CHEMISTRY COMMUNITY

clairehathaway 2J

Additionally, if you think about this in terms of Ecell values you will see that they will cancel each other out. Because a concentration cell is made up of two identical half reactions when you add the anode and the cathode E naught values you will be left with 0. For example, the equation Pb2+(aq)...

clairehathaway 2J

I agree with Kailani, looking at reduction potentials would probably be the best way to solve this problem. In terms of the reduction potentials; the more negative a value, the better reducing agent, and the more positive a value, the better oxidizing agent.

clairehathaway 2J

I agree with what was said above and to add on, you can use the Ecell naught values to calculate the Ecell value with the Nernst equation.

clairehathaway 2J

I agree that standard reduction potentials are used mainly to calculate cell potential. Since they are all reduction potentials we know that a negative value indicates that the compound is likely to be oxidized. I also think it's important to remember that there is no special reason that standard re...

clairehathaway 2J

Standard reduction potentials are the values that we would use to calculate a standard cell potential. When doing the calculations for the standard cell potentials it is important to remember that in a redox reaction one equation is reduced and the other is oxidized. Because of this the reaction bei...

clairehathaway 2J

Are calorimeter constants (heat capacity of calorimeter) always positive?

clairehathaway 2J

Today in lecture Dr. Lavelle gave us the mnemonic "LEO" to remember that oxidation has a "loss of electrons". In high school I also learned the mnemonic OILRIG to help with redox reactions, it stands for “Oxidation Is Losing (electrons) and Reduction Is Gaining (electrons)"....

clairehathaway 2J

Yes, Dr. Lavelle specified a couple of times that the anode was on the left and the cathode was on the right so I would assume that unless otherwise marked that is always the case.

clairehathaway 2J

You use avogadros number when you have a mole of a certain molecule. This would make degeneracy raised to the power of avogadros number to account for each of the positions on each molecule. Sometimes when I do this I get an error on my calculator that says "overflow", does anyone know ho...

clairehathaway 2J

When calculating a change in entropy I think you would look at the values given to you and decide what equation to use based on that

clairehathaway 2J

Additionally, more work will be done along a reversible pathway than along an irreversible pathway.

clairehathaway 2J

What is the difference between residual entropy and molar entropy?

clairehathaway 2J

deltaS = q/T is the equation you use to calculate the change in entropy when the temperature is constant (ex. phase changes) deltaS = nR*ln(V2/V1) is the equation you use to calculate the change in entropy when there is a change in volume but the temperature is constant (ex. isothermal reversible ex...

clairehathaway 2J

If you are trying to find the change in entropy when there is a change in temperature you would use the equation deltaS = n*C*ln(T2/T1). If you were trying to find the change in entropy when there is no temperature change (phase change) you would use the equation deltaS = q/T.

clairehathaway 2J

The degeneracy equation is W = the number of states to the power of the number of molecules. So in an example where we had 6 identical molecules with 5 possible energy states we would say that W = 5^6

clairehathaway 2J

Yes! W is calculated by taking the number of states to the power of the number of molecules, so in this case 2^4 would be correct

clairehathaway 2J

Today in lecture, Dr. Lavelle explained that when a reaction was said to be spontaneous, he meant that it was favorable and not necessarily quick. So I believe that if an equation is spontaneous that means it would be favorable. Dr. Lavelle also briefly mentioned that knowing both the deltaU (or del...

clairehathaway 2J

That equation is used for reversible systems and Dr. Lavelle said he would explain how to use that equation in a later lecture.

clairehathaway 2J

That equation would be used for irreversible processes, I believe that the other equation (with the integral) will be used for reversible processes and discussed in a later lecture.

clairehathaway 2J

I think that since it's super simple to do the % calculation it can't hurt to do it every time. A rule that I tend to go by is since the cutoff is 10^-4, I don't do the percent calculation for anything smaller than 10^-7 because I assume that the K was small enough and that the % will be less than 5...

clairehathaway 2J

We won't need to memorize those values but that's because they vary based on many factors, such as the reaction itself or the temperature it occurs at, so I would focus on mainly understanding how to get a Ka/Kb value based on the information you are provided within the question.

clairehathaway 2J

I'm pretty sure that we will be provided with those values on the exam because this isn't crucial information to memorize, it would be more important to know how to use the values when they are given to you.

clairehathaway 2J

I was at a UA session earlier today and the UA said that the textbook is really inconsistent and that they will sometimes only write kJ instead of kJ/mol so I would just assume that when they write kJ they mean kJ/mol.

clairehathaway 2J

The equation for q involves multiplying the mass and specific heat by the change in temperature that occurred. We got +6.9 because 31.9(final temperature) - 25(initial temperature) is equal to +6.9. I think Dr. Lavelle specified that this was a positive value because it is also possible to get a neg...

clairehathaway 2J

I understand that temperature does impact K, whereas pressure and concentration do not, and that exothermic reactions favor product formation and endothermic reactions favor reactant formation. But I was wondering, if we aren't given a delta H value and we don't know if the equation is exothermic or...

clairehathaway 2J

At equilibrium would you say that the concentrations of reactants and products in constant (not changing)? I understand that at equilibrium the forward and reverse reactions are occurring at an equal rate so because of this would you be able to assume that the concentration is not changing because t...

clairehathaway 2J

Does anyone know up to what bullet point on Outline 3 will be covered on Midterm 1?

clairehathaway 2J

On the constants and formulas sheet it says that 1 L.atm = 101.325 J. Hope this helps!

clairehathaway 2J

Yesterday during his discussion section Dr. Lavelle said that Midterm 1 would cover material from weeks 1-3, but he also said he would send out an email to explain everything so look out for that!

clairehathaway 2J

If the rules are the same as last quarter, you should be allowed to use a graphing calculator on all exams!

clairehathaway 2J

Today during discussion section my TA said that for weak polyprotic acids we should calculate the pH using the 1st deprotonation and assume any further deprotonations are insignificant. He also said that for strong acids we should assume that the first deprotonation is complete and we would treat th...

clairehathaway 2J

Inert gases do not react with the gases already present so the concentrations of those gases won't change. There will be an increase in the concentration of inert gas because it wasn't present before, which will increase the pressure, but because it doesn't impact the gases that were already present...

clairehathaway 2J

I believe that during one of his discussion sections Dr. Lavelle said that a larger K value for a solid means that it is more soluble.

clairehathaway 2J

He means this in the context of finding the percent ionization. So when you divide X by the initial concentration of the weak acid/base and multiply it by 100% you will get a percentage. If that percentage is less than 5% you know that it was okay to have used an approximation in your calculation.

clairehathaway 2J

Yes! I was actually at a workshop earlier today and he explicitly said that a smaller pKa means it is a strong acid and a smaller pKb means it is a strong base.

clairehathaway 2J

Yes, when the K value is very small it means that there are very few products being formed and so the change is negligible. However, if K was very large it would not apply because many products would be formed so the change would definitely be something we want to notice. I think this ties in very w...

clairehathaway 2J

Super acids and super bases, while possible, I think are very rare and probably won't be covered in this course

clairehathaway 2J

Also, Dr. Lavelle specified in class today that Kw = 1 x 10^-14 when at 25 degrees Celcius. I'm not sure if we should always assume reactions with Kw are happening at this temperature but I do think it is something to keep in mind.

clairehathaway 2J

The constants and formulas sheet lists 4 different values for the R constant and each one has different units associated with it. When solving problems use the constant that aligns with the units you were given so that they cancel when you plug it in!

clairehathaway 2J

Do we assume that forward and reverse reactions at equilibrium occur at the same speed? For example, the reverse reaction wouldn't occur twice as fast as the forward reaction because then it would not maintain its equilibrium right?

clairehathaway 2J

I agree with what was mentioned above but you could always plug in the two values so you can get a better understanding of which one might be more correct!

clairehathaway 2J

I'm pretty sure it does still follow this rule because that is a rule that we can generally follow in math so I would assume it also applies in this case.

clairehathaway 2J

Most people use Chemistry Community to get clarification and ask questions they have about the content we are learning. It's super helpful to post any questions you might have from the lectures on here because questions tend to be answered super quickly since so many people are active on here. It's ...

clairehathaway 2J

I think it would work the same, but yesterday in a UA session the UA said that it's not good lab practice to titrate with a weak acid and a weak base so this likely wouldn't be something we need to know.

clairehathaway 2J

I think it's important to understand how acid rain is formed and what kind of impact it can have on the environment

clairehathaway 2J

I don't think you need to be able to thoroughly explain them but you should probably have a good general understanding of what they are/why they are significant. For example I think it would be useful to understand Cisplatin and the trans version and understand why Cisplatin is effective but not the...

clairehathaway 2J

Building off of what was said above, hydrogen bonds are stronger than dipole-dipole, dipole-induced dipole, and dispersion forces so it will require more energy to break a hydrogen bond. If more energy is required to break a bond it will have a higher boiling and melting point.

clairehathaway 2J

One thing that I think was from the textbook and helps me is that acids will usually start with H, like HCl or HNO3.

clairehathaway 2J

The way I understood it was that once the H is removed and we are left with Cl-O, Br-O, and I-O, looking at electronegativities will help determine which is the strongest. Cl has the highest electronegativity out of the 3 nonmetals and therefore will pull on the oxygen more than the other nonmetals ...

clairehathaway 2J

In lecture today (12/2) Dr. Lavelle provided us with examples of acid and base reactions. One was for acids, HCl(aq) + H2O(l) -> H3O+(aq) + Cl-(aq), and another was for bases, NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq). In both of these reactions, the reactants are neutral and the products are ions(char...

clairehathaway 2J

Just to clarify, the coordination number is the number of bonds the central atom forms with ligands, and the oxidation number of the central atom looks at charges from the ligands and from the overall molecule to determine the charge on the central atom. So in the example [Cu(CN)4]3- the coordinatio...

clairehathaway 2J

Chloro is more commonly used but I believe that chlorido is the preferred IUPAC name. Dr. Lavelle did say he will accept either one though!

clairehathaway 2J

The final will be cumulative, so it will have everything we've learned. What I am curious is the percentage of material covered (will there be an emphasis on week 7+ OR an equal amount of everything). Today during his discussion section Dr. Lavelle said the final will be cumulative and the percenta...

clairehathaway 2J

Hi, going off this topic, does anyone know how to identify when a molecule might have a ring structure? Does it have to be stated or is there a way to find out? Hi! I could be totally wrong about this but I think that if the two end molecules in a ligand both have lone pairs we can assume that they...

clairehathaway 2J

Delocalized pi bonds involve delocalized electrons which are typically found in resonance structures! So a species with a delocalized pi bond will be a species that has resonance structures that involve double bonds.

clairehathaway 2J

In his discussion section today Dr. Lavelle said that there would be no lecture posted on Friday! He also said if you have a discussion section Thursday or Friday they are not happening but the dicussion sections on Tuesday and Wednesday are.

clairehathaway 2J

Hi! I think that only the orbitals in the second shell are shown because they hold the valence electrons which are the most important in this case. I'm not 100% on this though!

clairehathaway 2J

In today's lecture (11/23) Dr. Lavelle gives us the example of benzene. We previously learned that benzene has resonance structures and that the most stable state is usually a combination of both structures, where the electrons are delocalized and spread across all bonds. Because the double bonds wo...

clairehathaway 2J

I had a question for this problem too: did we have to know how to write out the compounds? Will the midterm test us on this? Also, will we have access to the electronegativity chart on the midterm? I'm pretty sure that Dr. Lavelle said we would not need to have memorized any names for the test beca...

clairehathaway 2J

Hi! Polarizing power is an atom's ability to pull electrons (distort e- in other atoms), so cations are the ones that tend to have high polarizing power because their positive charge attracts negative electrons. There are two factors to remember when it comes to polarizing power: charge and size. T...

clairehathaway 2J

Hi! The general octet rule exceptions would be group 13 elements which usually only need 6 electrons to be stable (B, Be, etc). Elements such as H, He, Li need only 2 electrons be to be stable. As for an expanded octet, or elements that are able to accommodate more than 8 electrons, they have to be...

clairehathaway 2J

Is it safe to assume that if a molecule has lone pairs it will always be polar? Or could 2 sets of lone pairs cancel each other out?

clairehathaway 2J

In Outline 3 one of the objectives is "Know the relative magnitudes of intermolecular forces and hydrogen bonds". Just to make sure, this refers to the -2 kJ/mol for intermolecular forces and -20 kJ/mol for hydrogen bonds right?

clairehathaway 2J

Ben explains it very well. When determining structure it does not matter if the bond is single, double, or triple, it will still count as only one region of electron density. The number of lone pairs, however, is important. If you have 1 set of lone pairs vs. 2 sets of lone pairs vs 3 sets of lone p...

clairehathaway 2J

I understand that having a formal charge of 0 is the best for any molecule, but would this be attainable by every molecule? How do you know if a formal charge for a molecule is the lowest it can get? Adding on to this, is having the central atom with a formal charge ideal if there is an overall cha...

clairehathaway 2J

What about in something like ClO? If you were asked to draw that you know that there are only 13 electrons available to distribute between the two but how do you decide which one will get the lone electron? Does it have to do with electronegativity or electron affinity?

clairehathaway 2J

Hi, are you talking about electron-electron repulsion in reference to VSEPR theory? If so, electron-electron repulsion will actually make a molecule more stable, and therefore will be lower in energy. When electrons in a molecule are closer together, they are "excited" and are higher in e...

clairehathaway 2J

Does increasing electron-electron repulsion make it easier for an atom to lose electrons? And so would that mean that that atom with large electron-electron repulsion has more or less energy?

clairehathaway 2J

Yes these values are experimentally determined values that we will not need to know how to calculate. The reason we were shown them was to emphasize the energy required to break different kinds of bonds, I'm pretty sure we will not need to know how to find them.

clairehathaway 2J

London dispersion forces are always present but their impact/strength can vary. The atom will become slightly positive or negative for a period of time that will depend on how impactful the London forces are. Essentially, atoms with fewer electrons might experience London forces for shorter periods ...

clairehathaway 2J

Working with ions in the d-block can be confusing, it's important to remember that the highest n value will be the most unstable (in this case 4 is less stable than 3) and so the electrons will leave from that shell first. Like Rich explained above, only 2 electrons were removed which were taken fro...

clairehathaway 2J

In lecture today (11/6) Dr. Lavelle mentions that salts have ionic bonds and molecules have covalent bonds. Do all salts have ionic bonds and all molecules have covalent bonds? Or is it just a general trend that is observed? Conversely, can we say that all elements connected with ionic bonds are sal...

clairehathaway 2J

The earlier replies explain how to do this on Sapling, I just wanted to emphasize that putting brackets around a structure with a charge on the top right corner will be the overall charge of the molecule. When assigning charges to individual elements within the molecule, when drawing it by hand Dr. ...

clairehathaway 2J

I went to a step-up session today and they explained that ionization energies are very important and should be considered in drawing lewis structures but we should also consider symmetry because it will play a role in the stability of the molecule. I think Dr. Lavelle mentioned that we would be lear...

clairehathaway 2J

I dont want to repeat what is said above, so I wanted to add onto information about formal charge, the formal charge of each atom should add up to the total charge of the molecule So just to clarify, if the molecule has an overall charge then it will be most stable when it's total formal charge is ...

clairehathaway 2J

I think that knowing the 4th quantum number can be beneficial because then we are able to differentiate completely between each electron. If we only had the first 3 quantum numbers there would often be 2 electrons that shared all 3 values but by identifying the 4th each electron has its own set of n...

clairehathaway 2J

In addition to everything said above, because they are all metals that have low ionization energies they more easily will form cations (removing outermost electron). In lecture Dr. Lavelle also mentioned that the metals in the s-block were all reactive metals that form basic oxides and I think maybe...

clairehathaway 2J

I was going through the assigned textbook problems and in 1.13 they ask why oxygen has lower first ionization energy than nitrogen, but according to the ionization energy trends oxygen should be higher than nitrogen. So why does it have lower ionization energy? Also would that mean that in general a...

clairehathaway 2J

I worked out the problem and got a ratio of about 11.4 which I agree is hard to work with. I believe that at a UA session they said that problems like these on the midterm will likely be very straightforward and you would know if you should round up or down.

clairehathaway 2J

I also keep clicking on my name to view my points but it is not showing. Where do we click again to view our points? If I understand correctly you can get up to five points a week by posting on chemistry community so if you click on the number of posts under your name you will be able to see every ...

clairehathaway 2J

derek1d wrote:Won't the mass of the electron need to be given in order to solve A) of this problem?

This might be kinda obvious, but kinetic energy only applies to electrons right? So whenever we use the equation $KE = \frac{1}{2}mv^{2}$
the m will always represent the mass of an electron?

clairehathaway 2J

I was trying to calculate the energy of light absorbed from an electron (in a hydrogen atom) going from n=2 to n=4. In order to solve it I used the equation E_{n} = -\frac{hR}{n^{2}} and plugged in n=2 and then n=4 and then used plugged those results into the deltaE = Efinal - Einitial (deltaE = E(4...

clairehathaway 2J

Y e s , there is no point in the s-orbital where a nodal point exists, which means there is no point where e- ρ (electron density) = 0. In comparison to the p-, d-, and f-orbitals, s is the only orbital that lacks nodal points. Hope this helps. I might be way off on this but does the s-orbital have...

clairehathaway 2J

For the Balmer series the electron will always be going to n=2. In order for a line to be in the Balmer series, the electron must end at n=2, meaning that is its final state. It does not matter where it comes from, it can come from any n level (n=2,3,4, etc. like said in the textbook), it only needs...

clairehathaway 2J

In numbers 12 and 14 in the Sapling homework we are asked to calculate the wavelength (in nm) of the spectral line produced when an electron in a hydrogen atom undergoes the transition from the energy level n=3 to n=1. When I answered the questions I answered using 3 sig figs and got them right so I...

clairehathaway 2J

Multiple both sides of the E = (-hR)/n^2 equation by (1/h) Gives you 1/λ = -R/n^2 To find the difference in Energy you manipulate the equation. 1/λ = (-R/n^2) - (-R/n^2) where the second term pertains to the second energy level. Take out the -R and its 1/λ = -R[(1/n^2) - (1/n^2)] I believe the cons...

clairehathaway 2J

Like everyone above said, Planck's constant is 6.626 x 10^-34 Js. This value and a bunch of other ones that are commonly used in the equations we have been studying (ex. speed of light (c)) can all be found on Dr. Lavelle's website under the "Constants and Equations". Also, I'm pretty sure...

clairehathaway 2J

I believe in the lecture Professor Lavelle mentioned that anything with a wavelength of 10^-15 m or greater will have measurable wavelike properties and anything less than this will not. At a step-up session I went to today she said the same thing, anything equal to or greater than 10^-15 will be m...

clairehathaway 2J

The negative sign simply is to show that this is the amount of energy LOST by the electron. We changed it to a positive sign to show the amount of energy being ABSORBED by the photon or turned into light. This works because according to the conservation of energy, energy cannot be created or destro...

clairehathaway 2J

In today's lecture when working through the example asking for the frequency of light emitted by a hydrogen atom going from n=4 to n=2, we use the equation \nu = \frac{E}{h} and when we input the energy value (E) we change it from negative to positive (the negative value was equal to delta E). Dr. L...

clairehathaway 2J

So is delta v equal to the change in velocity, essentially the uncertainty they give us in the problem? Also, I was under the impression that when we were using uncertainties like ± 5.0 m/s we had to double it for it to be equal to delta v. I thought this because in this case the answer could be any...

clairehathaway 2J

For this problem, you would use Heisenberg's uncertainty equation: Δp x Δx ≥ h/(4π). We also know that Δp = m x Δv. Since the problem is asking for minimum uncertainty in the speed of an electron, we are trying to solve for Δv. The problem gives you Δx = 350. pm (but you should convert this to mete...

clairehathaway 2J

What exactly is threshold energy? In regards to the equation that was used in lecture today for an example: E(photon) = threshold energy + Ek Hi! The threshold energy is the same thing as the work function (Φ) which essentially is the minimum amount of energy needed to remove one electron from the ...

clairehathaway 2J

I'm currently in a step-up session and she said we should be familiar with the order of the different electromagnetic waves for exams. A good mnemonic she mentioned was: Rabbits Mate In Very Expensive Gardens (in order of increasing frequency). I was in the same step-up session! Here is what the mn...

clairehathaway 2J

I went to a step-up session yesterday and my UA explained that light is made of photons (which are particles) but photons travel in waves. She called this the wave-particle duality of matter and I looked it up afterward and found that there are so many websites and places that explain it in more dep...

clairehathaway 2J

I agree with Carolina, your process is not wrong because I also went through your method and got the correct answer. I know as I was working through the textbook problems I often would round the numbers after each step which progressively was making my final value less and less accurate and sometime...

clairehathaway 2J

I know that the products of a combustion reaction are CO2 and H2O, but will it always be one mole of each? Or will we need to balance it every time to see how many moles of each product is created?

clairehathaway 2J

I agree with the other answers to this question, and would maybe add that it's important to remember that the rules for adding and subtracting sig figs are different than for multiplying or dividing them. When adding and subtracting sig figs, the number with the lowest amount of digits after the de...

clairehathaway 2J

Hi! So I'm having a little trouble with figuring out how to start solving G.21 in the textbook. The question reads: A solution is prepared by dissolving 0.500 g of KCl, 0.500 g of K2S, and 0.500 g of K3PO4 in 500. mL of water. What is the concentration in the final solution of (a) potassium ions; (b...

clairehathaway 2J

Follow-up question, I completely agree that in order to maintain accuracy we should wait until the final step to round. Let's say you were doing a multi-step problem with lots of calculations, would you agree that after each set of calculations it would be best to round to the correct number of sig ...

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