Tuesday, October 4, 2011

Chapter 3

Any questions on Chapter 3 material? Ask them here.

28 comments:

Anonymous said...

question 8.45 on chapter 3 assignment:
how do you determine which substance is molecular and which is ionic.

Scott McIndoe said...

Essentially, electronegativity difference is a good first estimate (in the lecture, I said >2 = ionic) but this rule falls down when the oxidation state of the metal becomes high (i.e. it is >3+). At that point and beyond, compounds typically have a lot of covalent character.

Anonymous said...

question 8.17 on assignment 3:
how do you predict the chemical formula of ionic compounds formed between the pairs of elements, say Al and F for example. Do we use the idea of lewis structures here?

Anonymous said...

Hello

Is there any practice midterms that we can do?

Thank you!

Scott McIndoe said...

Predict formula using octet rule, i.e. Al needs to lose 3e to get octet, F needs to gain 1e, so AlF3.

Practice midterm available on course website.

Anonymous said...

Does anyone know how to remotely begin doing lab 3? I'm so confused!

Anonymous said...

where are you posting the 2006 midterm covered in class today?

Scott McIndoe said...

Just gone up; see main page of blog.

Anonymous said...

Lab 3: Iron in Vitamin Pill.

start by opening excel.

1) In box A1 type Concentration
A2, A3, A4, etc. type in your x values in g/L (should be 0.001g/L, 0.002 etc.

2)Box B1 type Absorbance
Box B2,B3 etc type in your y-values (absorbtion)

3) highlight all of these boxes

4) click insert tab. then hold mouse over scatter until scatter only with markers comes up. choose that

5) a graph will be made.

right click on one of your points, select "add trendline"

6)window comes up. at bottom of window, select display equation on chart. BOOM!

7) solve for x and use your unknown sample absorbance as y values to determine their [].
from there follow steps in manual

Anonymous said...

question 8.51 on assignment 3:

part B asks you to assign an oxidation number to each of O,C, and S in OCS. How do you do this?
thanks!

Anonymous said...

Hey!
8.53 assignment 3:
With what allotrope of oxygen is it isoelectronic?
(for NO2-)

Scott McIndoe said...

Don't worry about oxidation state.

Isoelectronic means same number of electrons... there is only one other allotrope of oxygen...

Anonymous said...

The midterm includes topics up to 3.5 am I correct? or is it 1,2 and all of 3?

Scott McIndoe said...

yes

Anonymous said...

will there be measuring dipole moments on the midterm? i am quite confused about this

Scott McIndoe said...

Just be aware that dipole moments are highest for long, polar bonds, and low (or 0) for short, non-polar bonds.

Anonymous said...

For the chapter 3 questions in the booklet, #2 specifically, why is I3 given an extra electron when drawing the Lewis structure which results in 22e instead of 21e. It was not explained in the video answer.

Scott McIndoe said...

That's an error, thank you for spotting it. It should be I3-. Sorry!

Anonymous said...

Why doesn't NO2 have delocalized bonding? If there were a triple bond between one oxygen and nitrogen, and a single bond between the other oxygen and nitrogen wouldn't the overall charge still be zero??

Scott McIndoe said...

I assume you're talking about question 5 at the end of the chapter? Another problematic solution. NO2 should be O=N-O, where the number of non bonding electrons is 4, 1 and 6 respectively, and the formal charges are 0, +1 and -1. The depiction shown gives N more than an octet, which is not allowed for second row elements, though it does predict the correct formal charge and shape. Sigh. Thanks for the questions & your attention to detail.

Anonymous said...

In a continuation on that last question..does that mean that NO2 does have delocalized bonding or not? And since giving N more than an octet gives you the "correct" formal charge and shape, is that the correct one to use even though it is not technically allowed?

Anonymous said...

In question 12 (chapter 3) how did you know which bonds of dimethyl ether were broken to form ethanol?

Scott McIndoe said...

NO2 should have delocalized bonding also, so the online solution is incorrect, unfortunately.

Scott McIndoe said...

Re: ethanol/dimethyl ether - just look at the difference between the molecules - ethanol has an O-H and a C-C bond where dimethylether has a C-H and an O-C bond; otherwise, the molecules have the same bonds.

Anonymous said...

On the sample exam, there is a question about bond energies (Part 2 Question 1) in which a C=C double bond is broken. I've triple checked the data sheet and only the value for C-C single bond is given (348kJ/mol). On a test situation are we to estimate that a double bond is twice the value of a single? (Even though it is less.. 614, not 696)? Please advise.

Anonymous said...

just another question regarding chapter 3, #12:
how are we supposed to know how these structures actually look? they tell us that they are structural isomers but I am confused about why they put the hydrogens where they do in the video.

Scott McIndoe said...

No, don't double it. The data sheet would have been different for that midterm (and included the C=C value!)

Scott McIndoe said...

Re Ch3, #12 - please come and see me, because explaining this is difficult to do with text, especially since I don't quite understand your question.