We've looked at redox reactions, voltaic cells, non-standard conditions and the Nernst equation, and converting between Eo, free energy and K. Tomorrow we will look at some electron counting (electroplating, etc.)
There is a new Mastering Chemistry posted, it will be your last MC assignment, and it's due Monday.
If you would like to take Exam 4 at any time other than Monday in class, you must let me know in advance. Since the semester has gotten shuffled around with the flood cancellations, I want to be flexible about Exam 4, but you must make arrangements with me in advance. If you don't talk to me about alternate date/time, I will assume that you will be taking the exam during class on Monday.
2009-04-28
2009-04-23
Exam 4
OK, the news you've all been waiting for...
After looking over the conflicts people sent me and thinking about the schedule,
Exam 4 will be on May 4th.
For the few of you who have significant other exams/quizzes/assignments/papers due on May 4th, I will be a little more flexible with allowing alternate times for the exam. Let me know if you are one of the people in that situation and we will make arrangements.
By the way, don't forget that there's a current Mastering Chemistry assignment, due Monday.
See you all tomorrow...
After looking over the conflicts people sent me and thinking about the schedule,
Exam 4 will be on May 4th.
For the few of you who have significant other exams/quizzes/assignments/papers due on May 4th, I will be a little more flexible with allowing alternate times for the exam. Let me know if you are one of the people in that situation and we will make arrangements.
By the way, don't forget that there's a current Mastering Chemistry assignment, due Monday.
See you all tomorrow...
2009-04-22
Redox!! It's shocking!!!!
We started talking about redox chemistry on Monday, reviewing oxidation numbers. Then we talked about a step-by-step approach to balancing redox equations. You don't always have to use every step for every redox equation, but if you run into a real "ugly" redox equation it's helpful.
Today we took another look at balancing redox and tried to decide how to determine if a redox reaction is spontaneous or not. We can physically separate the two half reactions of a redox process and assemble a voltaic cell. The direction of electron flow in the external circuit of a voltaic cell indicates the spontaneous process with the electrons flowing from the anode (oxidation) to the cathode (reduction).
I'm still trying to nail down a date for Exam 4. If you have a conflict (lots of exams, papers due, presentations, etc.) on April 29th, May 4th or May 6th, send me an email TODAY and let me know what your conflict is. I will look at everyone's conflicts and make a decision on the date tomorrow morning.
Today we took another look at balancing redox and tried to decide how to determine if a redox reaction is spontaneous or not. We can physically separate the two half reactions of a redox process and assemble a voltaic cell. The direction of electron flow in the external circuit of a voltaic cell indicates the spontaneous process with the electrons flowing from the anode (oxidation) to the cathode (reduction).
I'm still trying to nail down a date for Exam 4. If you have a conflict (lots of exams, papers due, presentations, etc.) on April 29th, May 4th or May 6th, send me an email TODAY and let me know what your conflict is. I will look at everyone's conflicts and make a decision on the date tomorrow morning.
2009-04-16
Moving heat (and energy)...
We've been talking about entropy, enthalpy, Gibb's Free Energy and thermodynamics in general.
There's a new Mastering Chemistry posted, due Monday.
There's a new Mastering Chemistry posted, due Monday.
2009-04-10
Friday...
Today in class we got started on Thermodynamics by reviewing enthalpy and looking at the other quantity that influences reactions, entropy. Entropy describes and quantifies the disorder of a chemical system.
I didn't have a chance to look at your exams yet, but you will get them back on Monday. There will also be a new Mastering Chemistry assignment on Monday.
Have a great weekend.
I didn't have a chance to look at your exams yet, but you will get them back on Monday. There will also be a new Mastering Chemistry assignment on Monday.
Have a great weekend.
2009-04-08
Squeezing in one more question...
I have a question about the Winter 2006 Exam (3). I'm looking at #2, asking whether a combination of compounds result in effective buffer solutions.One combination is 1.28mol Na2CO3(aq) + 0.64mol HCl (aq). I think this does make an effective buffer solution because the strong acid converts 0.64mol of Na2CO3 (a weak base) into its conjugte acid. Therefore you'd have equal amounts of a weak base and its conjugate acid. Is this correct reasoning so far?However, another combination is 0.90mol Na3PO4(aq) + 1.35mol HNO3 (aq). Here there is more strong acid than there is weak base, so all of Na3PO4 would be converted into its conjugate acid...that wouldn't result in a good buffer, correct? The answer key says that this combination would make a good buffer. Why?
But would it stop at monohydrogen phosphate ions? After all of the phosphate is neutralized to monohydrogen phosphate (a conjugate acid/conjugate base pair), half of the monohydrogen phosphate is neutralized to dihydrogen phosphate. This is an equimolar mixture of a weak conjugate acid (dihydrogen phosphate) and its weak conjugate base (monohydrogen phosphate). For most polyprotic acids (or bases), we can make more than one effective buffer by using each of the protonation equilibria. For phosphate, there are three buffers possible at three different pH's: phosphate/monohydrogen phosphate at ~12, monohydrogen phosphate/dihydrogen phosphate at ~7, and dihydrogen phosphate/phosphoric acid at ~2.
Likewise, carbonate can make the buffer mentioned in the problem (carbonate/bicarbonate at ~10) as well as bicarbonate/carbonic acid buffer at ~6.
But would it stop at monohydrogen phosphate ions? After all of the phosphate is neutralized to monohydrogen phosphate (a conjugate acid/conjugate base pair), half of the monohydrogen phosphate is neutralized to dihydrogen phosphate. This is an equimolar mixture of a weak conjugate acid (dihydrogen phosphate) and its weak conjugate base (monohydrogen phosphate). For most polyprotic acids (or bases), we can make more than one effective buffer by using each of the protonation equilibria. For phosphate, there are three buffers possible at three different pH's: phosphate/monohydrogen phosphate at ~12, monohydrogen phosphate/dihydrogen phosphate at ~7, and dihydrogen phosphate/phosphoric acid at ~2.
Likewise, carbonate can make the buffer mentioned in the problem (carbonate/bicarbonate at ~10) as well as bicarbonate/carbonic acid buffer at ~6.
2009-04-07
We're back!
Yesterday in class we reviewed for tomorrow's exam.
Another question:
What is the Kb of a base if 500.0mL of a solution containing 0.153 mol of the base and 0.191
mol of its conjugate acid has a pH of 6.114? Over what pH range would this conjugate acid/
conjugate base pair make an effective buffer?
How do you determine the pH range that the solution would be a good buffer?
A weak conjugate acid/conjugate base combination will be an effective buffer over a pH range that is within 1 unit of the pKa of the weak conjugate acid, so we have to work through to find the Ka. Plugging the numbers from the problem into the Henderson-Hasselbalch equation,
6.114 = pKa +log(0.153/0.191)
pKa = 6.210
NOTE: You can calculate concentrations of the conjugate acid and base, but the "500.0mL" volume mathematically cancels out, so you can just use mols directly in the H-H equation for this specific problem.
At this point, we can answer the buffer range question. Since the pKa of the acid is 6.210, this system will make good buffers from 5.210 to 7.210.
Once you know the pKa of the conjugate acid, the pKb of the conjugate base is:
14 - 6.210 = 7.790
And the Kb of the base is:
10-7.790 = 1.623x10-8
Mastering Chemistry Assignments 12 & 13:
A number of people have talked to me about the 2 Mastering Chemistry assignments that were due last week. I realize that the sandbagging efforts and evacuations caused some difficulties. At the same time, these assignments were posted to Mastering Chemistry and announced on the class blog before classes were cancelled so everyone should have had the opportunity to work on them before the due date. As a compromise, I have changed the grading settings in Mastering Chemistry so that the possible points on those assignments is decreased by 5% for every day they are late. This means that you can still recover most (but not all) of the points on these assignments even if you do them late.
Another question:
What is the Kb of a base if 500.0mL of a solution containing 0.153 mol of the base and 0.191
mol of its conjugate acid has a pH of 6.114? Over what pH range would this conjugate acid/
conjugate base pair make an effective buffer?
How do you determine the pH range that the solution would be a good buffer?
A weak conjugate acid/conjugate base combination will be an effective buffer over a pH range that is within 1 unit of the pKa of the weak conjugate acid, so we have to work through to find the Ka. Plugging the numbers from the problem into the Henderson-Hasselbalch equation,
6.114 = pKa +log(0.153/0.191)
pKa = 6.210
NOTE: You can calculate concentrations of the conjugate acid and base, but the "500.0mL" volume mathematically cancels out, so you can just use mols directly in the H-H equation for this specific problem.
At this point, we can answer the buffer range question. Since the pKa of the acid is 6.210, this system will make good buffers from 5.210 to 7.210.
Once you know the pKa of the conjugate acid, the pKb of the conjugate base is:
14 - 6.210 = 7.790
And the Kb of the base is:
10-7.790 = 1.623x10-8
Mastering Chemistry Assignments 12 & 13:
A number of people have talked to me about the 2 Mastering Chemistry assignments that were due last week. I realize that the sandbagging efforts and evacuations caused some difficulties. At the same time, these assignments were posted to Mastering Chemistry and announced on the class blog before classes were cancelled so everyone should have had the opportunity to work on them before the due date. As a compromise, I have changed the grading settings in Mastering Chemistry so that the possible points on those assignments is decreased by 5% for every day they are late. This means that you can still recover most (but not all) of the points on these assignments even if you do them late.
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