Review for Midterm 3
Friday December 4
4:40pm ISB 135 (Prof. Botch will lead)
Links to help you review bonding concepts:
Hybridization.pdf sigma/pi.pdf
movies for sigma/pi bonding in C2H4: sigma pi both
Lewis dot structures, formal charges, bond order, etc – review it.
Hi,
the answer key to MT3 is on posting#37.
is there going to be an answer key????
Christina-
winna!
(IBr2)1- has an electron pair geometry that is trigonal bipyramidal (two sigma bonds, and three lone pairs around I). The equatorial positions minimize the repulsion between the electron pairs, leading to a linear molecular geometry.
Hi Ross,
you’re on the right track. Localized means that the electrons are assigned to a specific bond or lone-pair, and will not move around when you draw a resonance structure. Delocalized means that the electrons can appear in different spots, depending on the resonance structure chosen. For example, the electrons used to form sigma bonds in benzene (page 361) are localized; the electrons used to form the pi bonds are delocalized because they switch locations in different resonance forms.
Great catch! That answer is an error.
The best answer has a double bond between S and one of O atoms, and a single bond between S and the other O atoms, with one lone pair on S. This structure gives a formal charge of +0 to S, and a formal charge of -2/3 to each O (due to the resonance forms).
Danielle, its reactants minus the products
justin,
formal charge is the charge on an atom in a molecule. to figure out this charge, you cut the bonds in half and count one end of a bond as one electron (normally a full bond counts as two electrons). so if carbon has 4 bonds stemming from it, you would count that as 4 electrons. then to figure out the formal charge, you take the number of valance electrons the the element should have (indicated by the group number) and subtract the number of electrons that you discovered the element has previously. so for the carbon example i said before: carbon should have 4 valence electrons minus the 4 electrons that are present from the bonds equals a total formal charge of 0!
this is useful to find out the proper lewis dot structure out of numerous resonance structure. the goal is to limit the amount of formal charges in the overall molecule and that tells you which resonance structure is the best one.
sorry that explanation was so long…
Alyssa,
I thought the same thing but then I reasoned that due to the electron lone pairs on I, it had 5 bonds making it trigonal bipyramidal. The two Br made a straight line Br-I-Br on the y axis, and the three lone pairs were in the trigonal planar formation connected to the I but on the x-axis. If you google it, maybe you could find a visualization to help you understand better. I know google is very useful for chem and calc.
I meant IBr2 ^1-^ !
does anyone know how IBr1- has a molecular geometry of linear? I thought it would be bent..but the practice exam is saying otherwise..
What does localized and delocalized electons mean? I could not figure out based on the textbook. Does it have something to do with being “stuck” or localized in a bond and lone pairs? Thanks!
Also I have a question about exercise 8.5 (b) in the book.
For SO3 2-, I keep getting 0 for S but in the back of the book, the answer is +2. Can anyone tell me how or what I would have to do to solve this answer? Thanks
can someone tell me the meaning of formal charge and what it is used for? Thanks
Danielle,
try an example problem (worked out in gory detail in chapter 8 of the book) to clarify this concept. The basic idea is that it costs energy to break a bond, but making a bond releases energy. Reason it out from there.
COunt it as two. In valence bond theory, the bond order is defined as the number of bonding pairs used between two atoms.
When you’re counting the number of bonds to determine the bond order, do you count a double bond as one or as two (one sigma and one pi)?
Thanks!
to clarify, reaction enthalpy is products minus reactants? or reactants minus products?
Is there no video for review session 3?
Could someone clarify the rule about the elements holding 10 valence electrons. It has to be in Period 3-Period 7 and must be a non-metal?
Is there any thing over the weekend I can attend or construct that will go over the test, like the study groups or meetings with TAs or Professors.
MikeDB
Imagine CH2O. The C is hybridized sp2 (trigonal planar geometry). However, it is polar.
Polar molecules depend on how polar bonds are oriented in space – a combination of geometry (trig planar) with polarity.
Is it true that a molecule with a trigonal pyramidal geometry will always be polar?
yup
Yeah, do we need to memorize the electron pairs gemometry and the molecular geometry for the exam?
Are there any other times for Review sessions? This directly conflicts with a class I have, but If there were any other sessions that would help.
Do we have to memorize the geometric shapes of the molecules? that would be hard