Name:________________________________________ Date:______________________________Blk:____Bonding Forces: Intermolecular Bonding

Recall that intramolecular forces are the strong bonding forces in which there is either an absolute donation of electrons(___________) or sharing of electrons(__________________ or ________________). Intermolecular forces are weaker forces in which particles are electrostatically attracted to each other but electrons are not _______________________, _____________, or _________________. It may help to think of intramolecular forces acting _________________________ the molecule and intermolecular forces acting ________________________ molecules, but bear in mind that molecules are not involved in all cases.

Indicate whether the following processes involve breaking of intermolecular or intramolecular bonds:

a) The melting of paradichlorobenzene

b) The burning of natural gas

c) The boiling of water

d) The electrolysis of water

There are six types of intermolecular forces (IMF). It is useful to think of the species involved as ______________________.

1) Ion-Dipole

An IMF caused by the attraction of an _____________ (partner 1) and one end of a ______________________________ (partner 2).

The partners must always be two different species.For example

This is the ___________________________ IMF and its strength depends on:1. The ______________________________within the polar molecule; the greater the

bond length the greater the dipole moment.2. The _________________________________ between the ion and the dipole on the

polar molecule; the greater the separation the weaker the force.

3. The ________________________________; the bigger the ion, the __________________ the IMF because dipoles can’t get as close to the _________________.

4. The _____________________________; the greater the charge, the ________________ the IMF

This IMF explains the solubility of salts in water and why positive ions in water will often attract water molecules to form hydrated ions like Al(H2O)63+.

2) Dipole-Dipole Forces

An IMF caused by the ____________________attraction of the _________________ end of one dipole (partner 1) to the ___________________ end of another dipole (partner 2).

Remember that dipoles arise within molecules due to the __________________________ _______________________________ caused by the differences in EN between the two atoms forming the bond.

Notice in the diagram below that the polar molecules orient themselves in such a way as to _____________________ attractions and ____________________ repulsions.

SHAPE \* MERGEFORMAT

This is a relatively strong IMF and is the reason why ______________ molecules have higher melting and boiling points than ____________________ molecules of similar size.

Molecules Type Dipole-Dipole? Boiling Point

CH3Cl Polar yes -24oCCH4 Nonpolar no -162oC

3) Hydrogen BondingA special type of dipole-dipole attraction that is so strong it is given its own category. H-bonding occurs between:

Partner 1 Partner 2

H-F Polar molecule with

H-O F, O, or N

H-N

(because F,O, and N are highly __________, the charge separation within the partner 1 molecule is ___________________ therefore the attraction between the + _________________ of partner 1 and - ________________ of partner 2 is very strong.)

It should be noted that partner 1 and partner 2 could be two different parts of the ______ molecule. This is common in large biological molecules like DNA.

Which of the following pairs could exhibit H-bonding?

Partner 1 Partner 2 H-bonding?

H2O N2

H2O H2ONH3 H2O

HF H2OHF HFNH3 F2

HCl HClCH3F CH3FKF KF

NO NH3

Many phenomena can be explained by H-bonding. For example:

The acidity of the hydrogen halides. HCl, HBr, and HI are all strong acids but HF is a _______________ acid. This is because the extensive H-bonding in a solution of HF “___________” the H making it hard for the H+ to break its H-F bond so it can be _____________________.

The boiling points of HF compared to the rest of the hydrogen halides.

HF 19.5oC

HCl -85.1oC

HBr -67.0oCHI -35.0oC

Except for HF, the boiling points of the hydrogen halides __________________as you go down the family. This is because the halide gets bigger as you go down the family so there is a larger _____________________________________ within the molecules creating a greater _________________________ and a more __________________ molecule. Because HF exhibits H-bonding the IMF between the molecules is even ___________________ and the boiling point __________________.

The boiling point of H2O compared with other hydrogen compounds in the oxygen family.

Normally, London dispersion forces in molecules get stronger with increased _______________________________. Thus you would expect the boiling points of the oxygen family hydrides to ___________________ as you go down the family. This is not the case.

Because hydrogen bonds are so much __________________ than London dispersion forces, water can withstand higher temperatures before its IMFs are _________________ and it enters the gas phase.

Water is less dense in the ________________ state (ice) than in the _________________ state.

This is a rare phenomenon and can be explained by the extensive H-bonding. The H-bonds in ice ‘lock’ the water molecules together in a framework that has a lot of open spaces.

4) Ion-Induced Dipole Force

This IMF is caused when the _______________________ distorts the electron cloud of a _______________________________ and creates a temporary dipole.

This is a relatively weak force. Its strength depends on:

The ______________of and _______________ on the ion.

The size of the _____________ molecule (the bigger the nonpolar molecule, the more electrons in its clouds and therefore the more _____________________ it is). Which is more polarizable CF4 or CBr4?

The ________________ of the nonpolar molecule (the more elongated the shape the more polarizable it is). Which is most polarizable? Least?

Note: Polarizability is the ease with which an electron cloud can be deformed. It increases with the number of electrons in the molecule, and is enhanced by the presence of pi bonding.

5) Dipole-Induced Dipole Force

This IMF is caused when a dipole on a _________________ molecule induces a dipole on a nearby ________________________ molecule.

This force is responsible for the slight _____________________ of nonpolar atmospheric gases like O2, N2, and CO2 in water.

The strength of this interaction depends on: The magnitude of the __________________ on

the ________________ molecules The ______________________________ of the

nonpolar molecule

6) Induced Dipole-Induced Dipole Force (aka: London Dispersion Forces)

This IMF is caused when an _______________________________________________ in the electron cloud of one molecule creates a temporary weak _______________ in a

neighboring molecule due to _____________________ interactions. . London dispersion forces are present between all atoms and molecules and although it is the weakest IMF,

it becomes increasingly significant as the ___________ of the molecule and therefore its _______________________ increases. Many induced dipoles can add up to hold a system together and, in large molecules, it is often the strongest net IMF at work.

It is this IMF that allows us to __________________ H2 and N2. Without this force, these substances would not hold together in the liquid phase.

Rank the following in order of decreasing boiling point: N2, H2, O2 (Hint: how will the size of the molecule affect its ability to remain in the liquid state?)

Identify the most important IMF in solids of the following:

a. Ar b. HCl

c. HF d. CHCl3

e. CH4 f. CO

g. BF3 h. CH3COOH

i. CF3(CF2CF2)nCF3 j. CH3(CH2CH2)nCH3

Rank the intermolecular forces in order of strength.

Strongest ---------------------------------------------------- Weakest

It is important to remember that although we can roughly rank the IMFs in order of strength, the specific structure and size of the particles involved can play a very important role in determining the overall strength of a particular IMF.

Biological Structures and IMFs

The structure and function of many biological systems depend on the strength and nature of the various Coulombic forces listed above. For example proteins are natural polymers made up of chains of each

of which has the form

There are _____ amino acids that make up most proteins by linking in various combinations to form the _________________ structure of the protein (the _____________ of amino acids)

The lone pairs on the ________________ in the ________________ groups can ____________________ bond with a hydrogen in an _______________ group on the same chain to give the protein chain a __________________ structure or on a different chain to give the protein a __________________ _________________ structure. This is called its _______________________ structure.

The side chain _______ can be ___________ or ________________________ and will dictate whether or not regions of the protein are ________________________ or ____________________________. This, along with a variety of different IMFs will dictate what a protein’s _____________________ structure will be in an aqueous or other environment.

Identify the interactions that stabilize the tertiary structure of the protein below.

NH3+ -O

CH H3C CH3

H3C CH3

CH

One type of protein is an ___________________. Enzymes _________________ important biological reactions. The _____________________ structure of an enzyme gives rise to an _________________ ___________ into which a _____________________ will fit in order to undergo reaction as shown:

The bonding between the active site and the substrate may be ___________________ ____________________, ___________________________ interactions, or ________________________ _____________________________ ________________.