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Intermolecular Forces: relationships between molecules. Polarity and Shape. Polar bonds do not necessarily always create a polar molecule. The shape of the molecule and the polarity of each bond is considered before stating if a molecule is considered POLAR or NON-POLAR
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Polarity and Shape • Polar bonds do not necessarily always create a polar molecule. • The shape of the molecule and the polarity of each bond is considered before stating if a molecule is considered POLAR or NON-POLAR • We will watch a 5 minute explanation to help us understand this difficult concept • polar molecules summary.rm
Intramolecular forces: forces that bond the atoms to each other within the molecule. • COVALENT BONDS!!!
Intermolecular Forces • Forces affecting the relationships between molecules • London Forces (Dispersion Forces) AKA van der Waals forces • Dipole-Dipole Interactions • Ion-Dipole Interactions (Salt dissolving in solution) • Hydrogen Bonding
London (dispersion) Forces • weakest intermolecular force • occurs between all types of molecules • It is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles
London Dispersion forces in a molecule These instantaneous dipoles may be induced and stabilized as an ion or a polar molecule approaches the non-polar molecule.
Dipole-Dipole Forces • Dipole = polar molecule • Molecules with dipoles will change their direction so that their oppositely charged ends are near to one another. • The electrostatic attraction between the ends is dipole-dipole force
Orientation of Polar Molecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules 11.2
Ion Dipole Forces • The force of attraction between an ion and a polar molecule. • NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl-
Polar molecules can interact with ions: • Ion - Dipole Interactions
water has the lowest molar mass water has the highest heat of fusion water has the highest melting point water has the highest heat of vaporization water has the highest boiling point The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group VIA.
Water exhibits these unusual properties because of hydrogen bonding between water molecules. http://www.northland.cc.mn.us/biology/biology1111/animations/hydrogenbonds.html
HYDROGENBONDING • Hydrogen bonding is usually stronger than normal dipole forces between molecules. Of course hydrogen bonding is not nearly as strong as normal covalent bonds within a molecule - it is only about 1/10 as strong. • This is still strong enough to have many important ramifications on the properties of water.
A hydrogen bond is the strongest form of dipole-dipole interaction. • A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to a small, highly electronegative atom: F, O, N. F—H O—H N—H
A molecule containing this combination of atoms, will be attracted to another molecule with the same combination. • A dipole-dipole bond bond will be formed between the two molecules which is called a hydrogen bond. hydrogen bond covalent bond covalent bond
Hydrogen Bonding • It is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. • The molecule involved with hydrogen bonding should have at least one lone pair. More lone pairs= stronger hydrogen bonds.
Polarity and Boiling Point: • The polarity of the molecules determines the forces of attraction between the molecules in the liquid state. • Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule). • Molecules have different degrees of polarity as determined by the functional group present. The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point.
What is boiling point? • Boiling point is the temperature at which a liquid turns into a gas (or vapour). • For a liquid to turn into a gas, intermolecular forces must be broken. • The stronger the intermolecular forces, the higher the boiling point of a substance.
The evidence for hydrogen bonding • Many elements form compounds with hydrogen - referred to as "hydrides". • If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group. The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.
The evidence for hydrogen bonding If you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.
Solubility and Intermolecular Forces • Like dissolves like • Polar solutes dissolve in polar solvents • Nonpolar solutes dissolve in nonpolar solvents • Molecules with similar intermolecular forces will mix freely
Comparison of the Properties of Substances with Ionic, Covalent, Metallic or Intermolecular Bonds
