CHEMISTRY COMMUNITY

claudia_1h

Regarding question 11: This is not representative of what would actually happen right? Because when temperature increased, K went up from 0.0177 to 0.0186, but isn't K supposed to decrease with increasing temperature?

claudia_1h

Remember that no matter the substitutions u may have to make for intermediates, the rate law will have the same order no matter what.

Can you explain what you mean by this?

claudia_1h

Can someone tell me the conditions that we use to check if a proposed reaction mechanism is possible/feasible? i.e. the conditions it must meet

claudia_1h

As far as I can tell, an elementary reaction is defined by it having one transition state. But a chemical rxn (i.e. an overall reaction) can be made up of multiple steps of elementary reactions, thus having multiple transition states in its reaction profile.

claudia_1h

In this class, we will always use the pre-equilibrium approach.

claudia_1h

For the Endgame worksheet, how is 14f done? How do you get rid of the concentration of B? See the original ENDGAME post for the problem details.

claudia_1h

Can someone please explain (conceptually, not with equations) the relationship between work, cell potential, and delta G? Like explain why wmax is = to delta G.

claudia_1h

How do we know when to include liquids in cell diagrams? In problem 6L.7b, H2O liquid is not included in the cell diagram even though it is involved in both half reactions. But in problem 6M.5, Hg liquid is included.

How do we distinguish this?

claudia_1h

I believe that because there are solid metals included in the half reactions, you don't need to add an inert electrode as those solids will already conduct e-s (unless it's something like I2 that's not a metal and won't conduct e-s well).

claudia_1h

Are you referring to Outlines 4 and 5 from Lavelle's website? If so, the second page from the thermo outline (4) and everything on outline 5 including any thermo is fair game for Exam 2.

claudia_1h

I was really confused about this too, just wanted to point you and anyone else who is confused to the video "Redox reactions part 1: explaining the galvanic cell." It does a really good job conceptualizing Galvanic cells and helped me visualize how the cathode and anode change when current...

claudia_1h

Standard reduction potential means K<1, the reaction is not spontaneous, and the cell will be electrolytic rather than Galvanic.

claudia_1h

Does anyone know if when balancing the redox reaction for an overall Galvanic/concentration/electrolytic cell, we are supposed to reduce the coefficients? I think my TA said in discussion not to in order to get the correct number of e-s to use in the equation for Ecell, but the solutions manual for ...

claudia_1h

Can someone confirm the equation for calculating K at 25 deg. C for me?

The solutions manual says lnK=nE(stand)/0.02569

In lecture, Dr. Lavelle said to use logK = nE(stand)/0.0592

Can we use either? And are we expected to memorize these because they are not on the constants and equations sheet?

claudia_1h

Can someone point me to where to find a table of reduction potentials for common half reactions? Either in the book or on Lavelle's website...can't seem to find it

claudia_1h

What is the rule for when we need to include a solid platinum electrode in our cell diagram?

claudia_1h

For 6L.3b, we are given the following cell diagram:

C(gr) | H2(g) | H+(aq) || Cl- (aq) | Cl2 (g) | Pt (s)

Can someone explain what the C(gr) part is and how we are supposed to use it when writing the half reactions?

claudia_1h

In the textbook, it says "In a galvanic cell, a spontaneous chemical reaction draws electrons into the cell through the cathode, the site of the reduction, and releases them at the anode, the site of the oxidation." Can someone explain the drawing of electrons through the cathode? Because ...

claudia_1h

I don't understand how to write out a reduction equation for chlorine in this case

I think that since in this case Cl2 gas is in liquid water, you assume that it dissolves into aqueous Cl- ions... That should be the product on the right side of your reduction rxn.

claudia_1h

I could be wrong, but I believe that since Cl2 gas is the only reactant, in this case it is both oxidized and reduced. It has products in both and oxidized and reduced state, making it both an oxidizing and reducing agent.

claudia_1h

A salt bridge allows for the flow of the negative ions that aren't participating in the redox reaction. Remember that we cannot just put aqueous ions in solution, they must dissociate from a solid salt. Those ions can make the charge unbalanced, so having a salt bridge essentially corrects that.

claudia_1h

The main difference we discussed in my discussion is that for alkaline conditions (basic), you add the same number of hydroxides as you just added H+s to both sides. This is because conceptually, we know that for a basic solution, there will not be more H+ than OH- in solution.

claudia_1h

When we are balancing the charge at the final step of balancing redox rxns (when we combine the half rxns), if we need to multiply by a constant to balance the e-s, do only the substances from the half rxn that produced those e-s get multiplied by the constant? Or does the entire side of the combine...

claudia_1h

we assume that the volume isn't changing and therefore use Cp.

^How does this make sense because if volume isn't changing, isn't that assuming "constant volume" which points to using Cv, not Cp?

claudia_1h

How do we know when to use Cv vs. Cp? I know one is for constant volume and one is for constant pressure, but what about for a sudden irreversible compression of gas where there is a temperature and volume change? For the temperature part of the entropy calculation, how would we know which c value t...

claudia_1h

How do we know when entropy of surroundings equals entropy of the system? Does this happen only when delta g = 0? I'm pretty confused about this..

claudia_1h

In the solutions manual explanation for 4h.9, it says that container A has the highest entropy because it has the "most particles". Can someone explain why this is because all the containers have 1 mole of gas?

claudia_1h

What causes there to still be some energy, allowing it to have some disorder? I thought that at 0 Kelvin, all particles stop moving, therefore there wouldn't be any disorder. At 0 K, there is no contribution to entropy from rotational/thermal motion. However, there is, for most molecules, residual ...

claudia_1h

Enthalpy is a state function (state values can be added/subtracted without worrying how they got there).

claudia_1h

Can someone give me a comprehensive list of the functions we have looked at so far that are intensive vs. extensive and state vs. non state? Also, are the two concepts related?

claudia_1h

Memorizing equations is not required as they will be given to us, but I would recommend memorizing the units for certain things (delta H, q, S, etc.) so you don't get points knocked off for mixing up units since those will not necessarily be given.

claudia_1h

How do we determine which equation to use when calculating entropy for a system for which there has been both a temperature change AND a volume change? (Refer to pizza rolls question 5)

claudia_1h

HF, unlike the other halogen acids, is not a strong acid due to the fact that Fluoride is highly electronegative and it has a small ionic radius, meaning the HF bond is more difficult to break and therefore does not dissociate easily like the other halogen acids (HBr, HI, etc.).

claudia_1h

Delta U is given by the energy supplied to the system by heat (q) plus the energy transferred by compression work (w). When the system is under constant pressure, q is equal to Delta H, so Delta H becomes a part of how Delta U is calculated.

claudia_1h

To check if the approximation is valid or not, you need to check if the percent ionization of the original substance is less than or equal to 5%. If it is greater, then you know that x will impact the original substance's molarity enough such that a quadratic must be used.

claudia_1h

I don't think so, I'm pretty sure if the energy of the bonds broken is greater than the energy of the bonds formed, then the reaction can be endothermic. But someone correct me if I'm wrong.

claudia_1h

Does anyone know if it's okay to leave units for work as L x atm? Or do we have to convert to Joules?

claudia_1h

Can someone please explain how to use given enthalpies of combustion for compounds to find the total enthalpy of the reaction? (An example of a problem like this is 4D.15)

claudia_1h

So basically: considering the heating curve for water (you can google this), we can see that it takes a lot of energy/heat to convert 1000 degree liquid water to 1000 degree steam. So, when the steam comes into contact with a colder surface (skin), it has to release a lot energy in order to condense...

claudia_1h

A state function is one wherein the state of the system depends only on the final and initial states, not on the path taken in between. For example, displacement is a state function. If you walk 25 paces forward and then 15 back, your total displacement is still 10 (the same as if you had just walke...

claudia_1h

Hi, does anyone know what the recommended order is to do the homework for the Thermochemistry unit? The problems for 4A and 4B don't seem to match up to what we've learned in class so far.

claudia_1h

Alexis Webb 2B wrote:Can you post a picture of the problem or your work for reference?

I didn't rlly write down any work, I just divided Kw (10^-14) by 1.5 M... I got 6.67 x 10^-15.

claudia_1h

This problem asks us to fill in a table for [h3o+] [oh-] pH and pOH. The given [h3o+] for the first one is 1.5 M, so I tried to use Kw=[hydronium][hydroxide] to get [hydroxide], but the answer I get (6.67 x 10^-14) is not what the solution manual says. Can someone tell me where I'm going wrong/how t...

claudia_1h

Can someone please explain when to not use the concentration of H+ to calculate pH? It's something about the dissociation due to the autoprotolysis of water but I don't remember the exact threshold...

claudia_1h

Yes, because the change to the initial concentration of the acid or base will be essentially zero. But usually the percent ionization doesn't come first, and is a step to confirm that approximating was okay (which you would do based on the value of K being <10^-3).

claudia_1h

It's not especially important. Focus on the fact that energy is required to break bonds, and since we can clearly see the breakage of bonds (and no formation of new ones), we know the reaction will be endothermic.

claudia_1h

Cl2 is more thermodynamically stable because it has a lower relative concentration of Cl products to its Cl2 reactants than F to its F2 reactants. So, Cl2 is more likely to remain than F2, meaning it is more stable. This can also be determined from the fact that Cl2 is a smaller Kc: less products on...

claudia_1h

^Yes, basically you have to account for the amount of H3O+ that already exists naturally due to autoprotolysis. Don't make the mistake of just calculating pH when [H3O+] is this small.

claudia_1h

Hi, not sure where you might be going wrong but I can try to explain how I did it. I set up an ICE chart where HCl changes by -2x and the other two gases change by +x. Then, because K is very small (it is less than 10^-3), I set it equal to x^2/(0.22)^2. We can do this because because K is so small,...

claudia_1h

Yes. Do not include solids or liquids.

claudia_1h

In the solution for problem 5I.23, I noticed that the values for the equilibrium concentrations of the reaction were rounded according to sig figs before they were plugged into the equation to find Kc. Is this a universal rule for rounding? If not, how do we know when to apply sig figs?

claudia_1h

So this problem basically asks you to calculate the reaction quotient given the amount of each substance in a container and the volume of that container. All of these substances are gases however, so don't we need to use partial pressures, not molar concentrations? The answer key just divides each n...

claudia_1h

Le Chatelier's Principle essentially states that "chemical reactions will adjust to minimize the effects of changes." Therefore, you would use it if a change is applied to a system to determine how the system will react in order to try and bring itself back to equilibrium. Ex: If you sudde...

claudia_1h

Also, the equilibrium quotient basically tells you how the system is going to react next in order to move towards equilibrium (or if it already is at equilibrium, when Q equals K). Q tells us if we have "too few" products or "too many", allowing us to predict which direction the ...

claudia_1h

This, combined with the fact that there are spacer atoms between the N's and that there are sigma bonds allowing rotation, make it tridentate.

claudia_1h

What is the difference between the definition of a chelating ligand and a polydentate ligand? Doesn't being polydentate (bonding to the Tx metal at more than one place) automatically form a "ring" and make it chelating?

claudia_1h

Cisplatin is one biological example of how coordination compounds are relevant. But I believe we should also know the biological functions of Cr, Fe, Co, Mn, Ni, Cu, Zn. The outline for this unit mentions knowing that Co functions in vitamin B12 specifically.

claudia_1h

Yes. This is shown when it bonds to Fe at 4 points to form a Heme complex.

claudia_1h

Step 1: Convert kJ to J -> 100,000 J. Step 2: We know the energy per photon is equal to 1.936... x 10^-18 from part a of the question. That 100,000 J of incoming energy is the total energy of all the photons, so if we divide 100,000/1.936...x10^-18, we get the number of photons. This also makes sens...

claudia_1h

This is the problem: Lyndon sees his crush and panics, accidentally dropping 382.7 mg of HCl and 147.1 mg CaO into a 1L flask. After filling that last with water up to the 1L mark, what is the pH of the resulting solution? I'm confused because HCl and CaO will form CaCl2, and I remember that the sal...

claudia_1h

I assume you are referring to kinetic energy of an electron ejected in a photoelectric effect problem? If you are, the KE is zero when the energy (E=hv) of the incoming light is equal to the work function. In other words, you don't have any excess energy to be released as KE or the wavelength of the...

claudia_1h

The chlorine inside the brackets is a ligand attached to the Co. So, since it is the first alphabetical ligand, it comes first. The suffix is not ate because you only add that if the charge of the bracketed part is negative. In this case, since it is bonding with Cl-, it is positive, so you do not n...

claudia_1h

chari_maya 3B wrote:is the answer to part a CH3COOK?

Yes, that's the correct answer. Can also just be written as CH3CO2K or potassium acetate.

claudia_1h

You know this because you know the overall charge of the compound you are naming. For example, say you are naming [Co(NH3)6]3+. You know there is an overall plus 3 charge. You have 6 neutral ammonias and therefore that one cobalt must get all of the charge. Therefore, the name of this compound would...

claudia_1h

Coordination compounds form when neutral molecules or atoms (or anions) supply a central cation (usually a Tx metal) with an electron pair to form a coordinate covalent bond. These often form when Tx metals in solution first form complexes with water as the ligands, which are then replaced by more e...

claudia_1h

Chlorine has a high electronegativity, meaning it has a high tendency to attract electrons. So, it stabilizes the molecule by drawing electron density away from the negatively charged oxygen in the O-H bond, which weakens that bond and makes it more likely to give off a proton.

claudia_1h

When writing the formula of a coordination compound, does the order in which the ligands are written matter?
Ex: In hw 9c.3d, for the name tetraaminediaquacobalt (III) bromide, does it matter whether the OH2 or NH3 is written first in the formula?

claudia_1h

How does one determine how many coordinate covalent bonds a Tx metal can form when forming a coordination compound? i.e. how do we know how many ligands can/will attach themselves?

claudia_1h

I don't fully understand the concept yet, but in lecture, Dr. Lavelle said we need to "know how cisplatin shuts down cell division." I'm assuming this means we need to understand that cisplatin (and NOT trans-diamine-dichloro-platinum" has the correct geometry to form a coordination c...

claudia_1h

The difference is between when you are talking about the species accepting/donating a proton or an electron. That is the difference between the Bronsted-Lowry definition of acids and bases versus the Lewis definition. In Lewis, acids are ELECTRON acceptors, whereas in Bronsted-Lowry, acids are PROTO...

claudia_1h

Also: sigma bonds form when two orbitals interact end-to-end, whereas pi bonds form when 2 orbitals overlap side-by-side. Note that pi bonds have electron density on each side of the internuclear axis, whereas sigma bonds have electron density along it. This will change the rotatability of the atoms!

claudia_1h

In 2E.21 d, we are asked to determine the bond angles of N2H4. The Lewis structure gives trigonal pyramidal geometry about each of the N atoms, and the answer key gives the bond angle as 107 degrees... But why is it not just <109.5 degrees? In what case to we assign a specific bond angle to trigonal...

claudia_1h

How is it possible for a molecule to be linear if the central atom has lone pairs? Wouldn't those electrons repel the electrons of the bonded atoms like in a trigonal pyramidal molecule? Example: AX2E3

claudia_1h

So 120 degrees would be the bond angle for a trigonal planar molecule. You can think of the lone pairs as acting like another atom attached to the central atom, whose electrons repels the electrons of the other two. However, the repulsion due to the lone pairs is STRONGER than the repulsion due to b...

claudia_1h

I believe that we don't need to know exact bond angles for certain molecules, but we might need to know their relationship to the expected bond angle based on electron density arrangement. For example, a trigonal pyramidal molecule has tetrahedral electron density arrangement, but because the lone p...

claudia_1h

Yes, Dr. Lavelle mentioned in class that the lone electron is considered as one region of e- density.

claudia_1h

No, lone pairs may be a common indicator of polarity, but it is not a catch-all rule. For example, molecules like I3 that are linear with lone pairs present around the central atom are still non polar due to cancelling dipoles.

claudia_1h

Based on homework problems and what we've done in class, it seems like we may be asked to determine whether hydrogen bonding would be present based on a molecular formula, to draw the orientation to other molecules based on H bonding, or to identify it as the most significant intermolecular force pr...

claudia_1h

The units of the dipole moment (C x m) have me somewhat confused. What does the calculation for a dipole moment (charge x distance) actually tell you? Is it the strength of the interaction?

claudia_1h

I believe I recall that hybrdization makes the orbitals suitable for electron pairing/bond formation, but I'm sure we will go into more detail on this in lecture this week.

claudia_1h

In the slides from the last lecture, Dr. Lavelle wrote Ep is proportional to a1a2/r^6 and then 1/r^6. Are these both for interaction potential energy? Do they show different things?

claudia_1h

When n is greater than or equal to 3 and the element is in the p block, an expanded octet is possible. This is possible because of the presence of the d sublevel.

claudia_1h

A dipole moment refers to the presence of two charges (di-pole, two charges). A temporary dipole occurs when in a covalent bond, the electrons momentarily are distributed unevenly, such that the distribution is not perfectly shared between atoms. This creates temporary partial charges.

claudia_1h

A coordinate covalent bond forms when one atoms supplies BOTH of the atoms that are shared in a covalent bond. In lecture, the example was given of F- providing both of the electrons to fulfill B's octet in the Lewis structure of BF3 to form BF4. The bond between the F that came from the F- and the ...

claudia_1h

Are induced dipole-induced dipole interactions different from dipole-induced dipole interactions? How so? Can an induced dipole then induce another dipole and is this a separate type of interaction?

claudia_1h

The large distortion refers to the tendency of electrons in the anion to be pulled towards the cation, creating more covalent character to the bond. Small, highly charged cations like Al3+ have small enough ionic radii and great enough charge that they have high potential to create this distortion o...

claudia_1h

Atoms have a tendency to want a full octet. A radical has an atom with unpaired electrons, therefore, they have a tendency to form bonds in order to complete their octet and pair their electrons. This is a more stable, lower energy state.

claudia_1h

Magnesium is not a transition metal, it is an alkaline earth metal, so knowing how to find the number of valence electrons for transition metals is not needed for this problem.

claudia_1h

Because the molecule possesses resonance, each structure will be essentially identical, save for the placement of a certain double or triple bond. You can draw the most "correct" structure by representing all these structures as equal using a double-headed arrow. But they are all equivalen...

claudia_1h

I know that past midterms have included molecular geometry. I'm guessing this is something we will go over next week when we study hybridization, but it all depends on the pacing of the class for the particular year.

claudia_1h

In lecture, Professor Lavelle drew a structure for benzene that he called "hybrid/line structure" or a "resonance hybrid" that looked like a hexagon with a ring in the middle. He said it was a better representation than the Lewis structure because of the experimentally observed b...

claudia_1h

There are different factors (increasing/decreasing nuclear charge, electron shielding, increasing/decreasing distance of electrons from the nucleus) that affect periodic trends in ionization energy/atomic radius/electron affinity. I remember talking in high school chem about which of these factors a...

claudia_1h

Double and triple bonds should be placed when the number of electrons does not allow for single bonds only + valence electrons for each atom, so electrons must be shared so they can be "counted twice" like he mentioned in lecture.

claudia_1h

Yes, it is still a Lewis structure even though P breaks the octet rule because it uses the same notation (lines for bonds and dots for valence electrons) as other Lewis structures.

claudia_1h

Z eff refers to the effective nuclear charge. In this case, Z eff e refers to the effective nuclear charge experienced by an electron. The equation for calculating it can be found in the textbook. Basically, it is asking you to evaluate how strong the attraction of the electron will be to the nucleu...

claudia_1h

The "n levels" are the principal energy levels. The n is the principal quantum number, which tells you how far away from the nucleus the electrons are as well as how much energy those electrons have. Increasing n means that distance from the nucleus as well as electron energy is increasing.

claudia_1h

In class today, Dr. Lavelle discussed an "exception" in writing electron configurations... He said "Half full d5 and full d10 subshells have lower energy". So in the example of Cr, the configuration is written as [Ar]3d5 4s1. Can anyone explain this to me? Why does the 4s shell n...

claudia_1h

You don't necessarily "know" when to use the previous element- it can just be more convenient if you are trying to write the electron configuration for an element with many electrons. Using noble gas configuration eliminates a lot of the writing and allows you to only write the valence ele...

claudia_1h

The work function is the energy required to remove an electron from a metal. Each type of metal has a specific work function. Think of it as the threshold energy or minimum energy to remove the electron. When you say E=hv is equal to work function, I think you may be referring to how the kinetic ene...

claudia_1h

Note that the dark lines show that the atom can only absorb radiation at certain frequencies. Each element has a specific, characterizing absorption spectrum.

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