chemistry - intermolecular forces and bonding

Intermolecular forces

• bonding between molecules

• Generally weak forces

• Covalent bonds are an intermolecular force → occur inside the molecule and are much stronger

THREE TYPES

• dipole-dipole

• Hydrogen bonds

• Dispersion forces

Electronegativity

• tendency of an atom in a covalent bond to attract electrons

→ high core charge, few shells

Polarity

• localised imbalances in electric charges within a molecule resulting in a negatively charged and positively charged end

Polarity - why do molecules bond together

• chlorine atone is more electronegative

• Chlorine atoms → attracts bonding electrons more than the hydrogen atom

Polarized bonds

• covalent bonds between atoms of different electronegativity have polarised bonds

• Atoms in polarised bonds have partial charge

• Covalent bond ← partial charge, ionic bond ← full charge

• The greater the difference in Electronegativity → more polar the bond

Polar covalent molecules - polarity of a molecule depends on

• the polar bond that is contains

• The way in which the bonds are orientation with respect to each other

→ eg: the shape of the molecule

Equal polarity & symmetrically arranged

• if bonds have equal polarity (eg: O2) and are symmetrically arranged → their charge separations (electronegativites) will oppose each other & will effectively cancel each other out

→ molecule will be non-polar even though it had polar bonds

Bonds of different polarity or not symmetrically arranged

IF EITHER:

• molecule contains bonds/different polarity

OR

• its bonds are not symmetrically arranged

→ bonds wont cancel out → molecules are polar

When is a molecule polar

To be dipole, a molecule must:

1. Have polar bonds

2. Have asymmetrical charge distribution

Intermolecular forces - dipole and polar molecules

• molecules with two charged ends (poles) = polar molecules or dipoles (two poles)

• Di[oles arrange +ve to -ve charge → intermolecular attraction

Intermolecular forces - dipole-dipole bonding

• only occur between molecules that are polar

• They are weak

→ when HCL melts → intermolecular forces hold it in liquid state → they’re lost in gaseous state

Intermolecular forces - hydrogen bonding

• special type of dipole-dipole bonding

• The strongest intermolecular force

• Holds many biological molecules together

→ DNA and protein complexes

Hydrogen bonding cont.

• occurs when H is bound to N,O or F

→ these are very electronegative atoms so H loses its valence electron and gains a large S+

→ they (N, O or F) have a free pair of electrons

Dispersion forces

• Due to random movement of electrons at any instant, it’s possible for them to be unevenly distributed → temporary dipole (instantaneous dipole)

• Instantaneous dipole → dissappears unless another atom happens to come close to the atom

→ if this happens → dipole will be induced in second atom

• weak attraction between atoms are dispersions forces → are found between molecules

Relative bond strength

• molecular substances may experience up to 4 types of interaction

→ all molecules - held together internally by covalent bonding

→ bcs this acts within a molecules → = intermolecular force

• its a very strong force and considerably greater than any of the other forces that could be experienced

• All molecules in liquid/solid states → also experience attraction to other molecules from dispersion forces. These are really weak forces that originate bcs of movement of electrons within molecule

• Other forces that might be experienced= dipole-dipole & H bonding

→ these two are due to polarity existing within some molecules

Properties

Hydrogen > d-d > dispersion

• stronger the bonding → higher the MP and BP → more energy needed to break bonds

• Larger molecules → more IMB → higher MP/BP

→ consider type of intermolecular bonding and number of bonds

Types of covalent structures

• covalent molecular structures - consist of molecules on their own. Low MP/BP. Covalent bonds btwn atoms within molecules = strong but intermolecular forces are weak

• Covalent network strucutre - made from network of repeating lattices of covalently bonded atoms. High MP/BP and insoluble in water

Graphite

• 2D network lattice with layers of hexagonal rings

• Within each layer, each C atom is bonded to three carbon atoms → leaving one electron per carbon atom not used in bonding

• Electrons are delocalised across the layer

• Each layer is only weakly linked to the next layer by London forces

Diamond

• a 3D network lattice of tetrahedrons

• Each carbon atom is bonded to four other carbon atoms