Hia! My prelim is in a week (not looking forward to it!) Yeah Unit 2 is defo the hardest- there's so much to remember!
I've haven't done as much revision as I should have for it, but the SQA course and unit support notes are always useful (see
https://www.sqa.org.uk/files_ccc/AHCUSNChemistry.pdf ) and Scholar is amazing- I don't know if your school has it but it's really good.
Ok, so, to answer your questions:
Molecular orbitals:When atoms approach each other, their separate sets of atomic orbitals merge to form a single set of molecular orbitals. Some of the molecular orbitals, known as 'bonding molecular orbitals', occupy the region between two nuclei. The attraction of positive nuclei to negative electrons occupying bonding molecular orbitals is the basis of bonding between atoms. Each molecular orbital can accommodate a maximum of two electrons. Molecular orbitals are usually constructed by combining atomic orbitals or hybrid orbitals from each atom of the molecule.
Pi bonds and sigma bonds: A covalent bond is formed when two half-filled atomic orbitals overlap. If they overlap along the axis of the bond (end on) a covalent bond is known as a sigma (σ) bond. Pi (π) bonds arise where atoms make multiple bonds, for example the double bond in a molecule of oxygen O2 is made up of one sigma and one pi bond. The triple bond in a molecule of nitrogen N2 is made up of one sigma and two pi bonds. Pi bonds are formed when atomic orbitals lie perpendicular to the bond and overlap side on. End to end overlap is more efficient than side on overlap and therefore σ bonds are stronger than π bonds.
Sigma and pi bonds specifically in hydrocarbons:Let's look at an alkane, ethane for example. Each carbon has three 2p orbitals and one 2s orbital which mix to form four degenerate (equal energy) hybrid orbitals. These are known as sp3 hybrid orbitals and point towards the corners of a tetrahedron in order to minimise repulsion from each other. The four sp3 orbitals on each carbon atom overlap end to end with one sp3 orbital on the other carbon atom and the three hydrogen 1s orbitals. This forms 4 σ bonds. The bonding in ethane can be described as sp3 hybridisation and sigma bonds.
If we take a look at the bonding in the corresponding alkene ethene we can see that the 2s orbital and two of the three 2p orbitals mix on each carbon atom to form three sp2 hybrid orbitals. To minimise repulsion these orbitals form a trigonal planar arrangement. The carbon atoms use the three sp2 hybrid orbitals to form sigma bonds with two hydrogen atoms and with the other carbon atom. The unhybridised 2p orbitals left on the carbon can overlap side-on to form a pi bond.
Hope this helps!