Nature of solutions

We took a little step away from colligative properties today to make sure we're all familiar with some of the terminology of solutions.  Solubility is not a "yes/no" question.  Is potassium nitrate soluble?  Yes, all potassium salts are soluble and all nitrate salts are soluble, so potassium nitrate must be soluble.  If we take 1 gram of KNO3(s) and add it to 1L of water, it will dissolve.  What if we take 1 kilogram of KNO3(s) and add it to 1mL of water?  Will it dissolve?  Yes, but will ALL of it dissolve?  I think not.  If we start with 1L of water and slowly add KNO3(s), at some point the excess KNO3(s) will no longer dissolve because the solution has become saturated.  A saturated solution represents the maximum amount of a solute that can dissolve in a given amount of solvent.  Sometimes, a solution can become supersaturated when "extra" solute is dissolved; supersaturated solutions are unstable and will form precipitate if a nucleation site is present.  This can be a speck of dust, or a scratch in the glass of the container, or a seed crystal of the substance.

We also looked at the temperature dependence of solubility.  For solids dissolving in liquids, heating the solution will usually allow more solute to dissolve.  This is one way to make supersaturated solutions; a hot saturated solution is allowed to cool in the absence of nucleation sites.  For gases dissolved in liquids, the situation is reversed; cold solvent is usually able to hold more dissolved gas than hot solvent because the gas solute particles in the hot solution have more kinetic energy and are more likely to escape from the solution.

Returning to colligative properties, we began to discuss the most important colligative property in biological systems, osmosis.  When two solutions of differ in concentration are separated by a semipermeable membrane, solvent tends to flow from the less concentrated side to the more concentrated side.  What's a semipermeable membrane?  Cell walls.  Skin.  LOTS of biological things are semipermeable membranes and control function by regulating osmosis.  Awesome.

There's new OWL posted, and don't forget to take the pre-lab quiz before 8:00am tomorrow.

Freezing Point Depression and Boiling Point Elevation

Continuing with colligative properties, if the presences of a solute affects the vapor pressure of a solution, then it must also affect the boiling point.  Liquids "boil" when the vapor pressure of the liquid is equal to the atmospheric pressure on that liquid.  If a solute is added to a solvent, the vapor pressure is depressed, so the temperature must be increased further to increase the vapor pressure to the point that it is equal to the atmospheric pressure.  Boiling point elevation is determined by the equation:

ΔT_bp = k_bpe•m•i

Where m = molality = (mols solute) / (kg solvent)

i = van't Hoff factor

The van't Hoff factor, sometimes denoted as "n", describes the number of particles formed when a solute dissolved.  The presence of a solute also affect the freezing behaviour of a solvent by changing the energetics of the system.  For a solvent to freeze when solute is present, the average kinetic energy of the particles must be slowed even more (temperature must be lower) than for a pure solvent.  Freezing point depression uses essentially the same equation, although with different values for the constant.

ΔT_fp = k_fpd•m•i

For the problems we looked at in class, the answers are below.  The problem was "23.381g of “salt” dissolved in 500.00mL of water, what is the freezing/boiling point?":

Compound	m	i	ΔTfp	Tfp	ΔTbp	Tbp
KNO3	0.462522	2	1.72	-1.72	0.48	100.48
Na3PO4	0.331763	4	2.47	-2.47	0.69	100.69
Mg(ClO3)2	0.244565	3	1.36	-1.36	0.38	100.38
(NH4)2SO4	0.353884	3	1.97	-1.97	0.55	100.55
CaCl2	0.421340	3	2.35	-2.35	0.66	100.66

Southwestern Advantage

Many of you may have had a short presentation/survey in some of your classes from representatives of Southwestern Advantage during which they talked about a summer "internship" opportunity.  I did not have them come to our Gen Chem class because I knew that they came to the Bio class that most of you are in at 9:30 and I didn't think you all needed to have a second dose of their sales pitch.

It has come to my attention that the representatives of Southwestern Advantage may have been a bit overzealous in their tactics when strong-arming their way into some classrooms.  A message was sent to faculty from the Career Development Center :

It has come to our attention that representatives from Southwestern Advantage are visiting your classes to explain their Internship/ Employment Program and to request that a survey be completed by your students. We also understand that they are stating their visit has been authorized by The Career Development Center and/or our Director, Greg Toutges. This is not the case.

During their pitch in class, I have also noticed that the recruiter is very reluctant to describe exactly what this "excellent internship and independent business opportunity" is.  Southwestern Advantage is a door-to-door bookselling business.  They sell books and "study systems" that are intended to help pre-college students with their studies.  I have not seen these products, so I don't know whether they're good or not, but doing a quick web search leads me to believe that the products sold by Southwestern Advantage are quite expensive, and given the wealth of FREE information and tutorials available online, I personally would never pay the prices I saw mentioned even if my child was struggling.

Southwestern Advantage uses an independent contractor/seller model, so although the recruiter very likely spoke about earning $8000 during the summer (a number he used when talking to me), that number may not be realistic, and may require 10-15 hour days, 7 days a week for weeks at a time.  In addition, there will be living expenses that you would not incur while living at home and working for minimum wage, so if you are considering exploring a summer job with Southwestern Advantage, make sure you really analyze the numbers they provide, although I would expect that they will offer very few concrete details until you have signed a contract.  Sales can be a very good career for some people, but it's not for everyone.  Set up a spreadsheet to calculate income and expenses to compare your various summer option before you are coerced into signing a contract.

A few students have also mentioned that the Southwestern Advantage recruiter was asking for Dragon ID#'s and social security numbers.  I did not attend the presentation in any classes this year, but if the recruiter was really asking for this type of information, I would be EXTREMELY suspicious of their intent, or at the very least their tactics.  In addition, when Lucas Odegard, the "Corporate Recruiter" who probably talked to your class, met with me about coming into Gen Chem, he consistently referred to all of you as "kids".  Like it or not, you are not "kids", you are adults.  If Mr. Odegard considers you all to be "kids", it seems to me that he has a profound lack of respect for all of you, and merely sees you as another resource or product that he may be able to use to make money.

I am sure that there are pre-college students who have benefited from Southwestern Advantage's products, and I am sure that there are college students who have earned good money selling these products, but I have been extremely unimpressed with the tactics that have been used by Southwestern Advantage on our campus.  If you choose to explore this opportunity, please make sure you are fully informed and are not taken in by a slick and predatory sales pitch.

Colligative Properties

"Solubility" is not a yes/no answer, it's always a matter of degree.  Although we may describe something as "soluble", there obviously must be a limit to that solubility: NaCl is "soluble" in water, but you can't dissolve a bucket of NaCl in a glass of water.  Similarly, if we look back at the solubility rules we used in Gen Chem I, just because something is described as "insoluble" doesn't mean that none of it will dissolve, it just means that very little will dissolve.  It's an important distinction and we'll re-visit it in a few weeks.

When a solute is added to a solvent, the properties of that solvent are affected.  A colligative property is one that is dependent upon the number of solute particles present in the solution and not necessarily the identity of those solute particles.  If 0.1mol of sucrose and 0.1mol of fructose (0.2mols of solute particles) are dissolved in 10.0L of water, the colligative properties of the solution will change by the same amount as if 0.2mols of sucrose alone is dissolved in 10.0L of water.  We looked at vapor pressure depression as a colligative property; the presence of a solute decreases the vapor pressure of a solution due to solvent-solute interactions and surface blocking.

Next week we'll look at more colligative properties.  Have a good weekend.

The solution to solution is solution

When a volatile liquid is in an open container, it evaporates.  When a volatile liquid is in a closed container, it builds up vapor pressure.  Vapor pressure is the function of the temperature of the system (an indication of the average kinetic energy of the particles) and the intermolecular forces holding the particles together in the liquid state.  It represents a dynamic system, where the rate of liquid particles vaporizing to the gas phase is exactly equal to the rate of gas particles condensing to the liquid phase.

The properties of pure liquids are fascinating, but they become even more interesting when we add another component to the system.  A solution is a homogeneous mixture of two or more components.  The major component(s) is/are the solvent(s); the minor component(s) is/are the solute(s).  When a solute dissolves in a solvent, solvent-solvent and solute-solute interactions must be broken (requires energy) and solvent-solute interactions must form (releases energy).  If the energy released is greater than the energy required, a solution forms.

We also reviewed molarity and stoichiometry problems.  All stoichiometry problems follow the same 4 steps:
1. Write a balanced chemical equation
2. Convert the quantity of the known compound(s) to moles
3. Using the mol-mol ratio in the balanced chemical equation, convert moles of known substance to moles of interest
4. Convert moles of interest to whatever you're looking for

Wow, big day.  See you in lab tomorrow.

Friday - Solids and liquids

On Friday we looked at some similarities, differences, and various properties of solids and liquids.  The behaviour of many solids can be explained by comparing the relative magnitude of intramaterial intermolecular forces to the intermaterial intermolecular forces that would allow the solids to melt, sublime, or otherwise break apart.  When looking at the properties and behaviour of liquids, these comparative intermolecular forces become even more important because liquid is an intermediate state between solids and gases.

Surface tension and capillary action are due to relative liquid-liquid, liquid-atmosphere, liquid-surface IMFs.  Viscosity and volatility are due to the relative magnitude of IMFs compared to the average kinetic energy of a sample.

Sorry I didn't get this posted sooner, it kind of slipped away from me.  Enjoy the rest of your weekend.

Transition to transitions (!!)

Today we looked at Dalton's Law of Partial Pressures and did a little bit of derivation and/or proof of where this Law comes from.  {English note: ending a sentence with a preposition is not great, perhaps that should have been "proof of whence this Law came"}  An ideal gas follows all gas laws exactly and does not violate Kinetic Molecular Theory of Gases; when real gases are studied, there are deviations, especially at high pressure and low temperature.  Some of these deviations are phase changes.  The behaviour of a sample as energy is added or removed can be visualized as a series of heat capacity and enthalpy events in a heating/cooling curve.  If heating/cooling curves are observed at multiple pressures, a phase diagram can be constructed.

Don't forget to get signed up for OWL and look at the currently posted assignments.  And as mentioned below, Chem 210L labs will not meet this week.

Chem 210L this week

I just realized that I forgot to mention in class yesterday that Chem 210L labs WILL NOT MEET THIS WEEK.  We will get rolling in lab next week, January 19th.  There will be a pre-lab quiz for next week's lab, so make sure you keep an eye on your email for a message that the lab info is posted and ready in D2L.

States of Matter - Gases

As we begin looking at states of matter, we start with gases.  Gases are convenient to study because many real gases behave in a very theoretically "correct" manner, meaning that their behaviour can be understood and explained using the Kinetic Molecular Theory of Gases: 1)gas particles are very much smaller than the space between the particles; 2) gas particles move randomly; 3) except during collisions, attractive and repulsive forces between particles are negligible when compared to the kinetic energy of the gas particles; 4) collisions are elastic; 5) the average E_kin of the particles in a sample of gas is proportional to the absolute temperature of that sample.  KMToG can be used to explain a number of gas laws including Avogadro's (V ∝ n), Boyle's (V ∝ 1/P), Charles' (V ∝ T), and the Ideal Gas Law (PV=nRT).

Next time, we'll wrap up gases and move on to liquids and solid as well as the phase changes between them.

Chem 210 - Spring 2012 - IT HAS ARRIVED!!

The semester has started, I hope everyone had a restful break and is ready to dive in.  Remember to get signed up in OWL and take a look at the first assignments.  I will try to have smaller assignments more often, so there should be something active in OWL just about any time you log in.  Make sure you work on those assignments early.

I'm going to try a little experiment this semester, I will be tweeting class topics using the hashtag #GenChem2012.  If you're a Twitter user, you know what that means.  If you're not, don't worry.  I'll also be posting class info here on the blog.  And, of course, I'll be giving info IN CLASS.