AP Chem Catalyst

Catalyst Questions

1. Which of these is not polar? H20, CO2, NO2, SO2, NH3

To Do & Homework

Homework/Announcements: 1. Test Corrections due Monday (+.5/1 back for each question) if you identify and correct your mistake and provide an explanation for the correct answer.

2. Study for Quiz on IMFs

2.6 Intermolecular Forces

Kinetic Molecular Theory for Gases

Gases are said to be so fast and tiny that they experience no attraction as gas particles. This is a part of the Kinetic Molecular Theory for Gases. In gases attraction is negligible (can be ignored)

However, liquids and solids differ in properties b/c they experience intermolecular forces (attractions).

In solids, the intermolecular forces are strong enough to hold particles close together and lock them in place.

Kinetic Energy – energy in matter due to motion or movement.

Intermolecular Versus Intramolecular attractions

Intramolecular forces are bonds that hold atoms together within a molecule or substance such as covalent, ionic or metallic bonds.

Intermolecular forces are ATTRACTIONS between molecules!

Thinking and reasoning:

If intramolecular forces are stronger than intermolecular forces:

Will it require more or less energy to vaporize a liquid or melt a solid compared to breaking a covalent bond? Justify

But why are ionic bonds stronger than all IMFs?

Electrostatic interactions get stronger as the magnitude of the charges increases and weaker as the distance between charges increases. The charges responsible for intermolecular forces are generally much smaller than the charges in ionic compounds

Sound familiar? What law proved this? Now Rank these 3 products and justify.

1

2

3

How do forces affect boiling point, or melting point?

Many properties of liquids, including boiling points, reflect the strength of the IMFs.

The molecules of the liquid must overcome their attractive forces to separate and form vapor. The stronger the attractive forces, the higher the temperature at which the liquid boils.

Similarly, the melting points of solids increase as the strengths of the intermolecular forces increase.

Explain this trend and justify why the melting point/boiling points increase moving up the periodic table.

4 Types of IMFs to KnowThree types of intermolecular attractions

exist between electrically neutral molecules:

dispersion forces, dipole–dipole attractions, and hydrogen bonding

The first two are collectively called van der Waals forces after Johannes van der Waals.

Another kind of attractive force, the ion–dipole force, is important in solutions.

Dispersion Forces (London Dispersion Forces)

You might think there would be no electrostatic interactions between electrically neutral, nonpolar atoms and/or molecules. Yet some kind of attractive interactions must exist because nonpolar gases like helium, argon, and nitrogen can be liquefied.

The motion of electrons in an atom or molecule can create an instantaneous, or momentary dipole moment.

The instantaneous dipole on one atom can induce an instantaneous dipole on an adjacent atom, causing the atoms to be attracted to each other (Called Dispersion Forces!)

Works only if molecules are close to each other.

Dispersion forces tend to increase in strength with increasing molecular weight

Shape affects dispersion force strength

Linear > Spherical

Increasing surface area, increase dispersion force strength meaning that MORE thermal energy (heat) is required to melt or boil a substance.

Dipole Dipole Interactions The presence of a permanent dipole moment

in polar molecules gives rise to dipole–dipole forces . These forces originate from electrostatic attractions between the partially positive end of one molecule and the partially negative end of a neighboring molecule

For molecules of approximately equal mass and size, the strength of intermolecular attractions increases with increasing polarity. Notice how the boiling point increases as the dipole moment increases.

Hydrogen Bonding

A hydrogen bond is an attraction between a hydrogen atom attached to a highly electronegative atom (usually F, O, or N) and a nearby small electronegative atom in another molecule or chemical group.

Notice in each case that the H atom in the hydrogen bond interacts with anonbonding electron pair (lone pair)

The hydrogen MUST be DIRECTLY attached to the F, O or N!!!

Hydrogen BondHydrogen bonds can be considered a special type

of dipole–dipole attraction; way stronger than dipole dipole that’s why it’s classified differently.

Because N, O, and F are so electronegative, a bond between hydrogen and any of these elements is quite polar, with hydrogen at the positive end (remember the + on the right hand side of the dipole symbol represents the positive end of the dipole):

Hydrogen has no inner electrons

The hydrogen atom has no inner electrons. Thus, the positive side of the dipole has the concentrated charge of the nearly bare hydrogen nucleus. This positive charge is attracted to the negative charge of an electronegative atom in a nearby molecule.

Because the electron-poor hydrogen is so small, it can approach an electronegative atom very closely and, thus, interact strongly with it. This is why H-Bonds are so strong and polar.

Ion Dipole IMF or attraction

H–F H–F

H–F

H–F H–F

H–F NO3

–

NO3– NH4

+

NH4+

Polar molecules like water, can dissolve ionic crystal lattice structures! This ion and dipole molecule attraction is called an ion-dipole IMF.

Polar covalent

ionic

Polar molecules attract to ions formed when dissolving salts in water.

In which of these substances is hydrogen bonding likely to play an important role in determining physical properties: CH4, H2NNH2, CH3F, H2S (Yes draw the Lewis structure!)

10 minute Break

Comparing IMFs Dispersion forces are found in ALL substances since

electrons move randomly in all atoms. Don’t forget: The strength of these attractive forces increases with increasing molecular weight and depends on molecular shapes.

Dipole Dipole IMFs are due to polarity (partial charges)

H-Bonding due to H bonded directly to F, O, or N attracting another polar molecule.

Ion-Dipole: When a salt dissolves to form ions they attracted toward polar molecules w/ dipole moments.

Order of strength:

Intermolecular: LDF < Dipole Dipole < H-bond < Ion-Dipole

Intramolecular: Metallic < Covalent < Ionic (there are exceptions)

List the substances BaCl2, H2, CO, HF, and Ne in order of increasing boiling point.

Application #2: Viscosity The resistance of a liquid to flow is called viscosity

The greater a liquid’s viscosity, the more slowly it flows, the molecules move slower due to stronger attraction.

Viscosity depends on the attractive forces between molecules and on the shapes and flexibility of the molecules

For a series of related compounds, viscosity increases with molecular weight, sound familiar? Weak dispersion Forces!

#’s are increasing going down

Application #3: Surface Tension

Surface tension is a phenomenon in which the surface of a liquid, where the liquid acts like a thin elastic sheet.

The higher the surface tension, the stronger the intermolecular force. In this case, hydrogen bonds.

Application #4: Phase Changes

AP Chem Catalyst

Catalyst Questions

In which of these substances is hydrogen bonding likely to play an important role in determining physical properties: CH4, H2NNH2, CH3F, H2S (Yes draw the Lewis structure!)

To Do & Homework

Homework/Announcements: Review pi and sigma bonds.

Application #1: IMFs and Column Chromatography

The stationary phase is the substance used to hold onto the mixtures that you apply.

Here, stationary phase is a substance called ortho-nitro-phenol. It is polar but has hydrogen!!

If you apply a mixture, separation will occur because the substance with compounds that have stronger IMFs will remain on top while those with low IMFs will go to the bottom. Then you extract each layer to purify the substances.

The same happens with paper chromatography except those with stronger IMFs remain at the bottom.

Can hydrogen bond!

Intermolecular Forces

Station 1: Surface Tension

Station 2: Volatility

Station 3: Surfactants

Station 4: Polarity

Station 5: Magic Sand

Understand real life application of intermolecular forces

Apply and compare the following forces: London Dispersion Forces, Dipole-Dipole H-Bonding, and Ion-Dipole Force

Quiz #9

① At room temperature, Si (silicon) is a solid, and CCl4 is a NONPOLAR liquid. Which exhibits a stronger intermolecular force leading to a higher boiling point? What is the name of this force?

② Water dissolves a salt such as KCl. What is the name of the intermolecular force involved once the salt dissociates (separates) into ions?

③ Which type of intermolecular force operates only between polar molecules?

④ Describe what a London Dispersion Force is.

Volatility, Vapor Pressure and Temperature: 

When a liquid turns into a gas by disrupting the intermolecular forces, a pressure is exerted (released) by the vapor in the space above the liquid itself.  This of a Coke, shake it, the vapor pressure is the pressure exerted when you open the lid.  This is called vapor pressure. Increasing temperature, increases vapor pressure. 

The higher the boiling point, the higher the vapor pressure due to stronger intermolecular forces.  

Liquids that can evaporate readily are said to be volatile. The weaker the IMFs, the more volatile the substance is. Such as the acetone and ethanol in the lab, they were easily evaporated because their volatility was high and so the IMFs were weak.  

AP Chem Catalyst

Catalyst Questions

Determine the electron configuration for magnesium. Both the noble gas configuration and the long version.

Contrast sigma bonds and pi bonds.

To Do & Homework

Homework/Announcements: Review pi and sigma bonds.

Hydrogen’s PES

Helium’s PES

PES Analysis: Focus Questions

Do energies in the same orbital have the same or different energies?

What is the electron configuration for each spectra?

What is the identity of the element for each spectra?

Why do the energies increase for inner electrons or electrons closer to the nucleus?