Activation energy is the energy required to get a chemical reaction started.

 A catalyst is a substance added to a reaction to help lower the activation energy.  A catalyst does NOT appear as a reactant or product. Therefore, a the presence of a catalyst may be indicated above the arrow of a balanced chemical equation. For example: 2H 2 O 2 H 2 O + O 2 Pt

 Someone asked yesterday why heat travels from hot to cold and not the other way around….  It’s because of ENTROPY! Entropy is the tendency of the universe to seek randomness and disorder.

Entropy (S) = a measure of randomness or disorder MATTER IS ENERGY. ENERGY IS INFORMATION. EVERYTHING IS INFORMATION. PHYSICS SAYS THAT STRUCTURES... BUILDINGS, SOCIETIES, IDEOLOGIES... WILL SEEK THEIR POINT OF LEAST ENERGY. THIS MEANS THAT THINGS FALL. THEY FALL FROM HEIGHTS OF ENERGY AND STRUCTURED INFORMATION INTO MEANINGLESS, POWERLESS DISORDER. THIS IS CALLED ENTROPY.

 Think about it. If I drop a bunch of pencils on the floor, will they land in a random mess or will they land all lined up? Why? Which outcome requires the least amount of energy.? Remember…the universe is LAZY!  Similarly, quickly moving molecules will transfer some of their kinetic energy to neighboring molecules. Particles want to be lazy if you let them!

Intermolecular forces are interactions that hold the particles in a liquid or solid together. There are three types of intermolecular forces that we need to consider.  Dipole-Dipole  Hydrogen Bonds  London Dispersion

 Dipole-dipole forces: Interactions in which polar molecules stick to each other like little magnets. The more polar the molecule, the stronger the attraction!

Hydrogen bonds: A very strong dipole- dipole force that occurs when the lone pair electrons on O, F, or N interacts strongly with a hydrogen atom bonded to O, F, or N. Essentially, these bonds are so polar that the lone pair electrons on one molecule want to stick to the very positive hydrogen atoms on another molecule. The more hydrogen bonding that a molecule can do, the stronger this force is.  Water has a MP of 0 0 C, while methanol has a MP of C.

 London Dispersion Forces (also called Van der Waals’ forces): When nonpolar molecules are attracted to one another via temporarily induced dipoles. Essentially, nonpolar molecules stick together magnetically – like polar molecules. How can this work? The bigger the molecules, the stronger the force (because, after all, there are more electrons to become unbalanced and interact with each other).  Neon has a MP of , while F 2 has a MP of C and methane has a MP of C.

 The stronger the intermolecular force, the higher the melting and boiling points. Because melting and boiling both involve the movement of particles from their neighbors, anything that causes neighboring particles to stick together will raise them.

 Intermolecular forces make liquids almost as dense as solids: Because the intermolecular forces in liquids keep the molecules stuck to each other, liquids are nearly as dense (and in a few cases even denser) than solids.  Intermolecular forces allow liquids to flow – a property called “fluidity”. Because the molecules in a liquid are attracted to each other but not permanently stuck in place, the molecules can move from one place to another – flowing!

 Intermolecular forces cause differences in the viscosities of liquids: Definition: Viscosity is the ease with which a liquid can flow. High viscosity = slow flowing. The higher the intermolecular force, the more viscous the liquid. This is because molecules that are held tightly together want to move apart less than molecules that are loosely held. Viscosity also goes down with increasing temperature (i.e. things flow more easily at high temperatures) – this is because the energies of the particles in the liquid are beginning to get to the point where they can overcome some of the attractive forces.

 Intermolecular forces cause surface tension in a liquid. Definition: Surface tension is the energy needed to increase the surface area of a liquid – the higher the surface tension, the harder it is for something to push through the surface of a liquid. Stronger intermolecular forces cause the surface molecules to hold together more tightly, making the surface tension higher. This is why some things that are heavier than water (i.e. water bugs, leaves, etc) don’t fall through.

 Intermolecular forces cause capillary action. Capillary action: The tendency of some liquids to rise when placed in a small tube – this explains why putting one edge of a paper towel will eventually cause the whole towel to get wet. This happens because water molecules want to grab the surface of the walls of the tube with their intermolecular forces more than they want to grab each other. This causes them to move up the sides of the tube (away from each other). This causes the meniscus. Eventually, the pressure of the water height overcomes the attraction of the water for the sides of the tube and the water stops rising.

 Ranking of intermolecular forces (strongest to weakest):  Ionic interactions and covalent bonds (they are not intermolecular forces, they are intramolecular forces) – it takes a huge amount of energy to break these.  Example: The BP of NaCl is C.  Hydrogen bonding  Example: The BP of water is C.  Dipole-dipole forces  Example: The BP of H 2 S is C.  London dispersion forces  Example: The BP of methane is C.

 What intermolecular force is molecule X likely to experience? Draw the Lewis structure of the molecule. If the molecule is nonpolar, it’s Van der Waals forces. If the molecule has H bonded to O, N, or F, it’s hydrogen bonding. If the molecule is polar but H isn’t bonded to O, N, F, it’s dipole- dipole forces. [do some examples: CH 4, CH 4 O, HCN]

 Rank the following by increasing melting/boiling point. To solve, determine the type of intermolecular force that each molecule is undergoing. You may assume that molecules with stronger intermolecular forces will have higher MP and BP than those with weaker ones. Example: Rank CH 2 O, CO 2, and CH 2 O 2