CHEMISTRY COMMUNITY

Ashley Wang 4G

As requested, here's a high five to Dr. Lavelle!

Thank you so much for everything in the past two quarters! For both the informative lectures and the personal care you've extended towards us, as well as the inspirational poems and background music.

Ashley Wang 4G

For the properties we learned about in thermodynamics, Gibb's free energy, entropy, enthalpy, and internal energy are all state functions. Work and heat are path functions.

Ashley Wang 4G

Yes, you can use the Nernst equation, provided all the other info is given. If it's a concentration cell, the cathode is the half-cell with the higher concentration, so it'll be the reactant. If it's a normal galvanic cell, you would identify the reactant and product from the overall equation of the...

Ashley Wang 4G

They'll most likely both be given with an associated standard reduction potential (Eº), so you'll identify the half-reaction with the more positive Eº as the reduction half reaction, and the other when (when flipped) will be the oxidation half reaction.

Ashley Wang 4G

Gibbs free energy is associated with the given reaction of interest, which could be under any conditions. Standard Gibb's free energy is the Gibb's free energy for the same reaction, but under standard-state conditions, which is 1 bar for gases, 1M for aqueous solutions, and pure solids and liquids.

Ashley Wang 4G

I think in electrolytic cells you have to compare standard reduction cell potentials to determine which species (in this case, Ni2+ or water) will be preferentially reduced (higher Eº). You won't just pick which species will be reduced/oxidized.

I hope this helps!

Ashley Wang 4G

It is simply how a zero-order reaction is defined - the rate is independent of concentration of reactions.
For example, if the reaction requires an enzyme to proceed, but the enzyme is already saturated, increasing the concentration of the reactant does not affect the reaction rate.

Hope this helps!

Ashley Wang 4G

If you increase the concentration difference between the solution at the cathode and at the anode, you will increase the cell potential, and vice versa.

Ashley Wang 4G

Try writing out the oxidation and reduction half reactions for the cell. When you add them together for the overall redox reaction, you will see that the reaction has the cathode species (or concentrations, in a concentration cell) on the reactant side, and anode species/concentrations on the produc...

Ashley Wang 4G

Half life is inversely proportional to reaction rate. The larger the rate constant k, the shorter the half life.

Ashley Wang 4G

Hi, As I understand it, when identifying the cathode and anode in electrolytic cells, we would still take the cathode to be the electrode where reduction would naturally occur (more positive reduction potential), and anode where oxidation would occur, regardless of which way the electron actually fl...

Ashley Wang 4G

does the flow of electrons indicate which of the metal pieces is being eroded in the solutions? I think if the metal conductor is one of the species taking part in the reduction/oxidation, then its size will change. However, if there is no conducting solid in the reaction and platinum is used, then...

Ashley Wang 4G

We compare initial reaction rates because at the initial stage of the concentration we can assume only the forward reaction is occurring, and we can find the max rate at which the concentration of reactants are decreasing/concentration of products are increasing. If we compared reaction rates after ...

Ashley Wang 4G

Hi! I'm not sure what you mean... If you're referring to why we would write the unique rate for 2NO2 --> 2NO + O2 as -(1/2) d[NO2]/dt = (1/2) d[NO]/dt = d[O2]/dt instead of - d[NO2]/dt = d[NO]/dt = 2 d[O2]/dt It's just a convention because it allows you to easily determine the unique rate just from ...

Ashley Wang 4G

Hi, In my notes I've written that when K < 1, ∆Gº is positive, and when K > 1, ∆Gº is negative (based on the relation ∆Gº = -RTlnK at equilibrium). However, I was wondering why the sign of ∆Gº matters if the sign of ∆G is what ultimately decides if a process if spontaneous or not. Or is it simply st...

Ashley Wang 4G

A spontaneous reaction means it will tend to occur under the given conditions without an outside source of energy. Note that it says nothing about the speed of the reaction! When ∆S and ∆H are either both negative or both positive, that is when the temperature dependence of the reaction becomes very...

Ashley Wang 4G

We don't ignore ∆S/R when deriving the equation. Maybe the term appears to "disappear," but it's actually just cancelled out when we subtract one lnK1 equation from the other (lnK2). If you're referring to why we can assume ∆S and ∆H are independent of temperature, it is just an assumption...

Ashley Wang 4G

I think it's easier to avoid defining cathodes or anodes as positive/negative because it gets confusing with galvanic vs electrolytic cells). The ions flowing into each solution to prevent charge buildup, not because it is "attracted" by the charge of the electrode. For example, as the rea...

Ashley Wang 4G

If a porous disk is used in place of a salt bridge, would there actually be no separation between the two half reactions?

Ashley Wang 4G

Hi,

In the example in class, Cu(s) | Cu2+ II Fe3+, Fe2+ | Pt
are Fe3+ and Fe2+ not separated by a "I" because they exist in the same phase?

Thank you!

Ashley Wang 4G

Hi, I'm having trouble understanding the difference between the ∆G and ∆Gº terms in the equation ∆G = ∆Gº + RT lnQ. Is ∆Gº the standard Gibb's free energy difference between the reactants and products, and ∆G the Gibb's free energy difference between the initial state and equilibrium (since we deriv...

Ashley Wang 4G

I think this is from using Van't Hoff's equation to calculate K at a different temperature if ∆Hº is known. We assume the difference in entropy ∆Sº between the reactants and products in a reaction is the same even when the reaction occurs at two different temperatures. Even though the actual entropy...

Ashley Wang 4G

Because Boltzmann's equation calculates the residual entropy , which is the entropy of the sample at 0 Kelvin, we are assuming the entropy arises only from positional disorder in the sample (i.e. linked to the number of microstates) that survives at that temperature, we disregard the entropy resulti...

Ashley Wang 4G

Positional entropy is the entropy of a substance at 0 Kelvin, arising from the substance's positional only (i.e. eliminating thermal entropy). This boils down to the number of possible microstates, which is linked to the positional entropy by Boltzmann's Equation. Thermal entropy takes into account ...

Ashley Wang 4G

Because entropy is a state function, you can calculate the change in entropy (of the system) as though the expansion occurred reversibly! You would calculate the change in entropy from a change in volume, assuming constant temperature (∆S = nRlnV2/V1), and the change in entropy from the change in te...

Ashley Wang 4G

I suppose if the pressure inside the vessel is different from the external pressure, then there will still exist a definite direction of spontaneous change, which would make the reaction irreversible. That said, the reaction will somehow have to be kept at constant temperature in order to fulfill yo...

Ashley Wang 4G

q = ∆H(rxn) if the reaction occurs under conditions of constant pressure, just by the definition of enthalpy change (the heat absorbed/released in a reaction at constant pressure).

Ashley Wang 4G

The equation involving heat capacity isn't applicable to phase changes because ∆T would be zero during a phase change. I'm fairly certain the enthalpies of vaporization, fusion, etc will be given on the midterm. The ones for water are listed on the constants and equations sheet, and I imagine for an...

Ashley Wang 4G

You would use q=mc∆T when there is a change in temperature (∆T), i.e. in the slanted portion of a heating curve.
Where there is a phase change involved, the temperature does not change, so you have have to calculate the transfer of heat with q=n∆H(vap, fus, etc)

Hope this is helpful!

Ashley Wang 4G

My TA kindly put something together. Take a look here!
viewtopic.php?f=160&t=58744

Ashley Wang 4G

Work = -P ∆V when the external pressure is constant.
By the ideal gas law, PV = nRT, and P∆V = ∆(nRT)
So any work done under constant pressure = - ∆(nRT), where the change in volume can result from a change in the number of moles of gas (∆n) or a change in the temperature (∆T).

Ashley Wang 4G

For a system containing gases, work can be done by/on the system through the expansion/compression (change in volume) of a gas in a container. For a constant external pressure, the change in volume can either occur from a change in the number of moles or a change in temperature. In either case, the ...

Ashley Wang 4G

Work can be represented by the integral because infinitesimally small changes in volume is the mathematical equivalent of dV, and summing up all these changes occurring in a system would be the equivalent of taking an integral. However, for cases of work done by expansion against a constant pressure...

Ashley Wang 4G

Hi,

Could someone clarify what Dr. Lavelle was explaining about residual entropy towards the end of the lecture on Friday (Lecture 4)? Including its definition and relation to the orientation of atoms in a molecule, etc; I couldn't really follow in class.

Thank you!

Ashley Wang 4G

To add onto the responses above, that is why when a reaction occurs under conditions of constant pressure, the heat given off/absorbed by the reaction is the change in enthalpy of the system.

Ashley Wang 4G

You can use the difference in the sum of standard enthalpies of formation of the products and the reactants (method 3 in lecture).

Ashley Wang 4G

State functions are defined by its current state and not dependent on the path taken to reach it.

Because heat is a transfer of energy and not an intrinsic property of the system, it is not a state function.

Ashley Wang 4G

I expect you would have to include the enthalpy change that would result from the phase change of the substance in its current state to the standard state? I say this because in a different but related example, Dr. Lavelle mentioned the enthalpy of the reaction Br2 (l) --> 2Br (g) would be comprised...

Ashley Wang 4G

Hi,

What exactly are pure solids and liquids? This has come up time and time again, both in the equilibrium section and again when we discussed standard states.

Thank you!

Ashley Wang 4G

I don't think bond enthalpy is reactants minus products! It's the sum of all the bond enthalpies of reactants and products, but it just so happens that these values are negative for the products because energy is released when bonds form (while bond enthalpy is defined to be E required to break a bo...

Ashley Wang 4G

When there are more molecules, the particles collide with one another and the container more frequently, resulting in higher pressure. Thus, when applied to Le Chatelier's, if the pressure of the system is increased, the reaction will shift in a direction that will alleviate this added pressure, whi...

Ashley Wang 4G

A buffer solution is able to do either, depending on what the situation calls for. One species in the solution can give off a proton if a base is added to it; another can accept a proton if an acid is added. This is what allows buffers to resist changes in pH. Hope this is helpful!

Ashley Wang 4G

I think the situation your notes are referring to is when you pump an inert gas into a vessel of fixed volume. Because the concentrations of the reactants and products don't change in this case, neither the forward nor the reverse reaction are favored. In the case where you change the pressure via a...

Ashley Wang 4G

To add to the responses above, it's important to note that Le Chatelier's Principle only applies to systems that are at equilibrium! Hope this was helpful.

Ashley Wang 4G

To clarify, the temperature doesn't change as a result of the reaction being exothermic. The change in temperature is an external stress applied on the system (to explain it in terms of Le Chatelier's). If the forward reaction is exothermic, the reverse reaction would then be endothermic, so an incr...

Ashley Wang 4G

Hi, From my understanding, the reason that a change in pressure from adding inert gas (to a constant volume) doesn't cause the reaction to shift is because the addition of inert gas changes the total pressure but not the partial pressures of the reactants and products. However, I was wondering how e...

Ashley Wang 4G

Topic 5J in the textbook has problems on applying Le Chatelier's Principle! Specifically from the Outline 1, 5J #1, 3, 5, 9 are what you're looking for :)

Ashley Wang 4G

I think it's helpful (at least for me) to remember what Dr. Lavelle said about not thinking of Q > K as "overshooting". One example would be perhaps after reaching equilibrium, some reactant is consistently being used up in another reaction, so even though the backward reaction proceeds, Q...

Ashley Wang 4G

Since Q is calculated the same way as K, and is a ratio, it does not have units. However, as with K, we have to make sure the values we're plugging in are in terms of units taken by Q (eg. mol/L and not mmol/L for Qc).

Ashley Wang 4G

Yes, we can. With Kp, the ICE table would then represent the initial pressure (I), the change in pressure (C), and the pressure at equilibrium (E).

Hope this helps!

Ashley Wang 4G

A proton can never reach the speed of light because it has mass. If you remember, Dr. Lavelle used a similar example to illustrate why the electron cannot be contained inside the nucleus of an atom, since its Heisenberg uncertainty in speed would be grater than the speed of light, which is impossibl...

Ashley Wang 4G

Does only one O2 molecule bind to the heme complex at one time?

Ashley Wang 4G

For NO2- ion, how do you know when it would bind with which atom (N or O)? i.e. when would you name it nitro or nitrito?

Ashley Wang 4G

Hi, I was wondering what the difference is when H2O is bound to the transition metal cation and when the cation is "hydrated"? Or are they the same? Essentially, does it mean different things when the (OH2) ligand is within the coordination sphere and when it is written on the outside as *...

Ashley Wang 4G

Why are all the orbitals with the same principle quantum number in the H atom degenerate?

Ashley Wang 4G

Hi, In lecture, Dr. Lavelle went over how atoms with higher electronegativity delocalize and stabilize the negative charge on the oxygen atom in oxoacids, accounting for higher acidity. It made sense when he was explaining it, but I don't really understand why a stable anion means that it's a strong...

Ashley Wang 4G

In lecture, Dr. Lavelle mentioned two criteria/trends for determining relative acidity: 1. acids with a longer acid-H bond are stronger, as they can lose the H+ more easily. eg. HI is a stronger acid than HBr 2. the stability of the resulting anion: oxoacids where the oxygen is bonded to a more elec...

Ashley Wang 4G

Since denticity of a ligand is so strongly dependent on its shape, would we always have to draw out the shape of ligand in order to determine whether it's polydentate, since lone pairs aren't conclusive enough?

Ashley Wang 4G

Hi, Since hybrid orbitals are a blend of atomic orbitals, I imagine the "s-character" is the characteristic of the hybrid orbital that the s-orbital contributes to. For example, sp2 has more s-character than sp3 hybrid orbitals. As for the second question, the most straightforward way I wo...

Ashley Wang 4G

Hi,

For the final, are we expected to memorize the common ligand names? Or will we be given a chart similar to the one Dr. Lavelle sent out?
I was wondering because it might be considered too easy if we're given a chart, so I wasn't sure.

Thank you!

Ashley Wang 4G

I expect most problems will still be in terms of pH, but I suppose pOH is fair game since it was brought up in the lecture. It helps that it's not difficult to memorize!

Ashley Wang 4G

Hi, Could someone clearly explain vapor pressure to me? This is something I felt confused by in high school as well. They mostly come up in questions regarding intermolecular forces, and because of my confusion with its definition I've never been able to clearly understand why a high vapor pressure ...

Ashley Wang 4G

The two lone pairs are located in two of the three trigonal-planar positions because then they are only interacting with two of the bonded pairs at 90 degrees, and one at 120 degrees. This minimizes repulsion, whereas if they were located in the axial positions they would be interacting with all thr...

Ashley Wang 4G

Because lone pairs repel other lone pairs and bonded electron pairs more strongly than bonded electron pairs repel one another, the repulsion makes the bond angles smaller than they would be if the places the lone pairs occupy were taken up by bonded atoms instead.

Ashley Wang 4G

From what I understand, we are meant to identify hybridization based on the VSEPR model? As in the hybridization is adapted to explain the experimental evidence. Are there other tips for identifying hybridization? Do we always have to draw out the valence orbitals for each atom, or should we just me...

Ashley Wang 4G

Hi,

I'm confused as to when hybridization of orbitals occurs. Should we think of it as exceptions, or is it something that always occurs?

Thank you!

Ashley Wang 4G

Hi, In lecture Dr. Lavelle mentioned how tetrahedral shapes have bond angles of 109.5º if all 4 atoms bonded to the central atom are the same (eg. CH4). He then said that if one H atom were to be replaced by a fluorine atom, the bond angles would change slightly. Could someone confirm whether I'm co...

Ashley Wang 4G

I think you would consider the local geometry, so look at the shape about each "central" atom individually.

Ashley Wang 4G

What do equatorial and axial mean? In structures where regions of electron density form a "plane" around the central atom with more regions above and below it (such as seesaw, square planar, etc), - the axial lone pair would be one lying on the "axis" of the molecule, so above a...

Ashley Wang 4G

If I'm understanding your question correctly (why it matters where the lone pair is located on the central atom?), it depends on the molecule. For example, in an AX4E molecule, the electron arrangement achieves the lowest repulsion if the lone pair is equatorial, because then it only strongly repels...

Ashley Wang 4G

The shape of the molecule helps you figure out whether the dipole moments (if any) on the molecule cancel out, which will help you predict if the molecule is polar!

Ashley Wang 4G

I think a trigonal pyramidal geometry corresponds to AX3E VSEPR formula, while a T-shape geometry corresponds to AX3E2!
So because there are two lone pairs on the Br atom in BrF3, it has a T-shape shape, while NH3, which has one lone pair on the N atom, has a trigonal pyramidal geometry. :)

Ashley Wang 4G

Hi,

Does anyone know from upperclassmen and the like whether taking 14B and 14BL together would be too much in a schedule with 4 classes?

I'd appreciate any and all input! Thank you.

Ashley Wang 4G

Hi,

I was wondering if you can still draw dipole arrows in an overall nonpolar molecule, if the bonds themselves are polar?

For example, in CO2, the CO bonds are polar but the overall molecule is made nonpolar by the symmetrical arrangement. Can we still indicate the dipole moments?

Thank you!

Ashley Wang 4G

Are we going to be touching on sigma and pi bonds later on (perhaps when we start the Shapes section)? Because it seems that so far we have stuck to the topics on this website pretty closely, but we've skipped this topic save for a brief mention once in lecture.

Thank you!

Ashley Wang 4G

Hi,

I received a lot of replies on my previous post regarding ion-ion interactions. It seemed to me from the replies that ion-ion interactions are the same as ionic bonds? Would I be correct in saying that?

Thank you.

Ashley Wang 4G

Hi, I feel like we didn't really touch on ion-ion interactions much in lecture. I was confused as to whether ion-ion interactions is the attractive force between the ions themselves or between ionic compounds? For example, is it the interaction between Na+ and Cl- ions, or is it the interaction betw...

Ashley Wang 4G

I think during the review session today one of the TAs said the maximum would be 18 electrons (2 in s + 6 in p + 10 in d orbital)!

Ashley Wang 4G

Assuming you're referring to either 2B.21 or 2B.22, i think finding the "most likely" structure just means finding the most stable (formal charges closest to zero, negative FC carried by the outer atoms, etc) resonance structure and therefore the one expected to contribute the most to the ...

Ashley Wang 4G

If most bonds exhibit both ionic and covalent properties to varying degrees, do we ever draw ionic compounds as Lewis Structures instead of two ions next to each other?

Ashley Wang 4G

Hi,

In lecture it was mentioned that it it preferable for the non-central atoms to carry the nonzero formal charges when necessary. If the central atom has an expanded octet, is it then okay to have a nonzero formal charge on the central atom?

Thank you!

Ashley Wang 4G

Hi, I was wondering what it means for resonance structures to be "equivalent." Are structures considered equivalent resonance if they have the same formal charges on the atoms? Or do resonance structures need to have the same number of single, double, and triple bonds (where you're essenti...

Ashley Wang 4G

I think it might be more accurate to think about increasing nuclear charge in terms of protons as opposed to electrons! So when there is an increase in the atomic number as you move across a period, the increase in effective nuclear charge felt is stronger than the slight increase in electron-electr...

Ashley Wang 4G

In class, we discussed how atoms in period 3 or higher can accommodate additional electrons than described by the octet rule because they have access to d-orbitals. I was wondering why these d-orbitals can be occupied when bonding even though they aren't used when the atoms exist in and of themselve...

Ashley Wang 4G

Hi, In lecture, Dr. Lavelle touched on how resonance spreads multiple-bond characteristics over a molecule, lowing its energy and making the molecule less reactive. Does this imply that molecules with resonance are more stable in general? Or did he mean that the actual hybrid characteristic of reson...

Ashley Wang 4G

I think the exactly quote you're looking for is that "resonance exists when you can draw multiple structures for a molecule with multiple bonds, wherein the arrangement of atoms are the same but the arrangements of electrons between the atoms are different." :) I hope this helped!

Ashley Wang 4G

Hi,

I was wondering what exactly a wave function is. If it is a mathematical description of the particle, why does its square give the probability of the particle's distribution?
Is it a mathematical finding that I should just accept as it's likely too complicated for me to understand?

Thank you!

Ashley Wang 4G

Last week, when explaining why electrons have quantized energy states, Dr. Lavelle talked about how electrons are comparable to a "circular standing wave" around the nucleus, and that the two "ends" must be in phase in order for the energy levels to be stable. I can understand th...

Ashley Wang 4G

Thank you so much! Your explanation made things clearer for me :)

Ashley Wang 4G

Hi, It was only after learning about the quantum numbers that I realized I'd been using the terms "orbital", "subshell", and "shell" somewhat interchangeably. Are the magnetic quantum numbers the actual "orbitals" that exist in the subshells, or is it also cor...

Ashley Wang 4G

Hi, Early on, when describing the p-orbital, we seemed to describe orientation by the x, y, and z subscripts. After learning about the magnetic quantum number, which supposedly describes the different orbitals of a subshell and the number of possible orientations in space (eg. 5 for d-orbitals), I w...

Ashley Wang 4G

Hi, On Friday, Dr. Lavelle mentioned that the 4s shell is filled before the 3d shell, but "after the 4s state is occupied, then the 3d state has lower energy than the 4s state." The quote is what I have copied down exactly in my notes. Could someone clarify what this means (or correct me i...

Ashley Wang 4G

Hi! I think for me their applications become most apparent when I think about how they relate to electron configuration. Someone (kindly) correct me if I'm wrong, but it seems that these principles are essentially the basis for how we know the electron configuration of each atom, and can also help y...

Ashley Wang 4G

Hi, I was wondering what the question meant by the "sulfide" of the metal. Does it mean the compound containing the metal and S^2- ions? Thank you!

Ashley Wang 4G

In class, someone asked whether de Broglie's Equation for wavelength can be used for light/EM radiation, to which Dr. Lavelle answered no. However, when we derived the equation later in the same lecture, we began with the equation for the speed of light. I know Dr. Lavelle clarified that this deriva...

Ashley Wang 4G

I was wondering if there are any ways to determine what state (s, l, g, aq) a compound is in? Specifically, when a word problem describes a reaction and we are expected to write the chemical equation, how do I determine what state each compound is in if it is not stated in the question? Should I det...

Ashley Wang 4G

In this unit, we've talked a lot about the frequency/wavelength of light versus the intensity of light, when discussing the threshold energy needed to eject an electron. From what I understand, the definition is that if the energy of the light does not reach the threshold energy, electrons will not ...

Ashley Wang 4G

The threshold energy is the energy (corresponding to a certain wavelength/frequency of light) needed to remove an electron from a metal surface when you shine light on it. If the energy of the light doesn't reach the threshold, the electron is excited but not ejected from the surface, no matter the ...

Ashley Wang 4G

Hi, just to make sure, calculators are permitted on the test as long as they're non-programmable and non-graphing, correct? Specifically, a CASIO scientific calculator would be allowed? Thank you!

Ashley Wang 4G

Hi! I think generally it's better to be precise, but I would go with whatever value is on the formula sheet given to us on the test (assuming this conversion will be).

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