Unit 1 Module 1 SS 2 Notes

Global Chemistry Lessons

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2.1

State the various forces of attraction between particles.

Ionic bonds, covalent bonds, metallic bonds, van der Waals' forces.

Both bonding (intramolecular) forces and intermolecular forces arise from electrostatic attractions between opposite charges. Bonding forces are due to the attraction between cations and anions (ionic bonding), nuclei and electron pairs (covalent bonding), or metal cations and delocalized valence electrons (metallic bonding). Intermolecular forces, on the other hand, are due to the attraction between molecules as a result of partial charges, or the attraction between ions and molecules. The two types of forces differ in magnitude, and Coulomb's law explains why:

• Bonding forces are relatively strong because they involve larger charges that are closer together.
• Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart.

Ion-Dipole Forces

When an ion and a nearby polar molecule (dipole) attract each other, an ion-dipole force results. The most important example takes place when an ionic compound dissolves in water. The ions become separated because the attractions between the ions and the oppositely charged poles of the H2O molecules overcome the attractions between the ions themselves.

Dipole-Dipole Forces

When polar molecules lie near one another, as in liquids and solids, their partial charges act as tiny electric fields that orient them and give rise to dipole-dipole forces: the positive pole of one molecule attracts the negative pole of another (diagram below).

Polar molecules and dipole-dipole forces. In a solid or a liquid, the polar molecules are close enough for the partially positive pole of one molecule to attract the partially negative pole of a nearby molecule. The orientation is more orderly in the solid (left) than in the liquid (right) because, at the lower temperatures required for freezing, the average kinetic energy of the particles is lower. (Interparticle spaces are increased for clarity.)

For molecular compounds of approximately the same size and molar mass, the greater the dipole moment, the greater the dipole-dipole forces between the molecules are, and so the more energy it takes to separate them. Consider the boiling points of the compounds in the next diagram. Methyl chloride, for instance, has a smaller dipole moment than acetaldehyde, so less energy is needed to overcome the dipole-dipole forces between its molecules and it boils at a lower temperature.

Dipole moment and boiling point. For compounds of similar molar mass, the boiling point increases with increasing dipole moment. (Note the increasing color intensities in the electron density models.) The greater dipole moment creates stronger dipole-dipole forces, which require higher temperatures to overcome.

The Hydrogen Bond

A special type of dipole-dipole force arises between molecules that have an H atom bonded to a small, highly electronegative atom with lone electron pairs. The most important atoms that fit this description are N,0, and F. The H-N, H-O,
and H- F bonds are very polar, so electron density is withdrawn from H. As a result, the partially positive H of one molecule is attracted to the partially negative lone pair on the N, 0, or F of another molecule, and a hydrogen bond (H bond) forms. Thus, the atom sequence that allows an H bond (dotted line) to form is -B:····H-A-, where both A and B are N, O, or F. Three examples are

The small sizes of N, O, and F are essential to H bonding for two reasons:

1. It makes these atoms so electronegative that their covalently bonded H is
    highly positive.


2. It allows the lone pair on the other N, O, or F to come close to the H.

The Significance of Hydrogen Bonding

Hydrogen bonding has a profound impact in many systems. Here we'll examine one major effect on physical properties and preview its enormous importance in biological systems.

How To Use Red Mud As A Catalyst

Can chemists come up with better uses of mineral resources to make catalysts that are more sustainable? For a growing number of researchers, the answer is yes, and the key is taking advantage of materials that are already out of the ground. Red mud, the noxious by-product of the Bayer process for extracting aluminum from bauxite ore, makes a good case study.

The majority of material processed in mining operations ultimately goes to waste. For every ton of alumina extracted from bauxite, more than a ton of red mud is produced; aluminum mining leaves behind some 120 million metric tons per year of the salty, highly alkaline, heavy-metal-laden material, according to the International Aluminum Institute. Some 4 billion metric tons of the  material is lying about globally, much of it held in retention ponds.

Mining companies have long tried to find ways to recycle the environmentally problematic red mud. It is a classic problem in search of a solution. One approach is neutralizing red mud with seawater or treating it with CO2 or sulfur compounds. The modified materials have been tried as fill for mining and construction, as pigment and filler for bricks and cement, and as a sorbent for water treatment. Others have looked at extracting more aluminum from red mud, or obtaining other useful metals such as sodium, copper, and nickel. But so far there have been few safe and economical large-scale applications.

On a new front, some chemists are trying to go catalytic, focusing on iron oxide, the chief component of red mud. But given the purity and properties of red mud, researchers have found it typically is not an active enough catalyst to compete against existing commercial catalysts. That’s because the mineral composition, particle size, and surface properties are important in developing heterogeneous catalysts. With red mud, finding the right combination is a work in progress.

One early sign of success comes from Foster A. Agblevor of Utah State University’s USTA Bioenergy Center and coworkers in conjunction with Pacific Northwest National Laboratory researchers. They have been testing red mud as a bulk catalyst to replace zeolites in a fluidized-bed reactor to pyrolyze biomass to make crude oil.

The team processes the biocrude oil using a traditional catalytic hydrotreating process to make a gasoline- type fuel and has tested it on a lawn mower or lawn trimmer. “We are able to run an engine on the fuel without difficulty,” Agblevor says. The Utah State researchers have applied for a patent for their process. They are working with catalyst company Nexceris to scale up catalyst production and with Wildland Forestry & Environmental to harvest wood from pinyon-juniper range lands in the western U.S. to scale up biofuel production.

The team is also expanding the scope of using red mud beyond biomass pyrolysis, Agblevor says. The researchers have applied the catalyst to coal gasification, he notes, as well as to a process for catalytic pyrolysis of waste tires for fuel production. Despite raw red mud’s ultimate utility as a catalyst, its story points to other possibilities for recovering metals that have already been extracted and used. For example, industrial processing, the use of consumer goods and medicines, and even the wearing away of jewelry leads to measurable amounts of catalyst metals such as gold, silver, and platinum accumulating at wastewater treatment plants.

Weak Base – Strong Acid Titration Curves

The titration of a weak base (NH3) with a strong acid (HCl) is shown below. Note that the curve has the same shape as the weak acid-strong base curve, but it is inverted. Thus, the regions of the curve have the same features, but the pH decreases throughout the process:

Curve for a weak base-strong acid titration. Titrating 40.00 mL of 0.1000 M NH3 with a solution of 0.1000 M HCl leads to a curve whose shape is the same as that of the weak acid-strong base curve,

but inverted. The midpoint of the buffer region occurs when [NH3] = [NH4+].

Methyl red is a suitable indicator here.

1.  The initial solution is that of a weak base, so the pH starts out above 7.00.

2.  The pH decreases gradually in the buffer region, where significant amounts of base (NH3) and conjugate acid (NH4+) are present. At the midpoint of the buffer region, the pH equals the pKa of the ammonium ion.

3.  After the buffer region, the curve drops vertically to the equivalence point, at which all the NH3 has reacted and the solution contains only NH4+ and Cl-. Note that the pH at the equivalence point is below 7.00 because Cl- does not react with water and NH4+ is acidic:

                                 NH4+(aq) + H2O(l) ⇌  NH3(aq) + H3O+(aq)

4. Beyond the equivalence point, the pH decreases slowly as excess H3O+ is added.

For this titration also, we must be more careful in choosing the indicator than for a strong acid-strong base titration. Phenolphthalein changes colour too soon and too slowly to indicate the equivalence point; but methyl red lies on the steep portion of the curve and straddles the equivalence point, so it is a perfect choice.

Why Warm Water Freezes Faster Than Cold

Nearly 50 years ago, Erasto B. Mpemba and Denis G. Osborne reported that if samples of water at 90 °C and 25 °C are cooled, the one starting at 90 °C begins freezing first. Many explanations for the “Mpemba effect” have been proposed, including ones based on evaporation, temperature gradients, impurities, and dissolved gases.

In warm water, weak hydrogen bonds break (top, red squiggles), leaving fragments

that easily reorganize into an ice lattice (bottom), a new study says.

A new computational study suggests that the effect arises from the liquid’s hydrogen bond network (J. Chem. Theory Comput. 2016, DOI: 10.1021/acs.jctc.6b00735). Southern Methodist University’s Dieter Cremer and colleagues investigated clusters of 50 and 1,000 water molecules, characterizing the types and strengths of the clusters’ 350 and more than 1 million hydrogen bonds, respectively. In (H2O)1,000 , raising the temperature from 10 °C to 90 °C led to fewer hydrogen bonds, as weaker, predominately electrostatic bonds broke.

That left behind cluster fragments with strong hydrogen bonds with more covalent character and proportionately more “dangling” or terminal hydrogen bonds. That hydrogen bond combination enables the fragments to easily reorganize and form the hexagonal lattice of ice.

Apart from learning what the name of this effect is and why it occurs, you can now answer two unit 1 past paper questions with the knowledge that:

1. Hydrogen bonds are largely electrostatic in nature.

2. Each water molecule forms four hydrogen bonds (from diagram).