CHEMISTRY COMMUNITY

Tessa Lawler 1A

When finding the entropy of fusion of water, why do you take the entropy of fusion of the liquid state minus the entropy of fusion of the solid state, and not the other way around? Shouldn't it be final - initial state?

Tessa Lawler 1A

In the textbook, it says Because the solid and liquid phases are in equilibrium at the melting point, you should expect to find that ΔG=0 at 0.°C. Above that temperature the melting of the solid state is favored, so you should expect that ΔG will be negative at 10.°C. Why does ΔG have to be negative...

Tessa Lawler 1A

0.38 is the specific heat capacity of copper, not water.

Tessa Lawler 1A

Units! Check the units the given values are using.

Tessa Lawler 1A

Heat capacity is usually a given value. If you see something like this on the midterm, there should be a table or something somewhere that has the heat capacity for a bunch of different things.

Tessa Lawler 1A

Heat capacity is usually a given value. If you see something like this on the midterm, there should be a table or something somewhere that has the heat capacity for a bunch of different things.

Tessa Lawler 1A

You can search "hotdog" to find it. I don't think any solutions are posted though. Also I think someone put a link in the endorsed "midterm" post in this section.

Tessa Lawler 1A

Enthalpy of formation can be negative or positive, yes.

Tessa Lawler 1A

Dr. Lavelle is usually pretty good about always putting something into a class example or homework problem if we need to know it, so if you haven't seen any homework problems on it and we haven't talked about it in class, we probably won't be tested on it.

Tessa Lawler 1A

Thanks for clarifying this, guys. I was also confused about this during lecture.

Tessa Lawler 1A

If it doesn't tell you explicitly, just go off of what's present in the equation. Which do you end up with in the products - H3O+ or OH-? If it's OH-, then pOH. If it's H3O+, then pH. And of course you can always calculate one once you have the other using the equation pH + pOH = 14.

Tessa Lawler 1A

They don't have a large enough impact on the pH to significantly change your answer, whether you include them in the calculations or not.

Tessa Lawler 1A

We did an example in class on Monday of last week where we had: 0.0030M Ba(OH)2 (aq) at 25 degrees Celsius Then we needed to find the concentrations of H3O+ and OH- in the solution. So, the reaction is as follows: Ba(OH)2 --> Ba2+(aq) + 2OH- Since Ba(OH)2 is a strong base, we assume it dissolves com...

Tessa Lawler 1A

Yep. Just to add on, the only things in a reaction you have to worry about when we're calculating K values are the gases and aqueous solutions.

Tessa Lawler 1A

I also think it depends on what the question is asking for. I'm pretty sure that on the exam, Professor Lavelle will specify which one he's looking for. Since the only two states we can use in Kc/Kp calculations are gases and aqueous solutions anyway, we're bound to have problems with only gases rea...

Tessa Lawler 1A

This is part of the problems we'll have to do for the test. Calculating Ka/Kb values for substances based on the already known Ka/Kb values of similar compounds/molecules.

Tessa Lawler 1A

Dr. Lavelle also said in lecture that he doesn't like when books/students use this phrasing, and prefers the more accurate "this reaction favors the products (sits to the right)" or "this reaction favors the reactants(sits to the left)".

Tessa Lawler 1A

Just in case this helps focus your studying - while understanding the basic concept is important, Dr. Lavelle did mention in lecture that we don't need to know Le Chatelier's principle.

Tessa Lawler 1A

Dr. Lavelle also did a practice problem in lecture on Friday showing when we would use PV=nRT, using V and n to represent concentration.

Tessa Lawler 1A

I also had this question. Thanks for asking it!

Tessa Lawler 1A

That makes sense. Thanks for your help!

Tessa Lawler 1A

Hey guys! Quick question on bond angles in molecular shapes. When we're asked about the bond angles in a molecule, are we providing only the angle between the atoms branched off the center, or also the angles between those atoms and the central atom? To clarify: If you have AX2E, a bent molecule, is...

Tessa Lawler 1A

Okay. Thanks!

Tessa Lawler 1A

In 9C.9 (7th) there is a coordination compound [PtCl2(en)2] 2+ and [Cr(edta)] -
What do "en" and "edta" mean in this context?

Tessa Lawler 1A

I think it would suffice to just make note of any examples the textbook uses in the chapter/practice problems. We will probably only encounter the most common ones (like Fe).

Tessa Lawler 1A

I was wondering if someone could explain this question to me. It says: "Which of the following ligands can be polydentate?" a) HN(CH2CH2NH2)2 b)C03 2- c)H20 d)oxalate For the answers, the book says: a) Has 3 N atoms w/ a lone pair, so can be tridentate b) can bind to a metal ion through ei...

Tessa Lawler 1A

Great! Thank you!

Tessa Lawler 1A

Hello! On part c0 of this question (pg 731), the answer is aquapentacyanocobaltate(III) ion. Is it written "cobaltate" and not simply "cobalt" because of the negative charge of the compound? If so, I assume that's why a) is also hexacyanoferrate(II) ion. How would you name that c...

Tessa Lawler 1A

If the coordination compound is overall negatively charged, you add "ate" to the end of it.

Tessa Lawler 1A

Yes. ALso, I believe LDF is interchangeable with quite a few other names (such as induced-dipole induced-dipole), so if you look those up you'll be able to recognize that IMFs that occur in all molecules.
Also, it's helpful for the tests.

Tessa Lawler 1A

You'd probably want to draw the most stable structure (a good idea generally). So, the structure in which the most atoms in the molecule have a formal charge of 0.

Tessa Lawler 1A

Yes. We don't do it often on tests/homework I assume because it's just really simple. Hydrogen, for example, would just be "s". It's also kind of rare of an atom in a larger molecule (that isn't Hydrogen) to have only one bond and no lone pairs, so you don't see it too often.

Tessa Lawler 1A

You don't have to worry about finding the hybridization of specific types ("pi" or "sigma") of bonds. If an atom shares a triple bond with another atom, that atom has a sigma bond and two pi bonds. But it would still only get 1 level of hybridization (in this case, "s")...

Tessa Lawler 1A

Nope. The hybridization of an atom with a single bond would be the same as an atom with a triple bond (all other things being equal). Hybridization is really more about regions of electrons, so making a bond between the same two atoms stronger wouldn't change hybridization in the way a lone pair or ...

Tessa Lawler 1A

So if the Nitrogen atom in that molecule has a lone pair and is triple bonded to carbon, then it's hybridization would be sp, because there are two regions of electron density (the triple bond with carbon and the lone pair on Nitrogen itself). The Carbon atom bonded to N has a hybridization of sp, b...

Tessa Lawler 1A

I know that when you have a multiple bond like a double or a triple bond, only the first one is a sigma, and all the others are pi bonds. E.g. a triple bond contains one sigma and two pi bonds. But I'm not sure why that is the case structurally.

Tessa Lawler 1A

Someone who worked on this problem also posted a possible answer in the question "Lyndon's practice question 17"

Tessa Lawler 1A

In the textbook, there's a question that says "In which of the following compounds do the bonds have greater ionic character? a)P4O10 b)PCl3". The answer is A. I know that ionic character is determined by the difference in the atoms' electronegativity, but since P seems an equal distance f...

Tessa Lawler 1A

You could have more than one lone pair that would either balance it out like a square planar molecule or exist on a different access, and therefore not affect the bond angles.

Tessa Lawler 1A

For an example, you can take a look at a water molecule, which would have 180 degree bond angles if not for the lone pairs which cause the ultimate shape to be bent instead of linear, and the bond angles to be smaller.

Tessa Lawler 1A

The solution manual says 109.5, so that’s probably your safest bet. I think that double bonds don’t affect bond angles as much as, say, lone pairs do.

Tessa Lawler 1A

That’s a good question. It would probably be because of the polarity of the molecule as a whole and its desire to stay that way.

Tessa Lawler 1A

You'd probably want to draw all of them just to be safe, but there are usually only a few resonance structures for the molecules we're working with.

Tessa Lawler 1A

Another important note here is that a covalent bond is the sharing of electrons. So when two atoms are covalently bonded, their closeness to one another is because they are sharing electrons, which contribute to their radii. So there may be a sort of "overlap" of those outer valence shells...

Tessa Lawler 1A

Since an element with a bigger molar mass has more protons to give it that high mass, it also has more electrons. I would assume that that would be one reason.

Tessa Lawler 1A

They're a related concept. Molecules with resonance will ultimately form hybrid structures of all the different kinds of bonds they can make.

Tessa Lawler 1A

I think that overall the ultimate goal of two atoms in a covalent bond is to become neutral, even though they don't have the electrons to make that happen (fill the valence shell) independently of one another, which is why they share electrons, to become one, mostly neutral, molecule.

Tessa Lawler 1A

This is kind of a tricky one. In determining which molecule is more covalent, you do want to look to electronegativity, but almost more importantly, you want to look to the proximity of the atoms in the molecule. The closer the electronegativity values of the atoms in the molecule are to one another...

Tessa Lawler 1A

There are occasionally exceptions to the trend rules for ionization energy, electronegativity, and atomic radii, but since the general trend for ionization energy is that it increases going up and to the right, and I don't know of any exceptions where an element would have a higher ionization energy...

Tessa Lawler 1A

Actually, the central atom is typically decided by which atom is the least electronegative. That would be Nitrogen in this case, so the very basic outline of N2O would be N-N-O (not taking into account the eventual double bonds/lone pairs you'd add in drawing the structure).

Tessa Lawler 1A

2 main ways: 1- covalent is between 2 nonmetals. Ionic is generally between nonmetal and metal. 2- covalent bond is the sharing of electrons between two atoms/molecules i.e. they both share one pair of electrons to complete their valence shell. Ionic bond is the giving of electrons from one atom/mol...

Tessa Lawler 1A

As a general rule, elements with a full outer valence shell want to stay that way. Taking away an electron from a full outer valence shell is very difficult due to the amount of energy required to kick the element out of its ideal state (a full outer valence shell).

Tessa Lawler 1A

I don't think so. I believe that that rule is applicable to all elements in the periodic table.

Tessa Lawler 1A

Yeah I believe BrF3 is one of those weird molecules that is an exception to the rule.

Tessa Lawler 1A

It's probably good to know all the meter/liter/gram conversions we covered in week one. For example, nano-, mega-, pico-. Additionally, the meaning of each unit is important to determine final units in a problem - see Hertz, Joules.

Tessa Lawler 1A

Between 400-700 nm. For the record, the textbook has a really nice diagram (In the 7th Ed., page 6, figure 1A.9) that shows the ranges for all the different types of radiation, including visible light.

Tessa Lawler 1A

It's not necessarily uncommon. Sometimes, you'll need to use multiple equations to get an answer. For example, if you need to find wavelength but you're only given Energy, you can use E=hv to find the frequency (v) and then use that in the equation c=(frequency)(wavelength).

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