Intermolecular Forces: relationships between molecules

1. Intermolecular Forces:relationships between molecules Chemistry 11 Mrs. Kay

2. 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 • http://www.mhhe.com/physsci/chemistry/chang7/esp/folder_structure/bo/m4/s2/index.htm

3. Polarity • The atoms are sharing electrons, one atom will attract the electrons shared more closely, making it slightly more negative, and the other atom slightly more positive. • The overall shape is polar • POLAR Shapes always include: bent and pyramidal

4. Electronegativity • Atom is more electronegative, it is pulling electrons closer to itself. • Atom is more electropositive, its electrons have been pulled farther away.

5. Non-polar Molecules • A non-polar molecule is one that the electrons are distributed more symmetrically and thus does not have an abundance of charges at the opposite sides. The charges all cancel out each other.

7. Summary of Polarity of Molecules • Linear: • When the two atoms attached to central atom are the same the molecule will be Non-Polar • When the two atoms are different the dipoles will not cancel, and the molecule will be Polar • Bent: • The dipoles created from this molecule will not cancel creating a net dipole and the molecule will be Polar

8. Summary of Polarity of Molecules • Pyramidal: • The dipoles created from this molecule will not cancel creating a net dipole and the molecule will be Polar • Trigonal Planar: • When the three atoms attached to central atom are the same the molecule will be Non-Polar • When the three atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar

9. Tetrahedral When the four atoms attached to the central atom are the same the molecule will be Non-Polar When three atoms are the same, and one is different, the dipoles will not cancel, and the molecule will be Polar

11. Summary of Polarity of Molecules

12. Examples to Try • Determine whether the following molecules will be polar or non-polar • SI2 • CH3F • AsI3 • H2O2

15. Intermolecular Forces • Forces binding atoms in a molecule are due to chemical bonding • Intramolecular forces: forces that bond the atoms to each other within the molecule.

16. Two ways to form a solution 1. If there are strong mutual forces of attraction between solute and solvent particles in order to separate solute particles from each other and solvent particles from each other. 2. If there are very weak forces of attraction holding solute particles together and holding solvent particles together.

17. Dipole-Dipole Forces • Dipole = polar molecule • 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

18. 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-

19. London (dispersion) Forces • weakest intermolecular force between non polar 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

20. 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.

21. “Like Dissolves Like” • Ionic solutes dissolve in polar solvents • (ex: NaCl and H2O) • Non polar solutes dissolve in non polar solvents • (ex: solid I2 and liquid Br2)