CHEMISTRY COMMUNITY

kamalkolluri

Since both are neutral electrodes, i don't think so. Except I just use platinum because that's how it is in the solutions manual.

kamalkolluri

As of what I understood, a reaction rate is just the average change of concentration over time while a unique reaction rate is that value divided by the stoichiometric coefficient.

kamalkolluri

You multiply the rate in experiment 1 by (3.02/1.25)^2 and get around 50.78 which is approximately 50.8, so that data matches that B is second order. I don't think you do experiment 2 because you need to account for A also, which doubles.

kamalkolluri

If the Activation energy for the forward rxn is smaller than that of the reverse reaction, it is exothermic. If it's the other way around, it is endothermic.

kamalkolluri

The review sessions will be updated soon when rooms are figured out. He should also mention them in class!

kamalkolluri

k doesn't change because it is the same value for the equation.

kamalkolluri

Another way to tell the difference between zero and first is that for a zero order reaction, the plot of [Reactant] vs t is a straight line graph while for a first order reaction, the plot of ln[Reactant] vs t is a straight line graph.

kamalkolluri

If in one of the half reactions, all the reactants/products are gas/aqueous and there's no metal/solid, then you need to add in the inert metal.

kamalkolluri

Oxygen isn't in peroxide form anywhere nor is it in O2 form anywhere so it's oxidation number remains at -2. Cr goes from 6+ to 3+ so it is reduced and I think there's a typo in what you wrote, but I remember seeing this problem and Carbon is oxidized.

kamalkolluri

Galvanic and Voltaic Cells are the same thing

kamalkolluri

When writing half reactions for Redox rxns, are we allowed to add in molecules, or are we restricted to those given in the original equation?

kamalkolluri

How is the Gibbs free energy of formation for O2 0kj/mol even though the standard entropy isn't 0. Shouldn't G = H-TS, and by this the Gf for O2 have a value?

kamalkolluri

With hw, I like to do topics we learned in the last week. So for example, if your discussion is on Wednesday, choose topics/problems that you learned last Wednesday, Friday and this Monday.

kamalkolluri

Use 273.15 and don't round until the end of the problem.

kamalkolluri

For 4D15, the reason you write out equations is because you are given enthalpy of combustion and I think for the exam we should know how to write out combustion reactions.

kamalkolluri

To find W, you would draw out possible configurations for SO2F2 and get a value of 6. You then simply plug it into the equation getting S = kb*ln(6)

kamalkolluri

Entropy is the amount of disorder in a system. So a positive change in entropy is when disorder increases, while a negative change in entropy is when disorder decreases (order increases). Usually in reactions where the products are more ordered, the entropy has decreased (negative deltaS) while if t...

kamalkolluri

w = -P(deltaV) by definition. So an increase in volume (expansion), makes dV positive, so the work done is negative. However, if deltaV is negative (compression), then work is going to be positive (compression).

kamalkolluri

Yes, Lavelle keeps the same equation sheet and periodic table for every test

kamalkolluri

I think the answer in the book is wrong. Do you have the solutions manual, because in the solutions manual there is a detailed answer.

kamalkolluri

I don't think we've covered thermo or work yet?

kamalkolluri

You should be consistent with units. If they say kJ in the problem, use kJ if they say J then use J. I think it will be specified in problems on the test though.

kamalkolluri

Since Ka1 is large, all of the H2SO4 dissociates. So yes, you assume the HSO4- and H+ are both .15M and use that value in your ice table for the next step where you use Ka2. When you find the concentration in the second step, add that to the 0.15 from the first step and do -log[H3O+] to find the pH.

kamalkolluri

As of what I saw in lecture, you just use the numbers because K has no units. So just don't include concentrations in the expression.

kamalkolluri

With temperature, in an endothermic (reaction that requires heat), increasing temperature favors formation of products; In an exothermic reaction (a reaction that releases heat), increasing temperature favors formation of reactants.

kamalkolluri

It usually depends on numbers you're given. Kp is generally used if all compounds are gases, but you can also use Kc if the numbers are given in mols and you know the volume of the flask. When writing the equilibrium expression it is usually better to use partial pressures and Kp.

kamalkolluri

When you multiply a certain reaction by a number n, the new K for the new equation is the old K to the power of n. So here, the equation is multiplied by 2 so the K of the reaction would be K^2 so 41^2.

kamalkolluri

Lavelle told us in class that pressure doesn't affect K. K is only changed when the temperature changes

kamalkolluri

Solids and liquids have no concentration/partial pressures so they can't be included in the expression.

kamalkolluri

Another way to conceptualize this is that in Kc, products are the numerator while reactants are the denominator. If Kc is larger in 1 scenario than another, that means you have more product formed in that scenario. This means the reaction is proceeding forward.

kamalkolluri

Another thing Lavelle said is there's no such thing as a concentration/partial pressure for solids and liquids, so they won't be included in your Kc or Kp.

kamalkolluri

I think we just need to use the general Kp and Kc. You don't need the complex equation, I'm pretty sure that's for later.

kamalkolluri

For question 34, once I have the Molarity of both compounds how do I find the pH with the two Molarities of HCl and CaO? I'm not completely sure if this is right but, for #34, I did the neutralization rxn between HCl and CaO. One of them is limiting, so if you use all of it up, you form water. The ...

kamalkolluri

We were just beginning to learn this in class, but I can help with the first one. In class, Lavelle gave NH4Cl as an example. So If you write the equation for NH4Br reacting with water, you get NH4Br+H2O --> NH3 + Br- + H3O+. So NH4Br would be acidic because when you put it in solution, it creates m...

kamalkolluri

Another thing we learned in class is the stability of the ion when it is broken from the H. So ClO4 is the strongest because it has more oxygens, and hence, more resonance, making the ion stronger than the others.

kamalkolluri

I think Lavelle taught us in class to use chloro, fluoro, etc so I think using cyano should be fine

kamalkolluri

I don't think it was clear in lecture, but I'm assuming it's because transition metals are group 3, so they can access d-orbitals to make more bonds.

kamalkolluri

Haha I just typed out an answer to this XD. What you would do is since its written (CH3)2Be, you know the CH3 (methyl) groups are going to be separate. And technically, Be is the least electronegative atom in this structure, so it would be in the center connecting to each of the CH3's on its sides.

kamalkolluri

A lot of the time it's written a certain way which helps you figure it out. So for example in that one, it's written (CH3)2Be. This means the CH3 group is together. and since Be forms two bonds you use intuition to know that its in the middle and connects to CH3 on each side.

kamalkolluri

angelagd3l wrote:Do lone pairs also affect the way the model is oriented? Because of the repulsion?

Yes, they do because lone pairs repel the bonding pairs, which changes the orientation. The best example is how in a linear molecule, if you add a lone-pair, the molecule becomes bent.

kamalkolluri

The Oxygen gets the negative formal charge because it is more electronegative.

kamalkolluri

For an example in which dipoles don't cancel out, look at CH3Cl. C is the central atom with 3 H's and 1 Cl around it. Since Cl is way more electronegative than H, the dipole moment (or partial charge) on Cl is greater than the partial charges on any of the H's. Since the electrons are unequally attr...

kamalkolluri

I'm also a bit confused but as of what I understood, there are a total of 4 bonds on the Carbons. The electrons occupy 3 sp2 orbitals on the carbons which accounts for 3 bonds and then I think the fourth bond has electrons in an unhybridized p orbital

kamalkolluri

By adding a lone pair on Sulfur, and only forming single bonds with Cl, it makes the formal charge 0 for every atom

kamalkolluri

Actually, in discussion today, we learned about T shape molecules. IT is possible to have 2 or 3 or even 4 lone pairs around the central atom and then 2 other atoms connected to it to form a linear shape. So part b) would be that it MAY be possible to have lone pairs.

kamalkolluri

We don't need to know how to draw wedge and dash notation. We just need to understand that its a representation of the molecule's shape in 3D space. And it doesn't really matter where the atoms go I think

kamalkolluri

so NO3- is a resonance hybrid so the electrons in the double bonds can exist anywhere. This causes the bond length of N-O to be an average of the NO double and single bonds. The overall bond lengths are all the same

kamalkolluri

As the others said, the electronegativity of Carbon is lower than Oxygen, which causes oxygen to "hog" the electrons more, giving it a negative formal charge.

kamalkolluri

In class, we discussed the Lewis structure of HClO3, which has Cl in the center (with a lone pair) and two oxygens double bonded to it, an oxygen single bonded and then the Hydrogen attached to the single bonded oxygen. This minimizes formal charge. However, when you single bond the Hydrogen to the ...

kamalkolluri

Definitely ask Lavelle or the TAs, but my thinking is that we just need to know that the wave functions represent orbitals and the wave function squared represents electron density distribution or the probability of finding an electron. Know nodal planes and how there's 0 probability of finding an e...

kamalkolluri

Ga becomes 3+ because it is easier to remove 3 electrons than add on 5 more

kamalkolluri

Xenon has n = 5, which means l can be 0,1,2,3 and 4. This means it has access to s p d and f and g orbitals. Chlorine has n =3, which means it has l = 0,1,2 and this means it has access to the s p and d orbital. This means xenon has access to more orbitals than chlorine, which means more electrons

kamalkolluri

I think you're gonna have to write out the resonance structures. It'll definitely not be something crazy, I think something like the NO3 example will show up.

kamalkolluri

It is because the 2py is down spin because everything should be up spin in a ground state Nitrogen atom. Hund's rule says that the electron needs to be parallel in spin.

kamalkolluri

I think a lot of confusion comes from the fact that we call it the s-orbital, p-orbital, etc. S, P, D, and F are all subshells, and S has 1 orbital, P has 3 orbitals (Px, Py, Pz), D has 5 orbitals (Dx, Dy, Dz, Dx2-y2, Dz2) and so on. n = energy level, l = subshell (0=s; 1=p; 2=d) and then the amount...

kamalkolluri

I think all you need to know is that orbitals are wave functions that take values of n, l, and ml, and that the square of the wave function is the electron density distribution. Also I remember him saying in office hours to know the concept behind hamiltonian derivatives, but I was confused on this ...

kamalkolluri

Another helpful resource to see what will be tested for Quantum is the outline. Go to the "Quantum World" outline on his site and you can find everything he requires for this test.

kamalkolluri

We just go by what is taught in class. So for example, we are still on Sections 1A and 1B, which means just do problems from there. There are definitely 7 problems based on topics we learned over the last couple of days that you can submit as homework for this week.

kamalkolluri

Problem: "Protons and neutrons have nearly the same mass. How different are their wavelengths? Calculate the wavelength of each particle when traveling at 2.75*10^5 m/s in a particle accelerator and report the difference as a percentage of the wavelength of the neutron. I was confused as to wha...

kamalkolluri

The formula for Kinetic Energy is K = (1/2)mv^2

The formula you listed is the formula for the Kinetic Energy of an electron.

kamalkolluri

I used 3.00 *10^8 m/s because that's what Dr. Lavelle put in his notes I'm pretty sure.

kamalkolluri

I'm pretty sure they expected higher intensity of any wavelength to be able to excite electrons but it turned out only certain wavelengths of light had photons that had enough energy to excite electrons

kamalkolluri

Let's say you did an experiment trying to find out how much water is produced from a combustion reaction. And lets also say the actual value you should have gotten should be 1 Liter of water. So if you do your trials and you get 0.2L, 0.16L, 0.18L, and 0.22L, your data is precise because all of your...

kamalkolluri

So I asked Dr. Lavelle about this today. You are allowed to turn in however many problems you want except the TAs will grade the first 7 problems.

kamalkolluri

The first thing to notice is that the equation isn't actually balanced. So do that first. It should come out to C6H9Cl3 + 3AgNO3 --> 3AgCl + C6H9(NO3)3. After that convert the gram measurements to moles based on molar mass: use grams/Molar Mass. Then find your limiting reactant and use the ratios fr...

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