Day 44 – Daily Starter Why can we adjust the coefficients when balancing equations, but never change the subscripts in a chemical equation?

QUIZ: Electronegativity & Balancing Equations ANNOUNCEMENT!!! QUIZ: Electronegativity & Balancing Equations Wednesday, Nov. 5 Week 9 D.S. will go on Quarter 1 report cards Make sure you turned in your balancing EQ. worksheet

Intermolecular Forces (an introduction) Day 44 Intermolecular Forces (an introduction)

Intermolecular Forces Intermolecular Forces: the forces of attraction between molecules They vary in strength, but are generally weaker than the bonds that join atoms in molecules (INTRAmolecular forces) ex: covalent bonds, ionic, metallic bonds

Polarity In chemistry, polarity refers to a separation of electric charges (+/-) leading to a molecule having an electric dipole (2 opposite charges on a molecule) Molecular polarity is dependent on the difference in electronegativity between atoms in a compound. Polarity underlies a number of physical properties including surface tension, solubility, and melting & boiling-points.

Polarity and Dipoles Electrons are not always shared equally between two bonding atoms; one atom might exert more of a force on the electron cloud than the other (electronegativity) The unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge. Partial charges are denoted as δ+ (delta plus) and δ− (delta minus)

3 Types of IM Forces Hydrogen Bonds (strongest) Dipole – Dipole London Dispersion Forces – the weakest intermolecular force (one-hundredth to one-thousandth 1/100 – 1/1000 the strength of a covalent bond)

Hydrogen Bonds Hydrogen is a special element. Because it is really just a proton, (atomic # = 1, meaning 1 proton) If the hydrogen in a molecule is bonded to a highly electronegative atom in the second row only (N, O, or F), a hydrogen bond will be formed!

Dipole-Dipole What would happen if we had a beaker of polar molecules, like formaldehyde? The molecules line up; positive ends line up near the other molecule's negative ends:

Dipole-Dipole Interaction However, we only see this sort of attraction between molecules that are polar. It is usually referred to as dipole - dipole interaction.  Clearly, the strength of this attraction increases with the increasing polarity of the molecules.

London Dispersion Forces The forces that hold molecules together in the liquid, solid and solution phases Think of a "snapshot" of a molecule at an instant in time, you would see that there would be slightly different charge distributions caused by the different positions of the electrons in the orbital. We already know that the electrons in the orbitals of molecules are free to move around. 

Sports in Chemistry….kind of Think of a basketball court. If you were to freeze-frame during a game, would all the players be evenly distributed on the court, or bunched up closest to the ball?

Sports in Chemistry….kind of Chances are, players would be unevenly distributed, some blocking, setting picks, etc. London Dispersion forces are similar in that at any given moment, electrons can be here or there…THEY MOVE…really really fast!

Strengths of IM forces London dispersion< dipole-dipole< H-bonding As these increase in strength it becomes harder to remove the molecules from each other. One would expect the melting and boiling points to be higher for those substances which have strong intermolecular forces. As these intermolecular forces increase, so do the energies requires to melt, vaporize, or sublime (go from solid to a gas) a species.

Surface Tension The tendency of a liquid to resist increasing its surface area is called surface tension. (resist spreading out) Liquids with stronger intermolecular forces, such as polar liquids, exhibit more surface tension.

Lab 1-Exploring Intermolecular Forces These notes will be helpful for our upcoming “Intermolecular Forces” Lab and the analysis that follows STAY TUNED for our pre-lab discussion Homework: Work on the Test Review

Daily Starter Day 49 How do London dispersion forces compare to a basketball court, or playing field ?