Another question...

Can you explain this? I don't understand the differences between the interactions.
3. Which of the following is true regarding intermolecular forces?
a. Hydrogen bonds are as strong as covalent bonds
b. On average, dipole-dipole interactions are weaker than induced dipole-induced dipole
c. Induced dipole-induced dipole interactions are stronger for large atoms/molecules
d. Hydrogen bonds occur whenever a molecule contains hydrogen
e. Hydrogen bonds are just a special kind of induced dipole-induced dipole interaction

Let's take these one at a time.
a. Hydrogen bonds are as strong as covalent bonds
Covalent bonds are the bonds that hold molecules together. They are MUCH stronger than any intermolecular force. If IMFs were stronger than covalent bonds, the IMFs would rip molecules apart.
b. On average, dipole-dipole interactions are weaker than induced dipole-induced dipole
Induced dipole-induced dipole interactions (or if you prefer, you can call them London forces, or dispersion forces, or London dispersion forces) are temporary dipoles than change over time. Although some induced dipoles can be very strong and some regular dipoles can be quite weak, an average London force will be weaker than an average dipole-dipole interaction.
c. Induced dipole-induced dipole interactions are stronger for large atoms/molecules
Induced dipoles form when electron clouds are deformed. The bigger an electron cloud, the easier it is to deform. This can be because it's part of a big atom (like I or Po), or because it's part of a big molecule (like long hydrocarbons). The more an electron cloud can deform, the stronger it's induced dipole will be.
d. Hydrogen bonds occur whenever a molecule contains hydrogen
The hydrogen involved in a hydrogen bond has to be polar, that is, it has to be part of a polar bond. A molecule of H₂ contains hydrogen, but the H-H bond is not polar, so we wouldn't call its IMFs hydrogen bonds.
e. Hydrogen bonds are just a special kind of induced dipole-induced dipole interaction
H-bonds are a special/specific type of dipole-dipole interaction.

So it looks like the correct answer is "c".

I will not have access to the blog or email again until tomorrow morning, I will answer any questions I get overnight as soon as I get to my office tomorrow. {Usually by 7am}

Exam tomorrow...

Yesterday in class we (more or less) reviewed for the exam. We added normality to our list of concentrations, normality is closely related to molarity.

Since people didn't have any additional questions before the exam, we also started talking about kinetics, we'll dig deeper into rates starting Friday.

A few questions have come in:

How do we do a problem like this? (from Spring 2005 exam)
10. What is the mol fraction of sugar (C₆H₁₂O₆) in a saturated aqueous sugar solution at 25ºC?
(Solubility of sugar in water at 25ºC = 211.4 g/100.mL)
a. 0.1744
b. 0.6789
c. 11.73
d. 2.114
e. 0.2112

This is largely just a unit conversion problem. We're looking for mol fraction of sugar in water, so we need to know (mols of sugar), (mols of water), and (total mols). Looking at that solubility, we can convert 211.4g of sugar to mols (211.4g / 180.155g/mol = 1.1734mols sugar), and we can convert 100.mL of water to mols (100.g / 18.015g/mol = 5.5509mols water). The mol fraction of sugar is (1.1734mols sugar / (1.1734+5.5509)total mols = 0.1745) so the answer should be "a". Just a quick note on test strategy here, you know by definition that the only possible values of mol fraction are between 0 and 1; therefore, you know that "c" and "d" must not be correct. Even if you had to guess, eliminating those two impossible answers increases your odds from 20% to 33%.


Hi, for the Spring 2008 exam you gave, I was hoping you could post how to do number six regarding vapor pressure on the blog.
6. If each of the following solids is added to 500.0mL of water, which will change the vapor pressure the most?
a. 1.2mols sugar
b. 0.4mols calcium phosphate
c. 0.6mols sodium chloride
d. 0.5mols calcium nitrate
e. 0.7mols ammonium phosphate

We worked through this one in class on Monday. Since all of these solutes are being dissolved in the same amount of water, the one that provides the most solute particles will affect the vapor pressure the most. Sugar is a molecular solute, so 1.2mols of sugar will result in 1.2 mols of solute particles in solution. Calcium phosphate will break into 5 pieces, so 0.4mols of Ca₃(PO₄)₂ will result in 5(0.4) = 2.0mols of particles in solution. Sodium chloride = 2 particles, 2(0.6) = 1.2mols particles. Calcium nitrate = 3 particles, 3(0.5) = 1.5mols particles. Ammonium phosphate = 4 particles, 4(0.7) = 2.8mols particles. Since ammonium phosphate results in the most solute particles, it is expected to affect the vapor pressure the most.

Other questions, let me know.

Perhaps the most important force in biological systems...

Today we talked about osmosis, osmotic pressure, colloids and surfactants. These are all extremely important concepts in biological systems, so they should be pretty important to all of us biological organisms.

There is a new Mastering Chemistry assignment, due Tuesday.

Exam is next Wednesday. Old exams are posted on my MSUM web page (www.mnstate.edu/bodwin), if you haven't looked at them yet you might want to take a peek.

Have a good weekend, if you have any questions feel free to email me, I will answer all questions to the blog.

Solution behavior and colligative properties

In class Wednesday we reviewed concentration units and worked through a vapor pressure depression/boiling point elevation/freezing point depression problem using potassium bromide in water. We also talked a little bit about how temperature affects solubility of solids or gases in liquid solvents.

There is a new Mastering Chemistry assignment posted, due Monday.

Exam 1 has been moved up to next Wednesday, January 28th. Please make note of this and remind anyone else who is registered for this class.

See you all on Friday...

Phase diagrams and solution

On Friday we took the 2-dimensional heating/cooling curves we talked about on Wednesday and translated them into a phase diagram that relates pressure and temperature to the states of matter for a substance. Although understanding how pure substances behave is important, the majority of "matter" we encounter is not pure substance but various mixtures, so we started talking about solutions and how addition of a solute can change physcial properties. These colligative properties depend upon the number of solute particles present in the solution, not the identity of those solute particles.

See you all on Wednesday. (or Tuesday if you're in my lab section...)

Liquids and phase changes

Today we talked about some properties of liquids that can be explained using intermolecular forces including viscosity, surface tension, capillary action and vapor pressure. We then moved to phase transitions and looked at how a heating/cooling curve could be used to understand how much energy in transferred as heat is added or removed from a substance.

There is a new Mastering Chemistry assignment posted, due Monday. My web page is back up so you should be able to access all the information found there, please let me know if there are broken links or missing files.

See you all on Friday.

First day...

Today we ran through the syllabus and began looking at the interaction between energy and matter as related to states of matter.

Mastering Chemistry Course ID = C210SP09JB

If anyone has tried to look at my MSUM web page, I had a slight problem this morning and managed to destroy my page. I've gotten a few bits of info back up, but it will likely take a day (or 2 or 3) to get everything in full working order. Sorry about that.

Welcome to Chem 210

Welcome to Dr. Bodwin's blog for the spring semester. Check back for class announcements and other info.