Intermolecular Forces of Attraction

What holds molecules together?

• Intramolecular Forces – attractive forces within compounds that hold the compound together.

• Include ionic and covalent bonds.• Stronger bonds represent stronger intramolecular

forces. Consider the molecules below: N2 > O2 > Cl2.

• In general, Ionic compounds have much stronger attractions than molecular compounds.

• Ionic compounds consist of large numbers of ions in a repeating lattice structure. The attractions between positive and negative ions give ionic solids higher melting points than most molecular solids.

• This is why table sugar (sucrose – bottom left) melts at 186°C whereas salt (NaCl – bottom right) melts at 801°C

What attracts one molecule to another? • Intermolecular Forces (IMF) – the attractive forces

between neighboring molecules.• The further apart the molecules are, the weaker the

attraction between the molecules will be (i.e. molecules in a gas are not held together as strongly as molecules in a solid).

• Types of IMF include Dipole-Dipole Attractions, Hydrogen Bonding, and London Dispersion Forces.

Dipole-Dipole Attractions• Force of attraction between polar molecules.• Because polar molecules have permanent positive and

negative ends (dipoles), the positive end of one molecule will be attracted to the negative end of any neighboring molecules.

Hydrogen Bonding• Very strong force of attraction that forms between

the hydrogen atom in a very polar bond and a non-bonding electron pair on a neighboring molecule.

• Hydrogen Bonding only occurs in molecules that have H—O, H—N, or H—F bonds.

• The hydrogen bonds formed by H2O molecules are particularly strong, giving water many of its unique properties.

• Water molecules are small and can form up to four hydrogen bonds at any given instant (remember, one H2O molecule has two H atoms and two non-bonding pairs on the O atom, totaling four sites for hydrogen bonds to form).

London Dispersion Forces• Caused by the attraction between temporary dipoles in

neighboring molecules.• Because the electron cloud is not always evenly

distributed, it can become distorted forming temporary positive and negative ends in a molecule (dipoles).

• All molecules are capable of forming temporary dipoles and exhibiting london dispersion attractions.

• Larger molecules have larger electron clouds and are more polarizable. As a result, dispersion forces will be stronger in molecules that are larger (or have a higher molecular weight).

Comparing IMF StrengthWhen you are trying to compare the strength of the intermolecular attractions present in two different compounds, there are four questions you should ask…

Question #1

Is one of the compounds ionic? • Ionic compounds have much stronger forces of

attraction than any type of IMF found in molecular compounds.

• How do we know a compound is ionic? Recall that ionic compounds form between a metal and a non-metal whereas covalent compounds only contain non-metals.

• If neither compound is ionic go on to question #2…

Question #2

Does one of the compounds exhibit hydrogen bonding?

• Hydrogen bonds are typically stronger than the other types of IMF.

• What types of compounds form hydrogen bonds? Only those that have H—F, H—O, or H—N bonds.

• If neither compound forms hydrogen bonds, go on to question #3…

Question #3

Does one molecule have a larger

mass than the other? • If so, the molecule with the larger mass has the

stronger IMF. • Recall that larger molecules are more polarizable and

exhibit stronger london dispersion attractions than smaller ones.

• If both compounds have about the same mass, go on to question #4…

Question #4

Is one of the molecules polar?• Polar molecules form dipole-dipole attractions and

will have stronger IMF than similarly sized non-polar molecules.

Why are intermolecular forces important?• Boiling Points and Melting Points– Molecules with stronger intermolecular attractions are

held together more tightly, giving them higher melting points and higher boiling points than molecules with weaker IMF.

• Rates of Evaporation– The weaker the intermolecular attractions are, the more

easily a molecule can evaporate to the gas phase. – Liquids that have weaker attractions and evaporate

easily are said to be volatile.

• Surface Tension– A force that pulls adjacent parts of a liquid’s surface

together, decreasing the surface area to the smallest size possible.

– Surface Tension is caused by an imbalance of attractions at the surface of a liquid (shown in the diagram below).

• Molecules in the interior of a liquid are attracted in all directions by other molecules.• Molecules on the surface are only

attracted by other molecules that are at or below the surface. This results in a net downward attraction making the molecules pack more closely together.

– Because of it’s strong IMF, water has a high surface tension. This causes the surface of water to behave as if it had an “elastic skin” allowing bugs such as the water strider (bottom left) to walk across the surface.

–Water’s surface tension also causes it to form droplets, as seen on the leaf on the bottom right.

• Viscosity– Measures a liquid’s resistance to flow; a greater viscosity

means that a liquid will flow more slowly.– Stronger attractive forces usually make a liquid more

viscous, although size plays a more important role in determining viscosity. • Molecules with more

complex structures are more likely to become “entangled” and resist flowing past one another. • Viscosity increases with

increasing molecular weight.• Molasses (shown on the

right) has a high viscosity and pours very slowly.

• “Like-Dissolves Like”– Substances with similar intermolecular forces tend to be

soluble (dissolve) in one another.– Polar/ionic substances are the most soluble in other

polar/ionic substances. The positive charges in one substance will attract the negative charges in the other, helping the substance dissolve.

– Non-polar substances are most soluble in other non-polar substances.

– The cup shown on the right contains a non-polar oil and water. Because of the principle of like-dissolves-like they are not soluble in one another and form two separate layers.

• Some molecules are soluble in both types of solvents because they have polar and non-polar sections.

• Consider the ethanol molecule shown below; it has a non-polar region (the hydrocarbon chain) and a polar region (the O—H group). As a result, ethanol can dissolve in water as well as non-polar substances.

• In general, the larger the non-polar region (the hydrocarbon chain) is, the less soluble the compound will be in water.

• This same principle explains the way that vitamins are stored in your body: – Vitamins A, D, E, and K are soluble in non-polar fatty

acids. As a result these fat-soluble vitamins can be stored by the body for longer periods of time.

– Vitamins B and C are water soluble because they are polar. As a result, they should be part of a daily diet.

Cohesion Adhesion

Force of attraction between molecules of the same substance

Force of attraction between molecules of different substances

Water molecules have strong IMF so they attract each other forming droplets

Water is a polar substance, so it will adhere well to a polar surface

• Capillary Action– The ability of a liquid to flow in narrow

spaces in opposition to the force of gravity.– When a thin tube is placed into a beaker of

water, some of the water is drawn up into the tube as shown on the right.

– This occurs because of the intermolecular attractions between the water and the wall of the container. • Polar molecules will adhere to a polar

surface allowing capillary action to occur. • Polar molecules will not adhere as well to a

non-polar surface.