent net dipoles (polar)
- E.g. Due to its shape, water is a polar molecule. It has made of two hydrogen atoms and one oxygen bound by covalent bonds (intramolecular force). The oxygen has greater electronegativity (ability to attract electrons) than the hydrogen atoms. Thus oxygen attracts the electrons towards it and gains a partial negative charge and the hydrogens become slightly positive. It is this which makes water a polar molecule.
- If an electron is shared equally between the atoms forming a covalent bond, then the bond is said to be non-polar.
- For the requirements of the HSC syllabus know that dipole-dipole attractions occur between all polar molecules such as water, alkanols and alkanoic acids (the latter have polar and non-polar sections)
*Hydrogen bonding is stronger than dipole-dipole attractions (1/10 of strength of covalent bond)
- Essentially this works in the same way dipole-dipole attracts do but it is only called hydrogen bonding when the attractions due to the molecules being polar occurs between a hydrogen atom and an oxygen, fluorine or chlorine atom.
- The hydrogen nucleus is extremely small and positively charged and fluorine, oxygen and nitrogen being very electronegative strongly attract the electron from the hydrogen atom, leaving a highly localised positive charge on the hydrogen atom and highly negative localised charge on the fluorine, oxygen or nitrogen atom. This means the electrostatic attraction between these molecules will be greater than for the polar molecules that do not have hydrogen covalently bonded to either fluorine, oxygen or nitrogen.
- Hydrogen bonds are constantly being broken down and reformed in liquid water
- For the purposes of the HSC you will require the knowledge of hydrogen bonds and their applications such as solubility, interaction with other molecules and melting & boiling points.
*Water has all three intermolecular forces.
-It has dipole-dipole attractions as well as hydrogen bonds as the dipole-dipole bonds can be distinguished from hydrogen bonds as they are present even at angles where no significant hydrogen bonds are expected.
For the purposes of the HSC syllabus note how the intermolecular forces operate in water, alkanols and alkanoic acids. The absence or presence of some or all of these intermolecular forces greatly affects differences in melting and boiling points in covalent substances as well as the solubility of substances.
INTRAMOLECULAR FORCES
*Covalent bonds is the sharing of electrons between atoms
- E.g. H2O
*Ionic bonds are those that bind atoms together which are oppositely charged ions.
-For the purpose of the HSC syllabus know that ionic bonding occurs between a metal and a non-metal.
-E.g. NaCl: Na+ cation (metallic) and Cl- anion (non-metallic)
This is the approximate detail required for the HSC Chemistry syllabus
SOLUBILITY
*Non-polar solutes dissolve in non-polar solvents
*Polar solutes will dissolve in polar solvents
-E.g. Water in ethanol
*Ionic solutes generally dissolve in polar solutes
- E.g. NaCl in water
Cheers,
Chezza. ^[*.:.*]^
