CHEMISTRY COMMUNITY

Chloe Thorpe 1J

I thought we weren't supposed to use the Nernst equation on test 2?

Chloe Thorpe 1J

I cannot speak with certainty, of course, but I am guessing there will be at least one question that corresponds with each outline, plus a few quick multiple choice/true or false/very short answer questions concept. Certain outlines could also have multiple dedicated questions, like maybe thermo sin...

Chloe Thorpe 1J

I've seen it in atm most of the time, but the conversion is 1atm = 760 Torr if you need it.

Chloe Thorpe 1J

I'm not totally sure about the first question; but, as for the second, I do my calculations in mmol and then convert it to mol at the end. I don't think it matters as long as you're consistent.

Chloe Thorpe 1J

In a first order reaction, the reaction rate is proportional to the concentration to the first power. In a second order reaction, the reaction rate is proportional to the concentration squared.

Chloe Thorpe 1J

I don't think the O3 or O2 is charged since no charge was indicated.

Chloe Thorpe 1J

You have to figure out the oxidation states before and after by finding the charges of each element and matching it to what is given. If the oxidation number goes up, then it is oxidized. If the oxidation number goes down, it is reduced.

Chloe Thorpe 1J

On another post, Chem_Mod said that "if you have two ions (or an ion and a gas), you need a conducting metal that will act as the conductor of electricity for your cell so you have to add in an inert metal (Pt) that will act as the electrode. Solids (for test purposes) should always be placed o...

Chloe Thorpe 1J

7th edition 6K5 is a good practice problem for that.

Chloe Thorpe 1J

It usually works to add the amount of H2O needed to one side and double the amount of OH- to the other side [e.g. you need 4 moles of oxygen: add 4 moles of H2O to one side and 8 moles of OH- to the other]. It just takes a lot of trial and error, for me, but it gets easier the more you do it!

Chloe Thorpe 1J

Oxidation is when electrons are lost by something, while reduction is gaining electrons. You can remember this using OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Chloe Thorpe 1J

On question 3D on the midterm, how do we know the states of the reactants and products in the expression for Ka2? (like liquid, aqueous, etc.)

Chloe Thorpe 1J

I understand the function of a salt bridge, but how does the porous disk act in the absense of the salt bridge? Is it a different setup entirely?

Chloe Thorpe 1J

Why can we assume that ΔSº and ΔHº are constant in the Van't Hoff Equation?

Chloe Thorpe 1J

Sorry, I meant that heat capacity is extensive and molar/specific heat capacity is intensive!

Chloe Thorpe 1J

Lyndon said in the review today that the state functions are: PD TV HUGS (pressure, density, temperature, volume, enthalpy, internal energy, gibbs free energy, and entropy). You can remember this by thinking, "I'm under a lot of PRESSURE. I'm feeling DENSE. I just want to watch TV and get some ...

Chloe Thorpe 1J

Extensive properties depend on the amount of substance while intensive properties do not. For example, molar heat capacity is an extensive property since it is J*mol^-1*K^-1 while heat capacity is intensive since it is J*K^-1

Chloe Thorpe 1J

Saying a gas is an ideal gas assumes that it is under perfect conditions so that PV=nRT is true. Basically, it means there are no attractive forces between the gas molecules, and that they occupy all of the available space.

Chloe Thorpe 1J

My TA said that it helps to draw out the graph that Lavelle had on the slides showing why steam causes more severe burns than water at the same temperature. When the graph is diagonal, you use the specific heat of the compound. When it is horizontal, you use latent heat in the calculation.

Chloe Thorpe 1J

That's correct, expansion work requires that volume is changed as it is the work as a result of the change in volume. The pressure is constant as well, as Edgar said, since it is built into the equation.

Chloe Thorpe 1J

The conversion from Celsius to Kelvin is [K] = [C] + 273.15. So, 37.6ºC would be 310.75 or approx. 310.8ºK. Likewise, 157.9ºC is 431.05 or approx. 431.1ºK.

Chloe Thorpe 1J

It also helps if you look at the calculation. For atoms, Cv is 3/2*R (R being a constant), while for Cp it is 5/2*R.

Chloe Thorpe 1J

The change in volume in this question is given by -π*d*r^2. So, it would be -π*(20)*(1.5)^2 = -0.14 cm^3. Then, you multiply that by 1L/1000cm^3 to get it in the right unit, so you have -0.14 L.

Chloe Thorpe 1J

Yes, R is a constant. It only changes slightly due to different units used, but it is typically 8.314 J·K-1·mol-1. You can find this on the Constants and Equations worksheet on the class website.

Chloe Thorpe 1J

Pressure is in atm or bar, concentration is molarity. Gas has a molar concentration, but I'm not sure what you mean by pressure concentration?

Chloe Thorpe 1J

Like the other answer said, stronger acids have higher concentrations of H+, which leads to a lower pH (more acidic). A weak acid will not fully ionize, so there will be less H+ atoms in solution (lower [H+] = higher pH = less acidic).

Chloe Thorpe 1J

I think we are supposed to go off what Lavelle usually does, so rounding to sig figs at the very end. Not 100% sure, but it's probably a more accurate answer that way.

Chloe Thorpe 1J

Someone in lecture asked if, for example, the 1.3% ionization meant that only 1.3% of the acid reacted with the water. Is this correct? I didn't catch the answer.

Chloe Thorpe 1J

I noticed that too, it seems to be balanced if the stoichiometric coefficient on the product side for N2 is 5. Not sure if this is a textbook error or if there's another explanation for it.

Chloe Thorpe 1J

These links have some good explanations and examples of superacids and superbases. They say it better than I can, hopefully this helps!

Superacids: http://www.assignmentpoint.com/science/ ... racid.html
Superbases: https://www.thoughtco.com/most-common-s ... ses-603649

Chloe Thorpe 1J

I noticed that, too. It might just be a solution manual error, but it's not on the list of errors on the class website. Lavelle did mention in class that the equations have to be balanced to get the right answer, so I think it would be safer to balance it. Maybe the 7 in front of the N2 is supposed ...

Chloe Thorpe 1J

So it only means that the reverse reaction will occur?

Chloe Thorpe 1J

In the 7th edition textbook on page 416, it says "If Q>K, the reverse reaction is spontaneous...". What does it mean by spontaneous?

Chloe Thorpe 1J

I think that for part b, you would use the concentration because the products are aqueous solutions. Whereas if they were gases, you would use the partial pressure.

Chloe Thorpe 1J

In a review session, one of the TAs said you look at how many places the ligand can coordinate, and look at if the orientation allows for coordination. For example, NH3 is monodentate because there is a lone pair on the N which allows for coordination. H2O is also monodentate, even though it has two...

Chloe Thorpe 1J

As far as I know, we only look at the givens. Constants do not count for sig. figs.

Chloe Thorpe 1J

The Rydberg equation applies only to hydrogen atoms (or hydrogen-like atoms with 1 electron). The constant is R=3.28984*10^15 Hz, and is given on the constants and equations sheet attached to the tests.

Chloe Thorpe 1J

You can use either one, but you have to tweak your calculation slightly to account for how many mol of H there are in H2O. Using Molar Mass of H2: 0.3172 g H2O * (1 mol H2O/18.015 g H2O) * (2.0158 g H2/1 mol H2O) = 0.03552 g H Using Molar Mass of H (personally, I use this method): 0.3172 g H2O * (1 ...

Chloe Thorpe 1J

The Chem_Mod answer to this question on another post has a nice explanation for this: viewtopic.php?t=2892

Chloe Thorpe 1J

The solutions say the formula is K3[Cr(CN)6]3-. Is it K3 since the ion has a 3- charge?

Chloe Thorpe 1J

I believe AlCl3 and Al2O3 and common examples of Lewis acids that aren't Bronsted acids.

Chloe Thorpe 1J

Lavelle said that everything in the syllabus will be covered on the final, so I think that anything is fair game. I think he said at least 1 problem per outline, possibly 2 for some.

Chloe Thorpe 1J

I think H, He, Li, Be, and anything in group 13 can have an incomplete octet. Any element in row 3 or below can have an expanded octet because of the d-orbital.

Chloe Thorpe 1J

Can bent molecules or trigonal pyramidal molecules (and the like) ever be nonpolar?

Chloe Thorpe 1J

I noticed this too. I believe Lavelle said that, in class, we draw the dipole from the positive to the negative side. As long as you label the delta negative and the delta positive, I'm sure you'll be fine, but I think we are supposed to draw them positive to negative.

Chloe Thorpe 1J

I thought that pi bonds were weaker than sigma bonds since the amount of overlap in a sigma bond is greater than that in a pi bond. I try to imagine the lobes of the p-orbitals overlapping in a sigma bond versus a pi bond; the head-to-head overlay > side-to-side overlay.

Chloe Thorpe 1J

I think it would be non-polar since it has no positive/negative "side." It is made up of atoms of the same element, and molecules of this type are usually not polar, and its geometry is symmetric.

Chloe Thorpe 1J

I thought it was the central atom in both?

HCN: http://www.chemeddl.org/alfresco/d/d/wo ... 9f/hcn.png
ClCN: https://nonsibihighschool.org/intbasch11_files/ClCN.png

Chloe Thorpe 1J

From my understanding, we use the actual shape (trigonal pyramidal). The bond angle is slightly less than 109.5 because the atom arrangement (which I call the parent shape) is tetrahedral, which has bond angles of 109.5 like you stated. But, trigonal pyramidal shapes have a lone pair, which sort of ...

Chloe Thorpe 1J

What the other people said is right, I think. Usually, Prof. Lavelle says the molecular geometry first (the one including lone pairs), then determines molecular shape (the one only including bound atoms).

Chloe Thorpe 1J

In short, the difference is that sigma bonds overlap end-to-end and are the strongest covalent bonds, while pi bonds overlap side-by-side. I believe "component orbital" refers to the orbitals that sort of make up the hybrid orbital. For example, in 2sp, the component orbitals would be 2s, ...

Chloe Thorpe 1J

I believe London Dispersion forces are a subcategory of Van Der Waals forces, so it would apply to them as well. Not totally sure though.

Chloe Thorpe 1J

So I know that sigma bonds are end-to-end and pi bonds are side-to-side, but what exactly does this mean? What does are we refering to when we say “end-to-end”?

Chloe Thorpe 1J

Are we supposed to memorize bond angles for different shapes? Or is there a way to calculate them? Just wondering in case it comes up on an exam.

Chloe Thorpe 1J

It helps me to just remember that tetrahedral has a general formula of AX4 while trigonal planar has a formula of AX3. (A being the central atom and X being bound atoms)

Chloe Thorpe 1J

I think I remember Professor Lavelle saying that we would almost always be given the formula, but it may help to memorize some common ions, prefixes, and suffixes. To find out which bonds are ionic vs. covalent, I believe you'd have to look at the components of each compound and compare it to what w...

Chloe Thorpe 1J

I believe that answer would suffice, especially since you brought in formal charge to explain your reasoning.

Chloe Thorpe 1J

Is one dependent on the other? I know they follow the same trends but I don't quite understand the difference between the two.

Chloe Thorpe 1J

Sorry this isn't a question.

Chloe Thorpe 1J

I believe you would divide by Avogadro's number, then convert the grams to kilograms by adding *10^3.

Chloe Thorpe 1J

Can someone explain why we place the atom with the lowest ionization energy in the middle of a Lewis structure?

Chloe Thorpe 1J

So I know that atomic radius decreases as you move right across a period, but can someone explain why? Is it because there are more protons so they have a stronger pull on the electrons?

Chloe Thorpe 1J

The subshell 2d cannot exist since the greatest possible value of l is n-1. So in this case, since n=2, l can only be 1 or 0. l=1 corresponds with p and l=0 goes with s.

Chloe Thorpe 1J

I was wondering this as well. The textbook says it's initial-final, so I've been using that for my calculations.

Chloe Thorpe 1J

The question is: How many orbitals can have the following quantum numbers in an atom: c) n=2

I thought it was 3 (1s, 2s, 2p), but the textbook solutions say the answer is 4. Can someone explain this to me?

Chloe Thorpe 1J

My TA said that we should always use whatever is on the formula sheet so we can avoid as much rounding error as possible. Same goes for other constants (h, R, etc.).

Chloe Thorpe 1J

Yes, you have to convert the units to SI units in order to cross out the units. It helps if you write out all the units, like 1 J = 1 kg*m^2*s^-2, and then cross off as you go to make sure you get the right conversion.

Chloe Thorpe 1J

It gives a range of possible values that the velocity could take on. So, for example, if you have an initial velocity of 5m/s, and you calculated the change in velocity to be +/-2, the final velocity could be between 3 and 7m/s. It can sort of be compared to the margin of error, if that makes sense.

Chloe Thorpe 1J

Wavelengths, by convention, can’t be negative as far as I understand. For instance, wavelength is equal to the speed of light over frequency, both of which are always positive values; so, wavelength couldn’t be negative. It’s also usually measured in a unit like meters, which can’t have negative val...

Chloe Thorpe 1J

Dr. Lavelle just clarified in lecture that this relationship only applies to electromagnetic radiation, so it can’t be used for all waves.

Chloe Thorpe 1J

I believe 1 keV is equal to 1.6022 x 10^-16 J, so you'd just multiply the amount of keV by that number to convert the units.

140.511 keV * (1.6022*10^-16 J / 1 keV) = 2.25*10^-14 J

Chloe Thorpe 1J

As far as I know, electrons have the same mass of 9.109 x 10^-31 kg.

Chloe Thorpe 1J

Yes, that's correct. Frequency and wavelength have an inverse relationship, so frequency increases as wavelength decreases.

Chloe Thorpe 1J

@Melissa:

The initial volume is 0.020L because only 0.020L of the original solution is diluted. The 0.150L is there so that you can find the initial concentration of the solution. Hope that makes sense!

Chloe Thorpe 1J

In the homework question F.3 (7th edition), part a, it asks you to write the formula for nitric acid. How would you find this? And will there be questions/parts of questions on exams that ask you to start with writing the formula for a compound from only the name? Sorry if this is a dumb question, I...

Chloe Thorpe 1J

I have done the post-module assessment for the Audio-Visual Focus Topic a couple times, and I keep getting question 18 wrong. It reads: Determine the limiting reagent if 21.4g NH3 reacts with 42.5g O2. 4 NH3 (g) + 5 O2 (g) -> 4 NO (g) + 6 H2O (g) I know I'm doing something wrong because I get that N...

Chloe Thorpe 1J

This will be quite similar to Layal's post, but it may help to see it in a slightly different way! First, I found molar mass of KMnO4: (molar mass K) + (molar mass Mn) + 4(molar mass O) = 39.10g/mol + 54.94g/mol + 4(16.00g/mol) = 158.04 g/mol Then, use KMnO4's molar mass to find how many moles of KM...

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