Chemistry - Intermolecular forces

VSEPR

Valence shell electron pair repulsion

Why do electron pairs determine the shape of a covalent molecule

-Valence electron pairs in the shell of an atom will repel each other and position themselves as far as possible

-Shape is therefore determined by the number of shared/lone electron pairs

Interaction between lone pair and bonded pair

-Lone pairs repel 3x bonded pairs more than each other

Shapes

-Linear

-Trigonal planar

-Tetrahedral

-Pyramidal

-Bent/V-shape

Linear

Symmetrical molecules, all diatomic molecules

Trigonal planar

-3 bonded pairs with no free pairs

-120^o Bond angle

Tetrahedral

-4 bonded pairs with no free pairs

-All 4 bonds evenly spread in 3 - dimensions

-109.5^o bond angle

Pyramidal

-3 bonded pairs with 1 free pair

-Free pair ‘pushes’ bonded pairs away

Bent/V-shaped

2 bonded pairs and 1-2 free pairs

Polar and non-polar

refers to weak positive or negative charge that CAN exist for a covalent molecule only

Why do polar molecules exist

-When electrons are not shared equally between elements in the covalent molecules

-Cause by differences in electronegativity

Signs for non-polar molecules

-Made up of one element (e.g. N₂, O₂, Cl₂)

-Molecule that only contains carbons and hydrogens

-If molecule has symmetry (e.g. CF₄, CO₂, BH₃)

-If electronegativity difference is less than 0.5

Signs for polar molecules

-If hydrogen is bonded to NOF (N, O, F) (e.g. H20, NH3, HF, CH3OH, CH3COOH)

-If bond is asymmetrical and bond is polar

-If electronegativity difference is above 0.5

Types of IMF

-Hydrogen bonds (Only in polar)

-Dipole-Dipole bonds (Only in polar)

-Dispersion forces (In all covalent molecules (polar and non-polar)

Dipole-Dipole

-Form between polar molecules due to charged polar ends

-The + and - ends of molecules attract each other and bond

-Bonds vary in strength based on polarity of molecule

-More polar/higher EN diff → higher MP/BP

Hydrogen bonds

-Particular type of dipole-dipole bond where hydrogen is attracted to N,O,F

-Because these connections have very high electronegative difference (H-F, H-O, H-N)

-H-bonds are ~10x stronger than dipole-dipole

-Increase BP and water solubility of molecule

Dispersion forces

-Electrons shared in covalent molecules are in constant motion

-Will sometimes clump together temporarily and from temporary dipoles

-Thus, temp +/- areas of molecule and form attractive forces

-These temp dipoles bond non-polar molecules together

-Dispersion forces increase as molecule size/mass increases

Dispersion forces increase in strength

1.As you go down a group (number of electrons increases)

2.As molecular mass increases (i.e. methane vs ethane)

Key properties of IMF: MP/BP

-Increases with molecules size (higher dispersion forces)

-If similar mass - depends on type of IMF present

Key properties of IMF: Vapour pressure

Pressure exerted by gas particles of substance

-Low MP/BP higher vapour pressure

-Higher vapour pressure with lower IMF strength