imf and solubility

i am a dispersion force

What are intramolecular forces?

Covalent bonds within a molecule that hold atoms together.

What are intermolecular forces?

Forces of attraction between molecules.

Strength of intermolecular forces - MP and BP

to melt or boil a substance, imf between the molecules of this substance have to overcome

the stronger these forces, the more energy is required to do so and therefore the mp and bp increase with the strength of the imf

What is a dipole?

A separation of charge creating a partially positive end and a partially negative end.

Dipole dipole forces

some molecules exhibit permanent dipoles with permanently slightly charged positive and negative ends

the oppositely charged ends of two molecules can therefore be attracted to each other

What causes a polar bond?

A significant difference in electronegativity between bonded atoms.

When do molecules with a net dipole occur:

In all molecules with a single polar bond (arising from a significant difference in electronegativity between the atoms involved in the bonds)

In molecules with more than one polar bond where the bond dipoles don’t cancel each other out (non-symmetrical)

What is a permanent dipole?

A dipole that exists at all times in a polar molecule.

Dispersion forces (temp/induced dipole forces)

A temporary dipole that exists as a result of fluctuation of the electron cloud: electrons are constantly moving and are not always evenly distributed

Uneven distribution of electrons in one molecule induces an uneven distribution in a neighboring molecule

are the only force present in non-polar molecules, and present between all molecules

How do dispersion forces occur?

Electrons are constantly moving and can become unevenly distributed.

When this happens a temporary or instantaneous dipole is formed

Once that temporary dipole has been generated it will influence the orientation of electrons within the molecules close to it

Electrons in neighboring molecules will be repelled by the negative end of the temporary dipole, creating a new induced dipole

Size of molecule (number of electrons present)

the larger a molecules and so the greater the number of electrons present,

the larger the difference in positive and negative charge

thus the dispersion forces will be more pronounced

thus the stronger the bonds and more energy required to overcome it

Packing Density

the points of contact between molecules also affects the strength of the dispersion forces

the dispersion forces between molecules are stronger as these molecules can pack closer, meaning force can act over a larger surface area

Describe the trend in boiling points

As the electronegativity of the halogens decreases down the group, it would be predicted that the BPts would decrease as the strength of the dipoles decreases

This is, however, not the case

There the dipole dipole cannot be the dominant force

The dispersion force increase with the size of the molecules going down the group and are the cause for the increase in BPts

Hydrogen Bonding

imf that occurs between molecules where hydrogen is bonded to the most electronegative elements- Fluorine, Oxygen, Nitrogen

The positive H end of a dipole is strongly attracted to the lone pairs on the negative FON end of the dipole of another molecule

2 requirements:

H bonds to F, O or N

A lone- pair on F, O or N

Vapor Pressure

is a measure of the tendency of a substance to evaporate

as the temp of a substance increases, so does the kinetic energy of the particles

more particles evaporate and the vapour pressure increases

If the vapor pressure of a liquid is equal to the atmospheric pressure, the liquid starts to boil

For a substance to evaporates, its molecules have to escape the imf that exist in the liquid phase

therefore, liquids with stronger intermolecular forces tend to have lower vapour pressures and therefore higher boiling points

For a solute to dissolve in a solvent:

Solute-solute interactions need to be broken

Solvent-solvent interactions need to be broken

Solute-solvent interactions need to be formed

If the solute-solvent interactions are of similar strength of stronger, the solute will dissolve in the solvent

Why does ammonia dissolve in water?

Both water and ammonia have hydrogen, disp and dipole dipole bonding.

Hydrogen bonds between ammonia and water molecules form

Ammonia has the ability to form hydrogen bonds

When the hydrogen bonds between water molecules are broken, they can be replaced by equivalent bonds between water and ammonia