For students taking Chemistry 101 at the University of Victoria in the Fall 2013 term.
hey, this is actually about the midterm. question 23 on version A. Why is a seesaw considered a tetrahedron when its trigonal bipyramidal? that seems like a trick since trigonal bipyramidal was also an option, and I get that seesaw is a better answer except i don't understand why it said tetrahedron in brackets, it seems misleading.
should we memorize the structures and polymer names of the 2 charts?
1154: I think it was listed as "distorted tetrahedron" or somesuch. It wasn't intended as a trick question; rather we were acknowledging that there are alternative terms for this shape.1205: No, but do make a note of the fact that condensation polymers often have different types of name than addition polymers (which are usually named simple "polyX" where X is the monomer).
did we talk about alternate names in class, or where would i look to find out alternate names for shapes?
You only need to know one of them. It's as if I said "isobutane" (2-methylpropane)" - provided you know one of the two, you're good.
Regarding the sale of old exams in Elliott...Is this not shocking to anyone else? As if $500 for a semester of chemistry wasn't enough, making students pay for practice exams so that they can study more effectively seems unethical, especially considering many of these students are simultaneously working entry-level jobs to barely get by (including myself). Who is really profiting?-First year chem student who believes education is a debt sentence.
Thanks for the feedback. Education is expensive, it's true. I publicized this event on behalf of the undergraduate chemistry student society - this is their major fundraiser for the year, and we consider this a good cause.
Knowing that the money is going towards UCSS makes a little more sense. Tuition is a different cup of tea, however. Back to chemistry...Does it matter where to divide a molecule into monomers so long as one end of one monomer can include hydroxide and the other end has hydrogen?
It does matter, yes, and it will depend on whether it is an addition polymer (in which case you want a monomer with a double bond) or a condensation polymer. For the latter, if you see an amide you need to make an amine + a carboxylic acid; if an ester, an alcohol + a carboxylic acid. So it won't always be a hydroxide, no.
If you want to dope GaAs to make an n-type semiconductor with an element to replace Ga, which element(s) would you pick?The answer to this question in mchem is Ge, but could you pick other group 14 elements, or other elements in general (with more than 4 valence electrons).Why/ why not?
"CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light they emit light equal to the band gap energy."Could you explain what happens here? The ultraviolet light travels down the band gap in the form of an electron....what happens to the rest of the energy? And i thought only light that fit exactly the band gap could travel through?
"CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light they emit light equal to the band gap energy."Why can appropriately sized CdS chunks emit blue light but not red light?If we make the particles bigger wont that make the band gap smaller?
"The n-type semiconductor has a partially filled valence band."I said the statement is true, why is it false? There are electrons in the valence band...Question 14/25 in chapter 7
How does making n type semiconductors increase conductivity?You're not making the band gap any smaller, and the valence band is still completely filled. It's not any easier for electrons to get across, and free up space
Part E of question 18...I have no idea...Do we need to know how to distinguish properties of isotatic polypropylene, syndiotactic propylene and atactic polypropylene?
could you elaborate on what the "zinc blende" structure is? Is it only found in semi-conductors? Is it considered covalent, ionic, etc?
Mg has 2 valence and O has 6. the average is 4. Why is it an insulator and not a semi-conductor?
358: Si would also be a good guess. Ge is closer in size, though, so is likely to be a better fit to the lattice.410: Basically, you promote an electron across the band gap (a wide range of energies are possible, since both the bonding and antibonding bands are wide), then it relaxes to the lowest energy of the unoccupied MOs (losing energy through non-radiative means). When it drops across the band gap, that is a large amount of energy (that can't be taken up by vibrations, etc) and is emitted as a photon whose energy = the band gap.416: Once the crystals are large, the band gap won't change, since you essentially have a continuous band already. You can't make the band gap any smaller than the bulk maximum. 436: Because that's the conduction band.438: You can think about conduction as being like in a metal, but because the concentration of electrons is very dilute, it's not AS conductive (but still more so than a pure undoped intrinsic semiconductor).451: No. These just refer to the ways in which the methyl groups on the backbone are related to one another.526: Not much, no. The electronegativities don't differ that much (less than 1), so it's best thought of as a covalent network solid.530: Because the electronegativity difference is so large it is an ionic solid rather than network covalent.
Re:530: how are we supposed to know what this "threshold" is? The book says as long as they average 4. Re:436 my bad...so the valence band is completely filled?
To increase the bandgap in semiconductors, the book says that as elements in the periodic table with larger electronegativity differences and farther horizontal distamce from eachother have larger band gaps.But there are many examples in the book where we are asked to compare two semiconductors where electronegativity difference and horizontal distance oppose eacother...the answers in the book seem to say only the horizontal distance matters.Is this true?
1120: EN differences of >2 are a decent ballpark for ionic behaviour.Yes.1123: I'm sorry, I don't really understand your question. Certainly ionic compounds have large band gaps and so are insulators as a result.
I'm hoping I had wrote the notes down wrong from class. But I thought I would mention this.."vulcanized rubber is more elastic and less reactive than natural rubber."Vulcanized means that it has been cross-linked, correct? Wouldn't it be that vulcanized is less flexible and less reactive? (Pg 90)
Elastic and flexible are not synonyms. Elastic just means a tendency to return to the original shape after being deformed. So, for example, the Liquidmetal I demoed in class (the bouncing ball bearing) is highly elastic but not what you would consider flexible. A flexible substance has low resistance to deformation, but may not have any tendency to return to its original shape. Play dough is flexible but inelestic. A tire is flexible but elastic. Liquidmetal is inflexible but elastic.
Professor for the chapter 7 quiz there was a question: Nylon is formed by the reaction of a __________ with a __________. And I answered diamine and dicarboxyllic acid but the quiz said that its wrong even though when I looked it up that's the answer. Am I right or wrong?
You're right. Spelling mistake?
One of the questions on MasteringChemistry said this: If you want to dope GaAs to make an n-type semiconductor with an element to replace Ga, which element(s) would you pick?The answer was Ge.But, aren't all the elements in group 5A would be suitable options too?
Nickel metal crystallizes in a cubic close-packed structure (face-centered cubic cell). What is the coordination number of each nickel atom?The answer is 12, but why?
1254: look at answer to comment 358, this has been answered before.1216: In a cubic close packed structure each atom has 12 neighbours, corresponding to a coordination number of 12.
I'm: December 5, 2013 at 12:25 AMSo should I tell Dr. Briggs to fix that on my quiz since I lost that mark for no reason
Your chemistry is fine, but spelling it incorrectly is still wrong.
Do we need to know the uses for addition polymers and condensation polymers?
referring back to comment 1216, are we expected to know these structures in this much detail?Like, i have a general idea of what stuff looks like but i wouldnt be able to tell the coordination number of like zinc blende vs something else..I think you went over it in class, but its not in our textbook, nor did we write any of it down in our notes
what makes nanomaterials so special?Is water a nanomaterial? A single water molecule is small enough?
how can we tell what semi conductors are better than others? Are there trends in the table?I think the book goes over this but it wasnt in your notes.Are we expected to know things that were in the book but not in your notes?Ex...saponification reaction back in chapter 6
Do we need to know what the buckyball and stuff is? Fullrenes, etc.And, how crazy are you about knowing the properties of specific materials. Like, on the quiz it asked us the properties of graphene....would we be expected to just know that on a test?Also, the book says that for compound type semi-conductors, the farther apart the 2 elements are horizontally, the bigger the band gap. And, the higher up the elements are in the column, the bigger the band gap as well.But...the explanation for this is that as bond distances get bigger, there is less overlap of orbitals, which makes antibonding and bonding orbitals closer together. But this doesn't make sense. Should less overlap of orbitals further separate antibonding and bonding orbitals? Like, when we have a metal, the more orbitals we add, the more overlap, which lowers band gap and what not. This is why for metals, its easier for electrons to circulate, right?
If the band gap of CdS is given as 2.4 eV, how is it determined that the light emitted when illuminate with UV light will be green? Ie, how do we calculate the band gap energy??
For the masteringchemistry assigment, how is it determined that Sn is a metal, InAs and HgS are semiconductors, and MgO is an insulator? I thought that Sn would be a semi conductor because of its 4 valence electrons, and the others I'm just not sure about.
Consider doping the semiconducting material (Ge). Addition of a very small amount of which element would make the germanium a better electrical conductor?a)Galliumb) Phosphorousc)Arsenicd) Aluminume) Any of the aboveThis question is from a past exam and the answer said it was d) aluminum. Can you please explain why the answer is d) ?
955: No. Basic stuff only about lattices.956: they have interesting properties that arise from them being neither discrete molecules nor bulk solids. No, a molecule is not a nanomaterial.957: The book is the full set of notes; my lectures highlight what we consider to be the important stuff. Generally, this is the stuff that gets examined, but some questions are always "hard" (though I prefer "deep"), and require more thought and/or preparation.1142: No. Recall that the quizzes are open book but the exam is not, so they naturally generate different sorts of questions.No; good overlap generates large differences between bonding and antibonding orbitals. This can be envisaged fairly easily if you consider two hydrogen atoms far apart from each other - the antibonding and bonding orbitals will be close to each other in energy.For metals, you are decreasing the *spacing* between orbitals as you add more atoms. For a semiconductor, the band gap is generated by the difference in energies of the bands constructed from sp3 vs. sp3* orbitals. Those bands themselves are quite wide.700: you need to work out what 2.4 eV is in J.1002: Basically from EN values. Sn has a low EN. In/As and Hg/S are close in EN. MgO has a big EN difference and is ionic.303: It's wrong. It should be (e).
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