Intermolecular Forces: relationships between molecules Chemistry 11 Mrs. Kay & Ms. Cleary

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

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

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

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.

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

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

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

Summary of Polarity of Molecules

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

How are molecules held together? There are two types of attraction in molecules: Intramolecular forces Intermolecular forces Intramolecular Forces – the forces that bond the atoms to each other within a molecule. Covalent Forces binding atoms in a molecule are due to chemical bonding Intermolecular Forces – the forces that bond molecules to each other Think interstate highway = connecting one state to another

How are molecules held together?

Intermolecular forces are weak relative to covalent bonds Intramolecular forces are strong covalent bonds We can conclude that it does not take very much energy to break the bonds that hold molecules to each other (intermolecular forces)

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.

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

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-

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

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.

“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)