Monday, February 8, 2010

12.1.4 How Do You Define Equilibrium?

1st post = 10 points
2nd post (reply to someone else's) = 5 points
total = 15 points

When we explored stoichiometry earlier in the course, we worked with reactions in which the limiting reagent was entirely consumed. This is characteristic of reactions that go to completion. The products are formed until one of the reactants has been completely consumed.

As we've just seen, many reactions don't go to completion. They reach a state in which there is a mixture of reactants and products. We say that the reaction is reversible because when equilibrium is reached, the forward and reverse directions of the reaction are occurring at the same rate.

In theory, all reactions are reversible, as long as the reactants and products remain in contact with one another. But what does this mean in practice? Did we make some incorrect assumptions about the reactions we studied earlier in the course?

Consider the combustion of methane:

CH4 (g) + 2 O2 (g) CO2 + 2 H2O (g)

Notice that we've shown this as a reversible reaction. However, when we studied the ideal gas law, we used the reaction stoichiometry to calculate the volume of carbon dioxide that would be produced from a certain mass of methane. In this case, we assumed that the reaction went to completion.

Now that you know about dynamic equilibrium and the equilibrium constant, consider the following question: When is it safe to assume that a reaction goes to completion, and when must you treat it as a reversible reaction? What information would you need in order to make a decision, and how would you use that information to decide whether or not a reaction is reversible?

Saturday, December 5, 2009

8.2.6 Where is the Boundary?

Ideal gases must satisfy two conditions: the gas particles must occupy negligible volume relative to the container volume, and there must be no attractive or repulsive forces between gas particles. At high pressure, the volume between particles (the "free volume") is actually much less than the container volume. Equally important, when gases are subjected to high pressure, the particles can be close enough to interact.

The Dutch physicist Johannes van der Waals developed an equation to correct for the non-ideal behavior of gases. It's based on the ideal gas law and includes a factor that adjusts P upward to compensate for the decrease in P due to intermolecular attractions in real gases. The van der Waals equation also includes a factor that adjusts V downward, to give a measure of only the free volume in the container by excluding the volume occupied by gas particles. The van der Waals equation is:

The a and b terms are called the van der Waals constants and depend on the identity of the gas. For hydrogen gas, the van der Waals constants are:

a = 0.244

b = 0.0266

Imagine that you have a 1.00 mole sample of hydrogen gas in a 1.00 L container at 0.00C. Discuss whether you believe hydrogen gas is an ideal gas under these conditions. How do you decide? If you believe the gas is not ideal under these conditions, under what conditions of P, V, or T would the gas behave ideally? And what criteria do you use to decide? Explain your reasoning to your classmates, and respond to their explanations.

Scoring

This discussion is worth a maximum of 15 points. You'll get 10 points for participating. Your instructor will give you another 5 points if you post a follow-up comment or question that furthers the discussion.

Wednesday, November 25, 2009

7.2.2 What Kind of Isomer

We have just introduced four different types of isomers, which fall into two main classes. Structural isomers include linkage isomers and coordination isomers. Stereoisomers include geometric isomers and optical isomers. Structural isomers are compounds with identical molecular formulas, but different connections between the atoms. Stereoisomers are compounds with identical molecular formulas and identical connections between the atoms, but different arrangements of the atoms in space.

Linkage isomers are compounds that differ in the atom of a ligand that binds to a metal ion in a coordinate covalent bond. Coordination isomers, on the other hand, are compounds that differ only in the way the ligands and the counter-ions are distributed around the central ion. This seems confusing. Have you come up with a way to keep from confusing linkage isomers and coordination isomers?

Geometric isomers are compounds that differ only in the relative positions of some atoms. For example, trans-2-butene and cis-2-butene differ only in the positions of the CH3 groups relative to the double bond. Optical isomers differ only in the way they rotate plane-polarized light. These pairs of molecules are called chiral. Once again, the distinction between geometric isomes aand optical isomers seems confusing. Have you thought of a way to keep geometirc isomers and optical isomers straight?

In this discussion, first compare notes on the different types of isomers we have covered to make sure that everyone agrees on the different classes of isomers. Use an example if possible to help explain your answer. Then tell your classmates the ways that you have developed to determine what ype of isomer a pair of compounds represents. Also, use this as an opportunity to discuss what you find most confusing about isomers and see if your classmates have suggestions about making isomers more understandable.

Monday, October 26, 2009

5.1.3 What is a Color?

In the study called "Light: Waves and Photons," you explored the relationship between light and color. Within the visible spectrum, the shortest wavelengths of light (roughly 400 nm) are blue, and the longest wavelengths (around 700 nm) are red. These numbers represent the wavelengths of light not absorbed by compounds having these colors. For instance, a solution of copper sulfate appears blue because when it is exposed to electromagnetic radiation it transmits light with wavelengths near 400 nm and absorbs all other wavelengths. It's really the reflection of blue light, and the absence of colors at wavelengths that are far from 400 nm that our minds interpret as blue.

Now let's look at the color green. According to the visible spectrum presented in the study, green light has a wavelength of about 500 nm But does this make sense? Perhaps when you were younger and just learning to paint and draw, someone told you that yellow and blue make green. You figured out how to mix yellow and blue paints to get green paint. So is green a single wavelength or a combination of two different wavelengths?

Consider three green objects: a green sweater, the leaves on a tree, and the coating on green M & M's™. Imagine that these are all the exact same shade of green. Why are they green? Are they "pure" green or a mixture of yellow and blue? Is there any way you could tell the difference? Follow the directions below to go online and share your ideas with your classmates!

Scoring

This discussion is worth a maximum of 15 points. You'll get 10 points for participating. Your instructor will give you another 5 points if you post a follow-up comment or question that furthers the discussion.

Tuesday, October 13, 2009

4.2.7 Heat and Temperature: What Is the Difference?

Discussion Topic

Heat and temperature are often used interchangeably in everyday life. It won't come as a surprise to anyone that when you increase the heat of an object, the temperature will go up. But do heat and temperature really mean the same thing? Should the terms be used interchangeably?

We have introduced the concept of heat energy. Heat is the energy that flows into or out of a system because of a difference in temperature between the system and the surroundings. Heat always flows from a hot object to a cooler object. Heat transfer occurs until all objects are the same temperature. If a hot object is put in surroundings that are the same temperature, no heat energy flows, because there is no temperature gradient.

The temperature of an object is due to the internal kinetic energy of the particles in it. When heat flows from a hot object to a cooler object, the particles within the hot object slow down. This means the object's kinetic energy decreases. So temperature reflects the amount of heat energy in the system.

One striking difference between heat energy and temperature is their dependence on the amount of substance in the system. The amount of heat energy required to change the temperature of a substance depends on the amount of substance in the system. Imagine you have a 100 gram sample of pure water at 50C which has a heat energy of q. If you divide the water into two equal portions, what will the temperature and heat energy of each of the portions be? Consider the definitions of heat and temperature as you formulate your answer, then go online to share your ideas with your classmates.

Scoring

This discussion is worth a maximum of 15 points. You'll get 10 points for participating. Your instructor will give you another 5 points if you post a follow-up comment or question that furthers the discussion.

Saturday, October 3, 2009

3.2.10 Discussion

Great job on the last discussion! Very interesting comparisons for the size of a mole. Here is the next discussion question, with one difference: you only need to pick only 1 thing to discuss for your first post (I broke them into 9 separate questions)--try to choose one that has not already been picked and list which number you choose (#1-9)--10pts. Then comment on somebody else's post (is it easy to understand? did they make it sound more confusing? is there anything else to add to it? etc.) 5pts.

In this unit, you have heard about many different types of stoichiometry problems. This discussion gives you an opportunity to share your insights about solving stoichiometry problems with your classmates. Generally, if you are having trouble with a particular topic, someone else in the class is having the same problems. More importantly, it is likely that someone in the class can explain the problem in a way that you can understand. Use this opportunity to get help on the subjects you find difficult and give advice to others about problems they are having.

  1. Talk about how to convert from grams to moles, and
  2. how to determine the percent composition by mass of elements in compounds using both laboratory data and molecular formulas.
  3. Tell your classmates how you go about determining the empirical and molecular formula of a compound given combustion data and the molar mass of the compound.
  4. Discuss the simplest way to relate the amount of reactants and products in a chemical equation by looking at mole ratios.
  5. Solution stoichiometry is often confusing at first, so take a minute to talk about how to make a solution of a given molarity and the how to make dilutions from it.
  6. Discuss how to determine the limiting reagent in a reaction and how to find the theoretical and percent yield.
  7. Gravimetric and volumetric analysis can both be used to determine the amount of a substance dissolved in a solution. Talk about why you might choose one method over the other.
  8. Discuss how a titration works and what types of things need to be considered before doing a titration.
  9. Finally, remember to discuss how to calculate how many particles there are in a mole.

Friday, September 25, 2009

3.1.4 Discussion: How big is a mole?


Discussion Topic:

In the study "The Mole Concept", we introduced the mole, the counting unit for atoms, molecules, and small particles. A mole is the amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. One mole contains 6.0221 X 1023 particles, which is called Avogadro's number. Avogadro's number allows chemists to connect unimaginably small entities to sizes that are found in the laboratory.

In the study, we looked at the size or mass of a mole of everyday items like pennies and donuts. We saw that a mole of pennies would stretch most of the way across the Milky Way. In this discussion, you're going to have a chance to devise your own fun "mole" problem. Come up with an everyday object and figure out how large or massive a mole of that object would be. Try to relate this size to something the other students will be familiar with. Explain to the other students how you solved your "mole" problem. Also, respond to the examples given by other students and propose alternate solutions and comparisons.