CHEMISTRY COMMUNITY

Emma Healy 2J

Hi! We can only use the stoichiometric coefficients for an elementary reaction or an elementary step in proposed reaction mechanism. The slow step will also be the rate determining step. In the end game question, we can treat step 1 like it is at equilibrium and can get a relationship between the tw...

Emma Healy 2J

Hi Eileen! I'm wondering the same thing. Since Dr. Lavelle should give us the values that we need on the exam, I'm hoping that this won't be something that we need to worry about. However, I haven't known which reactions to use for questions like 6.45, and so I would love to hear any tips that anyon...

Emma Healy 2J

Hi Selena! In 14A, I don't think Dr. Lavelle ever took questions directly from the midterms and put them on the final.

There may be similar question on the 14B final to the ones we saw on the midterms, but they probably won't be exactly the same.

Emma Healy 2J

Hi Jessie, thank you for making this! Organic Chemistry Tutor and Khan Academy have some helpful videos!

Emma Healy 2J

Hi Sunny! Dr. Lavelle just confirmed in his office hours that the order of aqueous ions does not matter!

Emma Healy 2J

Hi Emily! I'm wondering if this just means that if a metal or compound is aqueous, then it should dissolve in solution?

Emma Healy 2J

Hi Sameer! I have been wondering the same thing this quarter, but Dr. Lavelle said that if we needed values that are found in Appendix 2B, then he should give them to us on the exam.

Emma Healy 2J

Hi! My understanding is that nonmetals are not conductors, but can any nonmetals act as conductors in some cases? And are metalloids conductors or just weak conductors? Are all metals conductors or just specifically d-block metals? And can all S block metals act as conductors? Are Gold, Copper, and ...

Emma Healy 2J

Hi! Are concentration cells always at equilibrium? Or only for certain cases?

Emma Healy 2J

Hi! When I used the Nernst Equation on Textbook Problem 6N.13, I got Q=2.67 x 10^6, but the answer key said the answer was just 10^6, and I'm not sure why the answer should just be 10^6? Thank you in advance! 6N.13 Calculate the reaction quotient, Q, for the following cell reactions, given the measu...

Emma Healy 2J

Hi Arezo! I'm wondering this same thing! And are all S block metals conductors or only specific ones? Also, are there any nonmetals that could act as a conductor?

Emma Healy 2J

Hi Samantha! Your work looks correct! I think you just need to convert J to kJ at the end and provide the answer in kJ on Sapling.

Emma Healy 2J

Hi Adam! I started by writing the unbalanced equation with the reactants on the left side and products on the right. The question states that "rust is caused by iron reacting with oxygen gas in the presence of water to create an oxide layer." So the reactants would be Fe + O2 + H2O. Then i...

Emma Healy 2J

Hi! Could someone please walk me through how to solve the textbook problem 6K.5 for part d? It is in basic solution, and I couldn't figure out how to balance the oxygens and hydrogens. Thank you in advance! The question is: Balance each of the following skeletal equations by using oxidation and redu...

Emma Healy 2J

Hi Scot! I started by writing the unbalanced equation with the reactants on the left side and products on the right. The question states that "rust is caused by iron reacting with oxygen gas in the presence of water to create an oxide layer." So the reactants would be Fe + O2 + H2O. Then i...

Emma Healy 2J

Hi Sasha! My understanding is that cell potential E is is sensitive to the concentrations of reactants and products. A galvanic cell typically has different species on both sides, which would have different concentrations. A concentration cell typically has the same species on both sides. If both sp...

Emma Healy 2J

Hi Ethan! I am not a Biochem major, but I know that Wesley Wu, one of the UAs, is also a Biochem major, and he took the 30 series. You could send him or a counselor an email or other students may have an answer for you. Good luck!

Emma Healy 2J

Hi Eric! Thank you for sharing this! I've heard of RED CAT before but haven't heard of AN OX, and so I'll definitely use this to help me to remember that anodes participate in oxidation.

Emma Healy 2J

Hi! A salt bridge helps to maintain electrical neutrality and charge balance with the positive and negatives in the Galvanic cell. It allows ions to pass through to help keep the cell running and neutral and keep the charges flowing within the cell so that they don't build up.

Emma Healy 2J

Aw I'm gonna miss not having Lavelle and Chem Community and the UAs for 14 C : (

Emma Healy 2J

Hi! Problem 4.43 in the textbook says if 120 mL of H2 gas at SATP combines with.... Since it says SATP, I thought that meant that it would be at 25 degrees Celsius, but the solutions manual says STP and used 0 degrees Celsius. I'm wondering what the temperature should be?

Emma Healy 2J

Hi! Isochoric means that delta V = 0 and that volume is constant. Isobaric means that delta P = 0 and that pressure is constant. Isothermal means that delta T= 0 and that temperature is constant. I think that isometric is when you have 2 or more compounds that are composed of the same number and spe...

Emma Healy 2J

Hi! I'm wondering the same thing and think that this is a great question.

Emma Healy 2J

Hi! In the textbook question 4H.1 part d, it asks about which substance has the higher molar entropy at 298 K: 1.0 mol of Ar (g) at 1.00 atm or 1.0 mol Ar (g) at 2.00 atm? I had originally thought that 1.0 mol Ar (g) at 2.00 atm would have a higher molar entropy since the pressure is greater because...

Emma Healy 2J

Hi! On question 4F.1 part a in the textbook, the answer key shows that delta S = - q / T because heat is leaving the body and releasing energy, but then it showed that the answer was positive. I thought that the answer was supposed to be - 0.341 J/(K * s), but I'm wondering if I'm missing something?

Emma Healy 2J

Hi! In the textbook question 4D.9, it asks us about finding the enthalpy density, and I thought it was just a matter of dimensional analysis and converting units, but the answer key used the enthalpies of formation to find the overall enthalpy of the reaction. Were we supposed to get those values in...

Emma Healy 2J

Did you hear that oxygen and potassium went on a date?

Yeah it went OK!

Emma Healy 2J

Hi! I think the first equation you listed may be for a calorimeter and not a molecule, and the mass of the calorimeter is not taken into consideration.

Emma Healy 2J

Hi! I think CO2 would be a linear molecule based on the Lewis structure and that Carbon, the central atom, has no lone pairs, which is a factor of its linear shape.

For my Sapling homework, I was given Ar (g) and just assumed that it was an atom.

Emma Healy 2J

Hi! What has helped me is thinking about what happens when we work out and how we expend energy when we exercise and do work, thus releasing energy. If you think about the example from lecture with pumping bike tires, it takes a lot of energy to pump them up, and so we release that energy into the s...

Emma Healy 2J

Hi! Is it possible to have a negative heat capacity? Or does it always have to be a positive value?

Emma Healy 2J

What's a kidney's favorite instrument?

The organ!

Emma Healy 2J

Why are skeletons good at chopping trees?

Because they're lumbar-jacks!

Emma Healy 2J

Never trust an atom...
They make up everything

Emma Healy 2J

What do you call an acid with an attitude?

A mean-oh acid!

Emma Healy 2J

Hi! I was looking at enthalpy of combustion problems and am wondering if the equation for the combustion of Nitrogen is just N2 + 2 O2 --> 2 NO2 ? or would it be N2 + O2 --> 2 NO ?

Also for a reaction like CxHxOxNx + O2 → CO2 + H20 + N2
would it yield N2 or NO2?

Emma Healy 2J

Hi! For table 5G2 in the textbook, I'm wondering if the K without a subscript is supposed to be Kp? It is clear that the other K value that is listed is Kc.

Emma Healy 2J

Hi! Are we supposed to know the standard states of molecules? Or just some of the most common ones? Or will they be given to us?

Emma Healy 2J

Hi! I know that for Kc equilibrium expressions we leave out solids and liquids, and we keep aqueous states and gases in the expression. I also know that when determining which direction a reaction will shift due to a change in pressure, we look at the amount of moles of gas on both sides and do not ...

Emma Healy 2J

Hi! When we're using method 2 or the bond enthalpies to find the overall enthalpy for the reaction, and we're looking at which bonds are breaking and forming, what should we do if we have resonance structures?

Emma Healy 2J

Hi! What should we know about Carbon in the graphite form?

Emma Healy 2J

Hi Jazlyn! I solved this problem the same way that you did. Since H2SO4 is a strong acid, I assumed that it would fully dissociate, and the concentration of [H30+] would be 0.15 M. The you should be able to do pH=-log(H3O+), which would be = -log(0.15)= 0.82. Dr. Lavelle also mentioned this in his l...

Emma Healy 2J

Hi! This is a great question! I'm wondering if neutralization would occur?

Emma Healy 2J

Hi Shana! I am not sure how to do this problem either, but Dr. Lavelle has not lectured on this topic yet, so I'm hoping that our lectures this week will help us to be able to solve it. It's not due for a little while, so it's good that we have more time to work on it.

Emma Healy 2J

Hi Jaylin! I started by converting pH to pOH and then found the concentration for [OH-] using 10^(-pOH). I then assumed that the concentration of [OH-] was at equilibrium and used that concentration of [OH-] to fill out my ICE table. In the ice table here, I used [OH-] = 3.673*10-5. We just don't kn...

Emma Healy 2J

Hi Margia! I also started by converting pH to pOH and then got a similar concentration for [OH-]. I just assumed that the concentration of [OH-] was at equilibrium and used that concentration of [OH-] to fill out my ICE table. We just don't know the initial concentration of the amine, so I used the ...

Emma Healy 2J

Hi! I think you're right! It seems like it should be 5 N2(g) for the products.

Emma Healy 2J

Hi! How many moles are there of reactants and products in this question? I was thinking that there were 6 moles of reactants and 6 moles of products, not including solids or aqueous. Unless the aqueous should be included for determining the number of moles on each side? 15. The photosynthesis reacti...

Emma Healy 2J

Hi! When finding the effect of increasing or decreasing partial pressure by determining how many moles are on each side of the chemical equation or how many moles there are of reactants and products, are aqueous molecules included? or just gases? For example, 2A (aq) + 1B (g) <--> 2C (g), would ther...

Emma Healy 2J

Okay! I tried doing that, but I still got the incorrect concentration for SO3.

Emma Healy 2J

Hi! So Dr. Lavelle has been talking about how temperature is the only thing that changes k but doubling the stoichiometric coefficients for a reaction will square the k value, so technically changing the temperature isn't the only way to change the k value?

Emma Healy 2J

Hi! On the Chemical Equilibrium Part 3 Post-Module Assessment, this is problem #20: A vial of SO2 (0.522 mol.L-1) and O2 (0.633 mol.L-1) react and reach equilibrium. Calculate the equilibrium concentrations of the products and reactants given that KC = 5.66 x 10-10 for this reaction: 2 SO2 (g) + O2 ...

Emma Healy 2J

Hi! No, we don't lose points, and there is no penalty for using hints! If you give up on the question, it will mark the question wrong though and deduct points.

Emma Healy 2J

Hi Amanda! I hope that you're doing well! I found this website to be helpful for me! I can look and see if I can find others. Naming can definitely be a challenge.

http://www.chemistry.wustl.edu/~edudev/ ... _comp.html

Emma Healy 2J

Hi! I'm just wondering what the maximum number of electrons would be in an expanded octet for an element like Iodine?

Emma Healy 2J

Thank you so much Brooke for positing this and for all of your time and help this quarter! I appreciate you so much and am looking forward to your sessions in 14B! Happy Holidays!

Emma Healy 2J

Thank you so much Dr. Lavelle and to all of the TAs and UAs! I am so grateful for all of the time, help, dedication, and commitment! I appreciate you all so much and am looking forward to seeing you all in 14B! Happy Holidays! : )

Emma Healy 2J

Thank you so much for posting your slides! They are so helpful!

Emma Healy 2J

Hi! I think we can still see our posts after this quarter ends because Chemistry Community has posts from 2011 and possibly earlier until now!

Emma Healy 2J

This is a great question! I have been wondering the same thing.

Emma Healy 2J

For example, if you had [Fe(CN)6]^4-, you know that the overall charge is -4. CN has an oxidation state of -1 and -1x6=-6. Using algebra, x+(-6)=-4, x=2. So Fe would have to have an oxidation state of +2.

Emma Healy 2J

For finding oxidation states, I would look at the charges that you know and use algebra to find the oxidation state of the metal.

Emma Healy 2J

I also enrolled in Chem 14BL and Chem 14B for next quarter and am wondering the same thing!

Emma Healy 2J

I saw that each TA has 2 review sessions where they will be discussing the same topic in each. Will both sessions be the same? Will the material they discuss in each session be the same? Or will each of their 2 sessions be different?

Emma Healy 2J

Are un-hybridized orbitals considered orthogonal? And why or why not?

Emma Healy 2J

Would a tetrahedral be considered the first 3D shape? Would molecules with 2 and 3 regions of electron density be considered flat?

Emma Healy 2J

I think just doing a lot of practice problems is beneficial in memorizing the shapes and bond angles and becoming more familiar with the molecules. I also think that being able to visualize the 3D shape of the molecule is helpful. One of the UA's shared this website with us: http://intro.chem.okstat...

Emma Healy 2J

An example of a resonance structure would be O3 because there is one single bond between an O and another O and one double bond between an O and another O. The electrons are delocalized and are shared in the molecule, and the single and double bonds could rotate between atoms and keep the same struc...

Emma Healy 2J

They are not considered resonance structures because the structures and number of single, double, and triple bonds were changing for each one, and the electrons were not delocalized between the atoms.

Emma Healy 2J

Yes, I counted all of the sigma bonds first for every bond and then all of the pi bonds after for the molecules.

Emma Healy 2J

I would also add that if each of the bond angles were 90 degrees, then the molecule would be flat and not 3D. Since there are repulsion forces between bonding pairs of electrons and the atoms are maximizing the distance, the structure is 3D, thus making the bond angles 109.5, which is greater than 90.

Emma Healy 2J

Hi! H2S has 2 single bonds, so it has 2 sigma bonds and 0 pi bonds. SO2 has 2 double bonds, and each double bond has 1 sigma bond and 1 pi bond, so there are 2 sigma bonds and 2 pi bonds in total for SO2.

Emma Healy 2J

Hi! I think we do need to memorize bond angles and make sure that we know that one of them is 109.5 degrees. I think just doing a lot of practice problems is beneficial in memorizing the shapes and bond angles and becoming more familiar with the molecules. I also think that being able to visualize t...

Emma Healy 2J

3F.5 Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point (Lewis structures may help your arguments): CHI3 or CHF3. I thought that CHF3 would have a higher melting point because F is very electronegative and has a higher ionization...

Emma Healy 2J

I had this same question. I think that O is more electronegative than Cl, and so it has a higher electron pulling power and really wants the full octet. Since O is more electronegative than Cl, it can handle having the negative formal charge.

Emma Healy 2J

Is there a trend for effective nuclear charge?

Emma Healy 2J

What does this bullet mean on the outline about the quantum world? Describe the factors affecting the energy of an electron in a many-electron atom.

Emma Healy 2J

If n=6, would l=5 be a valid answer? I know that l can be 0,...n-1 and that l=0 corresponds to s, l=1 corresponds to p, l=2 corresponds to d, l=3 corresponds to f. Based on this, l=5 would not correspond to s, p, d, or f. So would l=5 be valid because it fits for n=6-1=5?

Emma Healy 2J

How many nodes does the d orbital have? I think the p orbital just has 1 node and the s orbital has 0 nodes.

Emma Healy 2J

In the bond energy equation, what grows faster? The numerator or radius (denominator)?

Emma Healy 2J

Why do all molecules experience London Dispersion Forces?

Emma Healy 2J

Some molecules can have a resonance structure with one double bond, two double bonds, or three double bonds, and so do you count all of them as resonance structures even if the molecule with two or three double bonds is not as stable as the molecule with one rotating double bond?

Emma Healy 2J

What are some of the ion trends that we should know? And could someone please explain the ionic radius trend?

Emma Healy 2J

If two molecules have multiple Hydrogen bonds, do they all have to be the same length?

Emma Healy 2J

So if the electronegativity difference is > 2, then there is an ionic bond like NaCl. If the electronegativity difference is < 1.5, then there would be a covalent bond (C-F). What would happen if the value falls between 1.5 and 2?

Emma Healy 2J

Does it matter where you put each atom when creating a Lewis structure as long as the formal charges are okay and the correct number of electrons are used?

Emma Healy 2J

Knowing that Fluorine is the most electronegative element and that it's not in the very top right corner of the periodic table, would Helium be considered an exception to the trend since it is in the very top right corner?

Emma Healy 2J

Knowing that Fluorine is the most electronegative element and that it's not in the very top right corner of the periodic table, would Helium be considered an exception to the trend since it is in the very top right corner?

Emma Healy 2J

If you have a bigger cation, then the charge density is lower, and it has a lower pulling ability and lower polarizing power. What does this mean about the polarizing ability? And how is it affected?

Emma Healy 2J

Yes, I believe that only atoms with a d-orbital can have an expanded octet.

Emma Healy 2J

Are you a CATion? Because you're always so PAWsitive!

Emma Healy 2J

Hi Amanda! I am not sure which ones we should have memorized, and I have this same question too. I did find picometers on the formula sheet, which is given as 1 pm = 10^-12 m. I think an acronym could be really helpful if someone has one!

Emma Healy 2J

Would Carbon and other elements in the same column gain or lose an electron? And how does it affect its radius and the radii trends?

Emma Healy 2J

Hi! Does the 3s orbital shield the 3p orbital? Or instead of n=3, it could be n=2 or n=1.

Emma Healy 2J

Yes, okay thank you!

Emma Healy 2J

Hi! If we were looking at the 2p orbital, then n=2 and l=1, then m(l) could equal -1, 0, or 1, thus being 3 possible quantum values. If we made our own arbitrary scale, like p(x)=-1, p(y)=0, and p(z)=1, we could say that m(l) = -1 if we wanted the quantum number for p(x)? Or does that matter? I know...

Emma Healy 2J

Can the first 3 quantum numbers be the same?

Emma Healy 2J

Hi! I am wondering the same thing, but I think that n1 is n final and n2 is n initial, because it should always be final - initial.

Emma Healy 2J

Is there a way to find work function? Or does it just need to be given?

Emma Healy 2J

When would the kinetic energy be 0 for an electron when using:
KE(electron)=E(photon)- Work Function

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