Aim: What attracts molecules to each other? Do Now: Take out the homework. In Pairs discuss and correct your answers.

Review Molecular Polarity

Questions What holds the hydrogen and the chlorine together to form HCl? What holds the multiple HCl molecules together?

Discuss in Pairs How would you define an intramolecular attraction? How would you define an intermolecular attraction? Which attraction is stronger?

Intramolecular forces vs Intermolecular forces Intramolecular forces are the forces that hold atoms together within a molecule (bonds). Intermolecular forces are forces that exist between molecules.

Intermolecular Forces (IMF) Intermolecular forces are the forces of attraction between molecules The strength of these forces at a given temperature determine whether a substance will have the properties of a solid, a liquid, or a gas. Question: At room temperature, which substance has stronger attractions between its molecules? A solid or a liquid? Consider the relative distance between the particles in the solid and liquid phase.

Intramolecular forces vs Intermolecular forces Ionic bonds Metallic bonds Covalent bonds Polar covalent bond Nonpolar covalent bond Dipole-dipole Hydrogen bonding Dispersion Other types of attractions: Ion-dipole network

Ion- Ion Attraction (Ionic Bonding) Ion-ion interactions occur between ions of opposite charges, resulting in solids with ionic bonds.

Dipole-Dipole Interaction Class discussion: Predict how two dipole molecules are attracted to each other.

Dipole-Dipole interactions Occur when polar molecules that are attracted to each other. Opposite ends of dipoles attract, but not completely hooked like in ionic solids (ion-ion attraction). H F d+ d- H F d+ d- ||||||||||

Dipole-Dipole Interactions d+ d- d+ d- d+ d- d+ d- d+ d- d+ d- d+ d- d+ d-

Hydrogen Bond Special kind of dipole-dipole interaction Occurs specifically between a hydrogen atom bonded to either an oxygen, nitrogen, or fluorine atom.

Hydrogen bonding Are the attractive force caused by hydrogen bonded to F, O, or N. F, O, and N are very electronegative so it is a very strong dipole. (very electronegative and have small atomic radii) The strongest of the intermolecular forces.

Hydrogen bonding between water molecules Water H2O has high melting and boiling points because of the hydrogen bonds between the water molecules. H O H O H O H O H O

Hydrogen Bonding between ammonia (NH3) molecules

Ion-Dipole Attraction Class discussion: Predict how a lithium ion would be attracted to a dipole molecule.

Ion-Dipole Attraction The attraction between the ion of an ionic compound, such as salt, and molecules of water (or other polar liquids). When you put salt in water, the positive ion Na+ is attracted to the negative end of the water molecule, and the negative ion Cl- is attracted to the positive end of water. http://group.chem.iastate.edu/Greenbowe/sections/proj ectfolder/flashfiles/thermochem/solutionSalt.html

Ion-Dipole Interaction

Dispersion Forces Occur between nonpolar molecules. Electrons happen to momentarily move to one end of the molecule resulting in a temporary dipole. Depend on the number of electrons Bigger molecules have more electrons resulting in stronger forces, and decreasing the distance between the molecules. Explains why: Fluorine F2 is a gas Bromine Br2 is a liquid Iodine I2 is a solid

Dispersion Forces Dispersion forces even occur between monatomic molecules - molecules of noble gases, like helium, which consist of a single atom

Network Solids A Network Solid contains atoms that are all covalently bonded to each other. Carbon can form network solids. Ex: diamond and graphite

Relative strength of intermolecular forces of attraction Occurs between … Relative strength Dipole-dipole attraction Partially oppositely charged ions Strongest Hydrogen bonding H atom and N, O, or F atom As strong as dipole-dipole attraction Dispersion attraction Temporary or induced dipoles Weakest

How do Intermolecular Forces affect Physical Properties? They are what make solid and liquid molecular compounds possible. The stronger the attractions, the closer the molecules become. They contribute to the temperature at which a substance boils/melts The stronger the attraction, the higher the b.p. / m.p.

How forces of attraction affect properties of compounds Type of compound Intermolecular forces present Relative order of boiling and melting points Ionic compounds Ion to ion attraction between ions 1, highest) Covalent compounds containing hydrogen bonds Hydrogen bonds 2 Polar covalent compounds Dipole-dipole attraction between dipoles created by partially charged atoms 3 Nonpolar covalent compounds Dispersion forces 4, lowest

Practice Question 1 Which type of bonding accounts for the unusually high boiling points of water? Ionic bonding Covalent bonding Hydrogen bonding Network bonding The attraction between water molecules is called hydrogen bonding. Hydrogen bonds are relatively strong. The stronger the intermolecular force the higher the melting a boiling boints.

Practice Question 2 The attractions that allow molecules of krypton to exist in the solid phase are due to Ionic bonds Covalent bonds Molecule- ion forces Van der Waals forces (dispersion forces) Krypton does not have any poles (does not have positive or negative end). However, temporarily the electrons in krypton will shift to one side, resulting in a temporary dipole. The attraction between two temporary dipoles is a Vand der Waals force.

Practice Question 3 The dispersion forces of attraction between molecules always become stronger as molecular size increases, and the distance between the molecules increases Increases, and the distance between the molecules decreases Decreases, and the distance between the molecules increases Decreases, and the distance between the molecules decreases Bigger molecules and decrease in distance between molecules More electrons stronger forces