CHEMISTRY COMMUNITY

OwenSumter_2F

Whoops, my bad, it's actually k/k' (rate of forward / rate of reverse)!

OwenSumter_2F

I would assume though that there will be a focus on new material, with important factors of the other midterms included.

OwenSumter_2F

From Monday's lecture, we learned that the equilibrium constant K is equal to the rate of the reverse reaction (k') / the rate of the forward reaction (k) because of how the concentrations line up.

OwenSumter_2F

This equation is derived from ln k = - Ea/RT + lnA.

OwenSumter_2F

Hello,

I am having trouble with this problem:

A certain reaction has an enthalpy of ΔH=49 kJ and an activation energy of Ea=66 kJ.
What is the activation energy of the reverse reaction?

Where would I start?

OwenSumter_2F

The opposite applies to half lives, a larger k means a shorter half life.

OwenSumter_2F

And it also has the small r to differentiate it from the Boltzmann's constant - k. k_r is not a constant and has a different value based on the reaction.

OwenSumter_2F

The orders are similar to our calculations of K and Q at the beginning of the quarter, where if something had a coefficient of 2, it would be squared and so on. The order relates to that ^n on kinetics problems and specifies what specific calculations we do.

OwenSumter_2F

If you can find the k value linked to these two concentrations, you can use that to find the half life as well.

OwenSumter_2F

You can also experimentally find the order of a reaction, which you can try out in Sapling.

OwenSumter_2F

Yep, exactly!

OwenSumter_2F

And most times in real world applications, it will be under non standard conditions.

OwenSumter_2F

I'm pretty sure Lavelle mentioned in lecture that if the concentrations are the same, E is 0V.

OwenSumter_2F

On #5 of this weeks Sapling HW, one of my answer boxes has a yellow box in it that says "Missing superscript or subscript argument" and won't let me edit it or delete everything. Is there a way to contact support or have a work around?

OwenSumter_2F

I'm having a lot of trouble with this problem:

Separate this redox reaction into its balanced component half‑reactions. Use the symbol e− for an electron.

Cl2+2Cs⟶2CsCl

I'm not exactly sure where to start, even though I kind of understand other redox reaction balancing questions.

OwenSumter_2F

Constants are always derived somehow, they aren't a number randomly chosen. Most are experimentally found and required lots of work to figure them out, but now that we have them we can use them whenever we can!

OwenSumter_2F

It was strange to me when I did it and got a very large number, but trust your gut for the math and you should be good! :)

OwenSumter_2F

While using Kelvin to solve problems, just make sure you remember to convert back to Celsius at the end if that is what the question calls for!

OwenSumter_2F

To add on, both (galvanic/voltaic and electrolytic) are types of cells produced by an electrochemical cell.

OwenSumter_2F

Hi, I am having trouble with this question:

Write a balanced overall reaction from these unbalanced half-reactions.

Sn⟶Sn2+

Ag+⟶Ag

I thought by balancing it out with added electrons you would get

2Sn + 2Ag+ -> Sn2+ + Ag

But that is not correct. Can anyone help?

OwenSumter_2F

1. Yes first convert the watts to joules. 2. Then find the moles of molecules by finding the difference in the initial and final mass and dividing it by its molar mass. 3. Divide the energy(joules) by the moles to find the enthalpy of vaporization, and convert it to Kilojoules. 4. The molar entropy...

OwenSumter_2F

Yeah, the most important thing for these types of equations is to make sure everything is kJ or J. For Gibbs Free Energy problems, it seems like generally the only thing that is in J is the R, while everything else is in kJ.

Hope that helps!

OwenSumter_2F

Yes, Boyle's law states that pressure and volume are inversely related, so V=1/P and vice versa!

OwenSumter_2F

Exactly, and in the textbook it said that entropy normally increased as the complexity of a substance increased, which might be helpful!

OwenSumter_2F

Yes, I viewed it as the higher changes in entropy meant that the sample had to go farther to become a certain amount of disordered, meaning it was farther away from being disordered to begin with.

OwenSumter_2F

S_TOTAL = S₁ + S₂, which is equal to k_B * W₁ * W₂

OwenSumter_2F

So for the flask example, there are two states that a molecule could be in: either on the left side of the flask, or the right side. Degeneracy is the number of ways of achieving a given energy state, so with one molecule in that flask, degeneracy is 2, because the molecule could be either on the le...

OwenSumter_2F

Exactly, it can be a bit confusing, but capital W is degeneracy and lowercase w is work. Work is totally unneeded in this equation! :)

OwenSumter_2F

So for the flask example, there are two states that a molecule could be in: either on the left side of the flask, or the right side. Degeneracy is the number of ways of achieving a given energy state, so with one molecule in that flask, degeneracy is 2, because the molecule could be either on the le...

OwenSumter_2F

The Boltzmann Equation is getting entropy S, from degeneracy W by finding the natural log of W and multiplying it by the Boltzmann constant k_B. W is degeneracy while w is work, and work is not part of this equation :)

OwenSumter_2F

I think it's because the system is already balanced so any changes will only be small incremental ones experimentally.

OwenSumter_2F

I think at equilibrium, a reaction is reversible also.

OwenSumter_2F

We most likely won't be working with those processes when doing math anyway, I think it's just something to know.

OwenSumter_2F

I'm having trouble with this question: A sample of an ideal gas in a cylinder of volume 3.25 L at 298 K and 2.16 atm expands to 7.43 L by two different pathways. Path A is an isothermal, reversible expansion. Path B has two steps. In the first step, the gas is cooled at constant volume to 1.01 atm ....

OwenSumter_2F

Were we given PV=nRT or what R is? I'm working on the Sapling questions for this week and I'm having trouble remembering doing these.

OwenSumter_2F

Yep you are correct, if you look at the differences of slopes between the phases it should help visualize this! :)

OwenSumter_2F

One way to look at it is to visualize it in a different form. Say you need 5 Joules of heat to change an ice cube into a puddle of water. Anything below that 5 Joules of heat would increase the sample that much in temperature, but 5 is the maximum. Any more than 5, and the extra heat is used to chan...

OwenSumter_2F

An example would be to go from New York City to Los Angeles would be around 2,800 miles but if you were to take that trip it would most likely be more as you would not be able to go in a directly straight line. That first number (2,800) is a state function, but the actual miles you went would not be.

OwenSumter_2F

Yes it also has a lot to do with diffusion of the gas particles once it comes out in a daily example.

OwenSumter_2F

To go into more detail, in relations to phase changes vapor is more commonly used while gas is more commonly used in most other senses.

OwenSumter_2F

I think we will also learn more about how temperature affects things in a later chapter!

OwenSumter_2F

I'm having problems with Sapling Week 2 #5. The question is: The Kb for an amine is 2.666×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.976 ? Assume that all OH− came from the reaction of B with H2O. I tried to work backwards by finding the pOH from the p...

OwenSumter_2F

Yes, you'll have to memorize the strong acids and bases, they should be in your textbook. As for conjugate bases/acids, they are what's left of an acid/base respectively after taking away or adding an H+

OwenSumter_2F

Just be wary about using left/right as opposed to reactants/products as you may have a reverse reaction or something!

OwenSumter_2F

In addition, anything with both H₃O⁺ and OH^- will multiply to the K_W, just normally not the same number

OwenSumter_2F

In addition, anything with both H₃O⁺ and OH^- will multiply to the K_W, just normally not the same number

OwenSumter_2F

If it makes more sense, try working out the K value of a reaction one way, and then try working out the K value when it is reversed. You will find that the two K values are reciprocals of each other.

OwenSumter_2F

Awesome thank you, that clears up my confusion! :)

OwenSumter_2F

K is the more common equilibrium constant, and as a general rule we will most likely be working with K, as that is what a reaction will tend to level off to.

OwenSumter_2F

So with the equilibrium constant formula being K= [C]^c[D]^d/[A]^a[B]^b, how do we found those concentrations?

OwenSumter_2F

I am reading through textbook topic 5G and I am confused on what an activity is. Is it just a broader term for the concentrations of certain pieces of an equilibrium or is it something else?

OwenSumter_2F

Also make sure to take into account that Lewis acids and bases correspond to donating lone pair electrons, while Bronsted acids and bases correspond to donating H⁺ ions.

OwenSumter_2F

Also make sure given any of the four components (pH, pOH, OH^- levels, H⁺ levels) that you can get the other three, as seen in the Sapling homework for this week.

OwenSumter_2F

You would need more context dealing with bases. An acid only gives off protons if it has a base or bases to give them to. A polyprotic acid can release more than one proton but it doesn't have to. In fact, it is much easier for it to donate only one than two or more.

OwenSumter_2F

Also you can be pretty confident with weak acids or weak bases if they don't follow the general formula for the strong versions (i.e. a metal and OH for strong bases and a starting H for strong acids). Obviously some specifics will have to be memorized but you can use some process of elimination if ...

OwenSumter_2F

Also make sure you consider charge. If a base is neutral, its conjugate acid will be + charged. If a base is -, its conjugate acid will be neutral, and so on.

OwenSumter_2F

If it's polydentate, then it'll be able to bind to those sites, we won't be getting that deep into the actual reality of it in this class.

OwenSumter_2F

It's better if you know them, but you probably won't be tested on knowing the exact names and numbers and rather how to use them.

OwenSumter_2F

I'd definitely suggest mnemonics for some of the less common ones but try to remember some of the more common ones.

OwenSumter_2F

Also one thing I was struggling with is that if there are two or more ligands that need a prefix, put each in front of each ligand, rather than just 1 prefix.

OwenSumter_2F

So the question on Sapling Q1 is to give the systematic name of this coordination compound: [Co(NH₃)₄Cl²]Cl. I thought the answer was tetra ammine chloro cobalt (II) chloride, but that isn't correct. What part am I doing wrong?

OwenSumter_2F

This is also featured in multiple sapling questions about hybridization, where it focuses on one central atom and then another.

OwenSumter_2F

For the common shapes, it should be fairly easy to know those angles since we work with them regularly. For shapes with lone pair electron density areas, those bond angles would be smaller than normal but different for each molecule, and we won't have to calculate those.

OwenSumter_2F

Also check that you didn't accidentally switch the answers for the two molecules, because what you got is correct. Also you could try refreshing the page or something.]

OwenSumter_2F

From what I'm seeing, there is a PCL₃ question which would be sp3 and a PBr₅ question which would be the sp3d you are talking about.

OwenSumter_2F

You would have to find the formal charge to see what has the lowest formal charge, which is what you want.

OwenSumter_2F

To add on to what the others have said, if you are asked to give the most probable Lewis structure, you would give the last one with the lowest formal charge, however resonance is all possible structures.

OwenSumter_2F

It has to do with the chemical formula and composition. In this example, the H connects to the O for some reason, maybe it was already in a molecule with it and two molecules joined or because it is forming a Hydrogen bond, but it does not always have to bond with the central atom.

OwenSumter_2F

Furthermore, most questions of electron configuration won't ask for the electron spin since it is a 50/50 and you can't know more without more context. That being said, if you see a question where the answer for electron spin is something such as 0 or 1 you know it is wrong.

OwenSumter_2F

It is easier in my opinion to think of it as a spectrum rather than absolutes. For example, on homework problems asking for the molecule that is most ionic in character, it would be the molecule with the highest difference in electronegativities. So viewing it as such instead of strict rules helps m...

OwenSumter_2F

Also, as far as molecular shape goes, bonds don't change the shape whether they are single or double or triple, yet lone pairs can.

OwenSumter_2F

I found this guide for oxidation numbers online that I thought was helpful also.

OwenSumter_2F

Yeah, resonance structures are normally pretty redundant with large amounts of atoms, so it's pretty tricky to remember each atom you've used with double bonds and which you've haven't. Every individual atom gets a fair shot, not just the atom types.

OwenSumter_2F

Like others said, find the one with the least formal charge, but if resonance is in question, you'll want both types with an arrow pointing between the two.

OwenSumter_2F

This is also seen on Sapling Q10, where it asks you to arrange 5 bonds from most ionic to most covalent. This is found by the largest differences in electronegativity, or the farther away the two atoms are on the periodic table. It is more useful to know this as a spectrum.

OwenSumter_2F

Got it, so it relates back to orbitals and where on the periodic table it is?

OwenSumter_2F

And then the second number, the subscript, is the amount of orbitals in each subshell.

OwenSumter_2F

Yes, these are primarily theoretical and if new elements were discovered with these higher orbitals we would know how to handle them!

OwenSumter_2F

Normally for answers for problems in this class, we won't be getting really weird and large mole numbers, it will normally be something like 2:2:1, so if you get some really weird combo like 13:27:6 you may have done something wrong in rounding or in the calculations, but usually rounding will be ab...

OwenSumter_2F

As the title says, how do we know when an element will have more than 8 electrons? Is it just when making a structure and we realize we need more bonds, or is it something we can predict beforehand? Thanks!

OwenSumter_2F

You can think of them conceptually the same way if that helps you visualize it, but they are two different numbers and concepts.

OwenSumter_2F

Also, unless a question specifically asks for a certain unit, you can usually use the base unit (meters, grams, etc.). And if a problem does want for example picometers, having 1.5 x 10^-8 is just as correct as 150 x 10^-10.

OwenSumter_2F

To provide a bit of reasoning why, sig figs are used to show how certain you are of a certain number. So if a certain number i.e. 20 has two zeroes after the decimal place, it means you are certain that it is 20.00 up to those two decimal places. Rather than if you only had 20, you would be certain ...

OwenSumter_2F

This is something that was always kind of confusing to me, but what helped me was that the E=hv equation is the energy of a single photon. So to find the total amount of photons, you would take the given energy and divide it by the energy of a single photon.

OwenSumter_2F

Also, in equations, velocity and frequency shouldn't be used together in the same step for what we're working on. For example, velocity should only be found working with kinetic energy of an electron or with momentum in the Heisenberg Uncertainty Principle, otherwise frequency will be used such as i...

OwenSumter_2F

For our normal discussion section time, do we join the regular zoom link first and then follow instructions of the TA or do we just join the zoom through the midterm straight away?

OwenSumter_2F

The question will also most likely have the experimental yield shown somewhere, so if that is in kilograms or milligrams or something to that effect it would be best to put the theoretical yield into that unit as well, this will make it simpler if you have to do a percent yield equation.

OwenSumter_2F

Also some certain conversions such as between metric units (i.e. kilograms -> grams or meters -> nanometers) won't be on the equation sheet as that is something we are expected to know, so I would make sure you have that down. :)

OwenSumter_2F

It is also covered in textbook section 1D

OwenSumter_2F

And also, memorizing the names of chemical formulas is just for quicker work, in actuality it would be nearly impossible to memorize a whole lot of them, so you can eventually know some ones you work with often, but it's not a necessity.

OwenSumter_2F

To add on to what Truman said, they are the same formula, one is just shorter (E=hv). The other, E=hc/λ is the combination of E=hv and c=λv, it is just there to show how the two equations work together.

OwenSumter_2F

So I was just wondering, since when we use the mass of electrons in the de broglie equation we use 9.11x10^-31kg, do we always use kilograms, and then do we always use meters, or does it not matter?

OwenSumter_2F

I'm having some trouble with this problem. 1B.7 Sodium vapor lamps, used for public lighting, emit yellow light of wavelength 589 nm. How much energy is emitted by (a) an excited sodium atom when it generates a photon; (b) 5.00 mg of sodium atoms emitting light at this wavelength; (c) 1.00 mol of so...

OwenSumter_2F

Also, try not to round your numbers until the final answer, or else your answer may be off a couple of decimal points.

OwenSumter_2F

This was also confusing to me, but I was able to rationalize by thinking of Avogadro's number or moles not as just used for elements or molecules but as a number that can be used for any purpose. Therefore, you can look at it in simpler terms, such that if I had a ten pencil boxes with 3 pencils in ...

OwenSumter_2F

Also, as an additional note, try not to round your numbers until the final answer, otherwise you could be off by a few decimal points.

OwenSumter_2F

Also, using g or mg or any other unit like that is perfectly fine, it is just a convenience for us. It would be very annoying having to write 3.2x10^-6kg in every lab working with small amounts.

OwenSumter_2F

I think its mostly just how you work best and to make it simpler to you. If you want to memorize or get in the habit of knowing basic naming conventions for compounds, that will make it a quicker and easier process for you in the future, however you should be fine either way.

OwenSumter_2F

Yes, it depends on how precise your calculation is. For example, if your graduated cylinder measured to the hundredths place, you could measure exactly 100.00mL of water. But if the graduated cylinder only measured to the tenths place, you could measure exactly 100.0mL of water. Even if there is exa...

OwenSumter_2F

Formula units are the term for ionic compounds, such as molecules for a regular compound or atoms for atoms.

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