Exam 3 keys

The keys for Exam #3 are fixed and should work now, let me know if there are still problems. Also, I think the blank exam #3's that were posted were old copies that still had a typo in the big titration problem (asking for the concentration of selenic acid instead of potassium sulfite...). I posted the corrected blank exam #3's.

Let me know if there are other questions, I'll be in my office for at least part of the day today...

A couple process questions...

Questions:

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First off, for some reason, and this might be something just happening at this time, exam 3a and 3b answer keys are not available online.

Also, I was wondering if we are going to be given any formulas on the final in addition to what was listed on exam 4a with the periodic table.

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Yep, there's something not right with the keys for 3a and 3b, I'll get them fixed first thing tomorrow morning when I get to my office.

The cover page for your final exam will contain the same information as the cover page for Exam 4. If there are any other constants or equations you would like to see on the cover page, let me know and I'll consider it.

OK, everybody concentrate...

Email question...

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I am studying for the exams, and I just had a random question. Why are there Molarity and Molality? Aren't they almost always going to be the same number since 1 g H2O = 1 mL H2O? I guess I'm just curious of when you would choose one measurement over the other.

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In practice, yes, they are almost always the same (to our ability to measure them...), but the reasons we need them both are:

1. Molality is mols of solute per kilogram of solvent. Since the mass of a substance does not depend upon its temperature, molality is a concentration unit that is independent of temperature.

2. The density of pure water at (somewhere in the neighborhood of) 17degC is exactly 1g/mL, with many zeros of sig figs after the decimal point. For most dilute aqueous solutions at moderate temperatures, it's pretty safe to assume that the density of the solution is pretty close to 1g/mL, but there aren't nearly as many sig fig zeros after the decimal point. If you're working with a solution that's not dilute or not relatively close to that 15-20degC temperature, the density will start to vary significantly. That means the molarity (M) will change, but the molality (m) should not, therefore the conversion between these two units will not be 1:1. For an example you can try at home (or Kise...), take a little bit of water and add sugar or table salt to it until you get a very concentrated solution. Carefully (and slowly) pour that solution down the inside wall of a glass of "pure" water... the solution is more dense than the pure water and will sink to the bottom without mixing (if you're very careful).

3. What if your solvent is not water? In your freezing point depression experiment, your solvent was cyclohexane, so there was absolutely NOT a 1:1 relationship between the M and m of the solutions you were making.

Again, in many "real" situations the difference between M and m in an aqueous solution you're using will be so small that it's ignored. Similarly for other concentration unit conversions, we tend to simplify them in practice because the solutions we are most often working with in a lab (and especially a biology or biochemistry lab) are probably aqueous and probably relatively dilute. As long as you understand the assumptions you're using to simplify your in-lab calculations, you should be able to adjust to different situations or solvents...

Redox...

A question from email:

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Hey Dr. Bodwin I had a question on balancing equations. I can't tell if this equation is a redox or not

2 C2H2(g) + 5 O2(g) 􀀧 4 CO2(g) + 2 H2O(g)

I was looking for some guidelines on how to tell if an equation is redox or not, and if it is I am also looking for help on how to write the half reactions.

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OK, I'll answer the easier part of this question first. If a reaction is redox, then the oxidation numbers of some (or all) of the elements have to change. Let's look at ox#'s here...

In C2H2(g), let's use the "rules"... if each of the H's is +1 and the molecule is neutral, then the two C's must total -2. (The sum of all the oxidation numbers has to equal the charge of the molecule or polyatomic ion.) That means each C must have ox# = -1. {I'm assuming that the two C's are identical.}

Since O2(g) is an uncombined element in it's natural/standard state, the ox# = 0.

For CO2(g) it's back to the rules... if each oxygen is -2, then the carbon must be +4.

For H2O(g), the rules pretty much do all the work for us, H is +1, O is -2.

So in this reaction, carbon is going from -1 to +4 and oxygen is going from 0 to -2, there are changes in ox#'s, therefore this is a redox reaction. {Which one is reduction and which one is oxidation? Hmm...}

Now for the harder part of this specific reaction... writing half reactions for a combustion reaction like this is a little more involved than writing half reactions for metal/metal redox reactions because the oxygen is incorporated into both products. That means that our stepwise procedure for balancing redox reactions doesn't work all that great here... fortunately, hydrocarbon combustion reaction can usually be balanced by inspection/trial-and-error. If you have to balance a redox reaction (and write out half reactions) on the final exam, it will not be a hydrocarbon combustion. You should still be able to assign oxidation numbers and identify which element is being oxidized or reduced, but these are probably easier to balance by inspection.

Other questions? Let me know...

Strong acids and bases

The strong acids and bases you will be expected to know for the exam are:

Strong acids: perchloric, hydrochloric, hydrobromic, hydroiodic, sulfuric (first Ka), and nitric

Strong bases: any very soluble hydroxide can be considered a strong base, so any first-column hydroxide (LiOH, NaOH, KOH, etc). The second-column hydroxides are usually soluble enough to be considered strong (magnesium hydroxide, calcium hydroxide, barium hydroxide), but I don't expect to make any fine distinctions on those, if I want to use a strong base on the exam it will be either NaOH or KOH.

Keys posted...

The answer keys are posted for last year's exam 3.

Someone asked about the pH range for a good buffer (part of #5 on last year's exam). To be able to make a good buffer, the pH of the buffer should be within 1 unit of the pKa of the weak acid component of the buffer. If the pKa of the weak acid is 5.62, then you can make an effective buffer at any pH between 4.62 and 6.62.

Titrations and flood preparation...

We're into titrations and just touched on buffers. More on that after break. The Tuesday SI session after break has been cancelled. To address the potential flood issues, the University has asked all instructors to provide an addendum to the syllabus for each class. The syllabus addenda for Chem 210 and 210L are below. If you have any questions, let me know. Have a good and safe Spring Break.

Chem 210

In the event of class cancellation or campus closure due to flood, material will be presented online using assorted methods (videos, online notes/lectures, etc.). For information, refer to the class blog (http://msumgenchem.blogspot.com/) and/or Dr. Bodwin’s website (www.mnstate.edu/bodwin). There will continue to be Mastering Chemistry assignments, the exams may be rescheduled or reformatted. Dr. Bodwin will remain in contact via email (bodwin@mnstate.edu).

Chem 210L

In the event of class cancellation or campus closure due to flood, material will be presented online using assorted methods (videos, online notes/lectures, etc.). For information, refer to the class D2L page (https://mnstate.ims.mnscu.edu/shared/login.html) and/or Dr. Bodwin’s website (www.mnstate.edu/bodwin). If more than 1-2 weeks are lost to flood, “at home” experiments will be posted and quizzes/assignment will be posted in D2L. Dr. Bodwin will remain in contact via email (bodwin@mnstate.edu).

Acids and Bases...

We're into acids and bases...

There's a new Mastering Chemistry assignment posted due next Friday.

Reaching equilibrium...

When you study the kinetics of a reaction for a long enough time, the reaction reaches equilibrium. We've hit the high points of kinetics (rates, rate laws, integrated rate laws, activation energies, mechanism) and now we have reached equilibrium. Equilibrium is the state where the forward and reverse rates of reaction are equal, so although those reactions both continue to occur, the concentration of reactants and products remains constant.

Looking at the schedule, our next exam is coming up Wednesday Feb. 24th. Because of my schedule, it is extremely unlikely that I will ever be able to grade a Wednesday exam by class time on Friday, and given that kinetics and equilibrium are both pretty large topics, I think it would be best to move that exam back to Friday Feb. 26th. If anyone has a significant conflict with that day (other exams, plans to be out of town, etc.), let me know. At this point, the exam will still be scheduled for Wednesday Feb 24th, but if I don't hear of too many conflicts we will probably move it to Friday Feb 26th.

Arrhenius says...

We looked at a few examples of rate law problems and have determined rate law orders by comparing experiments with differing initial concentrations of reactants.

Kinetics is all about probability, and that probability is dependent upon the activation energy required to get a reaction started. Activation energy is calculated using the Arrhenius equation. We looked at the compact form of the Arrhenius equation (very elegant in its simplicity, but not always very practically useful) and derived a couple variations that are more useful, the comparative and linear forms.

As with so many things, kinetics problems become easier with practice, so keep up on your Mastering Chemistry and take a look at some textbook problems. The new MC assignment is posted.

Have a good weekend.

There oughta be a (rate) law!

We looked at average rates and instantaneous rates, almost did some calculus but not quite. The only unique instantaneous rate is the initial rate and we looked at Rate Laws as a relationship between the initial rate of a reaction and the concentrations of the reactants. Rate laws give us insight into the molecule-scale pathway of the reaction.

I handed back exams, if you didn't pick yours up I will have them with me in class on Wednesday and Friday.

Kinetics began, slowly...

Today we started talking about chemical kinetics, the study of the rates and pathways of chemical reactions. Kinetics can be explained using Collision Theory, and we're in the middle of looking at different types of rates.

Have a good weekend, I'll see you on Monday and I'll have the exams graded.

Exam Wednesday...

Today we talked about colloids a little and reviewed for the exam. I problem came up that I made a mistake on, but it wasn't the mistake I thought it was. We went through a quick osmotic pressure problem with approximately 1M MgCl2 in water and the answer I calculated on the board was around 74atm. That answer is correct. I was thinking about a vapor pressure problem that someone had asked about, and for a solution to have a vapor pressure of 74atm would be just goofy, at least under any reasonable conditions. An osmotic pressure of 74atm is also pretty high, but that's an indication of just how strong a force osmosis can be.

Because of the university closing, tonight's SI exam review session has been cancelled. If you have questions, feel free to email me, I will answer to the blog.

Stay warm, stay safe, and enjoy(?) the snow.

Colligativity...

Today we wrapped up the colligative properties and concentration units we're going to talk about before next Wednesday's exam.

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

More IMFs...

Today we looked at how intermolecular forces can be used to predict some properties of matter like viscosity, surface tension, and capillary action. We also looked at vaporization and vapor pressure and discussed heating/cooling curves and phase diagrams.

We will not be meeting for lab this week BUT there is an assignment up on D2L covering the syllabus and course policies. It's due by Friday at 11:59pm, but you should be sure to look at it before then, perhaps during the 3 hours of open time you have tomorrow because we are not meeting for lab...

Day One

Today we started looking at intermolecular forces, we'll continue on Wednesday.

Don't forget to sign on to MasteringChemistry and take a look at the current assignments as soon as you have a chance.

Fall 2008 Exam 4a key

I posted a key for last year's Exam 4a, but I don't have access to my mnstate.edu web page right now, so I wasn't able to put a link up yet or draw "nice" Lewis or VSEPR structures. Here's a direct link to the .pdf file

http://www.mnstate.edu/bodwin/courses/genchem1x/c150ce4ak.pdf

Electronegativity and Lewis Structures

Today we looked at electronegativity as a measure of a bond's polarity and a way to determine bond type. Then we started looking at Lewis structures, finishing up in the middle of NO2. We'll finish up NO2 on Friday and look at some other Lewis structures, including the Lewis structures of polyatomic ions. I know, you're on the edge of your seats...

There's a new MC posted.

Electron configs and properties...

We've been looking at how we can use electron configurations to predict physical properties including size, charge, ionization energy and electron affinity.

If anyone's interested in some more YouTube videos, the following links are a 3-part Bill Nye The Science Guy episode dealing with light and color.

http://www.youtube.com/watch?v=msf-DbYif-4&feature=related

http://www.youtube.com/watch?v=hEUtp2htuEs&feature=related

http://www.youtube.com/watch?v=Sbjuv-meMh8&feature=related

Bill Nye usually does a very good job of explaining scientific concepts in a (hopefully) entertaining way, and this show is no exception. If you're struggling with questions from lab this week, watching this might help.

There's a new Mastering Chemistry posted.

Quantum chemistry and orbitals...

Looks like I missed a couple days...We've been talking about quantum chemistry including the nature of light and radiation. This is so we can look at electrons a little more closely and understand their behavior as particles with wave-like character. A direct result of this wave-like character is that electrons exist in regions of space that are called orbitals. Understanding orbitals helps us interpret the chemistry that we have seen and allows us to predict various properties of atoms, ions and molecules.

There is a new Mastering Chemistry assignment posted, due Nov. 9.