More about Intermolecular Forces

SCH4U1The last day of February 2011 [unless I am

mistaken because it is a leap year]Mr. Dvorsky

From last class....

Intermolecular forces – forces of attraction between molecules-are always much weaker than any of the types of intramolecular bonding (ionic, polar covalent, non-polar covalent). For example if covalent bonds are assigned a strength of 100, then intermolecular forces are generally 0.001 to 15.

Intermolecular Forces

• At very low temperatures, the noble gases form atomic solids and liquids

• They already have the electron configuration other elements achieve during chemical bonding, therefore very stable and we would expect there to be little attraction between two atoms of noble gases.

• Proof of this: noble gases can be liquefied or solidified only at extremely low temperatures.

Intermolecular Forces

That being said, the fact that noble gases can form solids and liquids means there must be some forces of attraction between their atoms. -proof of the existence of van der Waals forces (intermolecular forces)

Intermolecular Forces

• Van der Waals forces refers to types of intermolecular forces (e.g. London force, Dipole-Dipole force)

Intermolecular Forces

• In a noble gas like He, an instantaneous charge imbalance can occur. –that means at a given instant, both He electrons may be found in the same side of the atom

• That side becomes slightly negative and the opposite side becomes slightly positive.

Intermolecular Forces

• this is a temporary fluctuating dipole• the atom can then induce a dipole in a

neighbouring atom and they can they attract each other

• called London Force or London Dispersion Force. Weakest of the van der Waals forces.

• The strength of the London force depends upon size of the atom.

• The larger the atom, the greater the number of electrons, the greater chance there is for a temporary imbalance of electrons and therefore the greater strength of force between that atom and neighbouring ones.

Intermolecular Forces

• With non-polar molecules, the force of attraction between them are these London forces that we discussed.

• This is because the atoms have filled energy levels just like the noble gas atoms.

• Therefore the only force of attraction between molecules is one which results from a temporary charge imbalance in the atoms which make up the molecules

• When comparing non-polar molecules with the same number of atoms, the size of the forces of attraction depends on the size of the atoms.

• As the size of atom increases, and thus the size of molecule increases, the number of electrons increases too. Therefore more chance of temporary charge imbalance arising making the forces of attraction stronger.

• When comparing non-polar molecules in which the atoms are the same size but there are a different number of them, the forces of attraction are greater between molecules with the greatest number of atoms.

• More atoms = more locations for van der Waals forces to occur in adjacent molecules

• When comparing non-polar molecules in which the same number and type of atoms are present, the forces of attraction are greater between molecules with the shape that allows for more places where forces of attraction can arise between adjacent molecules

Intermolecular Forces

• The dipole-dipole force is the attraction between dipoles.

• Recall a polar molecule has dipoles (oppositely charged ends).

• Opposite charged ends of polar molecules attract

Intermolecular Forces

• In polar molecules, the forces of attraction are London Forces AND the dipole-dipole force.

• Polar molecules such as HCl have a permanent dipole.

• The slightly negative end of one molecule can attract the slightly positive end of a neighbouring molecule.

Intermolecular Forces

• The more polar the molecule, the stronger the dipole – dipole force and therefore the higher the boiling point.

Hydrogen Bonding

• When hydrogen is covalently bonded to a very small highly electronegative atom such as fluorine, oxygen, or nitrogen, unusually strong dipole-dipole attractions are observed

• Why? Two reasons.

1. Because of the large electronegativity difference, the F-H, O-H, N-H bonds are very polar. Thus, the ends of these dipoles carry a substantial fraction of one charge

2. Because of the small size of the atoms involved, the charge on the end of a dipole is highly concentrated.

-this makes it particularly effective at attracting the end of opposite charge on a neighbouring dipole. -these two factors combine to produce attractions called hydrogen bonds. -hydrogen bonds are 1/10th as strong as covalent bonds but 10x stronger than normal van der Waals forces.

-If a substance has molecules with hydrogen bonds between them, it tends to have a higher melting point & boiling point. -hydrogen bonding also causes substances to form unusual structures in the solid state. When water freeze it forms an open expanded structure, therefore water expands when it freezes and ice has a lower density than liquid water. Thus ice floats. –for most substances solid state = more dense

Hydrogen Bonding• Why is this important?-if ice was more dense than water, ice forming at the top of a body of water would freeze and fall to the bottom, thus it would freeze solid.-aquatic life could not survive-therefore we would have not evolved

Hydrogen bonding in living systems

• the structure of proteins is controlled by hydrogen bonding

• Hydrogen bonding is an important factor in determining the shape of DNA

• Plant fibres are more rigid than animal tissue because of more hydrogen bonding

• We eat and wear things that are hydrogen bonded