CHEMISTRY COMMUNITY

Elena Chen 2E

Hi, I was just wondering how one becomes a UA for Chem 14A/B with Dr. Lavelle? Is there an application somewhere or is it invite-only? I'm interested in joining the program if possible! :)

Elena Chen 2E

Use the data in Appendix 2B and the fact that, for the half-reaction F2(g) + 2 H+(aq) + 2 e- ----> 2 HF(aq), Eº= +3.03 V, to calculate the value of Ka for HF. Here's what the answer key says: Screen Shot 2021-03-13 at 11.50.21 PM.png So I'm ok up to the point where they say to use lnK = nFEº/RT. Wha...

Elena Chen 2E

In an experiment, 0.020 mol NO was introduced into a flask of volume 1.00 L and the reaction 2 NO2(g) ---> N2O4(g) was allowed to come to equilibrium at 298 K. (a) Using the information in Table 5G.2, calculate the equilibrium concentrations of the two gases. So I got the right ICE table but I can't...

Elena Chen 2E

Hi, I'm a little confused on a part of the second problem that Dr. Lavelle goes over in the last lecture (#27). He says that we know the reaction is happening under standard conditions and at constant pressure because the problem gives us the delta Hº. Can someone explain why this is?

Thanks!

Elena Chen 2E

Hi, I just wanted clarification: when we calculate half-life, is the unit typically in seconds or hours? Thanks!

Elena Chen 2E

I think you're mixing up the ideas of having multiple reactants and having multiple elementary steps. The overall rate of a reaction would be the rate of the slowest elementary step, which would include any reactants involved in that singular step.

Elena Chen 2E

I haven't looked at the Sapling problems yet but from what I understand from Prof. Lavelle's lectures we don't need to know how to calculate anything concerning third-order reactions. It's most likely just conceptual.

Elena Chen 2E

Hi, how do we know which elementary step is the one that determines the overall rate? I understand that is it the experimentally slowest step in a reaction, but how do we know which one it is by simply looking at the elementary steps and not being given the rate of each one? Thanks!

Elena Chen 2E

What is the difference between differential and integrated rate laws, and how do we know in what situation to use each one? Thanks!

Elena Chen 2E

Hi, could someone please explain what the order of a reaction is conceptually? I understand how all the equations are derived, but I'm not quite sure what n is actually representing.

Thanks!

Elena Chen 2E

I'm a STEM major who writes poetry too! This is such a great idea and I'd love to support any other writers (or artists, musicians, etc) who are here. My poetry insta is @burnedoutpoet :)

Elena Chen 2E

Hi! So you're supposed to use the equation E=Eº-(0.05916/n)*logQ, just as if you were finding the potential of any other electrode with non-equilibrium concentrations under standard conditions (T=298 K). Keep in mind that the standard hydrogen electrode (SHE) is 2H+(aq) + 2e– → H2(g), with an Eº val...

Elena Chen 2E

Hi! I believe the error is in calculating your Eºcell value. First, Eº does not change with the number of moles used, so you don't multiply any of the Eº values by the coefficients. Also, when using the Eºcell = cathode - anode equation, there is no need to reverse the Eº value for the oxidation hal...

Elena Chen 2E

Thank you both so much!! I finally got it :)

Elena Chen 2E

Could someone please explain #7 to me? I can't figure out what I'm doing wrong and the only hint I get is "The anode should occupy the left‑hand position of the line diagram," which I think I'm doing.

Thanks!

Elena Chen 2E

Why was the electrochemical cell arrested?

Because he was convicted of battery.

Elena Chen 2E

The cathode is the electrode where the reduction half-reaction is occurring, and the anode is where the oxidation half-reaction is occurring. You can also think of the cathode as the "positive side" since it's pulling electrons and the anode as the "negative side" since it's the ...

Elena Chen 2E

A volt is considered 1 joule of work per Coulomb, where a Coulomb is the amount of electrical charge in 6.24 x 10^18 electrons. So measuring voltage is just measuring electrical energy. Hope that helps!

Elena Chen 2E

Hi, in the answer key for textbook problem 4.17 part e, it says:
(0.030 mol SO2)(-197.78 kJ/2 mol SO2) = -2.9667 kJ = -3.0 kJ (2 sf) = -3000 J

I was wondering what the (2 sf) part meant and how that turns the answer into -3000 J. Thanks!

Elena Chen 2E

Hi, for the textbook problem 4D.9, I got the right numerical answer, but the answer key says it should be a positive value instead of a negative value. Can someone please explain to me why this is? The problem is below: The enthalpy of formation of trinitrotoluene (TNT) is 267 kJ/mol, and the densit...

Elena Chen 2E

Currently binging Criminal Minds but I kind of want to rewatch Bridgerton haha.

Elena Chen 2E

Hi, could someone please explain conceptually what the Van't Hoff equation actually is and how we know when to use it? Thanks!

Elena Chen 2E

Just adding on to what was already said, one way to think of whether a reaction is favorable is to consider whether it would occur by itself naturally (ie. leaving a glass of water out will cause it to naturally evaporate over time).

Elena Chen 2E

The º indicates substances in their standard states.

Elena Chen 2E

I don't quite understand the relationship between K and deltaGº. I get why deltaGº=0 when K=1, but could someone please clarify conceptually why deltaGº is positive when K<1 and why deltaGº is negative when K>1? I remember Prof Lavelle saying something about the products and reactants.
Thanks!

Elena Chen 2E

You have to set the two equations qice and qwater equal to each other and add the enthalpy of fusion to qice because you're adding the energy needed to go from a solid to a liquid.

Elena Chen 2E

The negative sign goes on whichever side is exothermic (losing heat). For example, if you were combining two liquids of different temperatures, the negative sign would go on the side of the liquid with a higher temperature because when combined, it would be losing heat.

Elena Chen 2E

I don't quite understand the 3rd law of thermodynamics. Could someone give a summary or basic explanation of what we need to know about it? Thanks!

Elena Chen 2E

I believe there are actually four different states all with the same energy : 1) AB together on the left, 2) AB together on the right, 3) A on the left and B on the right, and 4) B on the left and A on the right. Also, I think the dog bone structure on the slides is more of a representation of the l...

Elena Chen 2E

At the end of one of the lectures, Prof. Lavelle mentioned that the residual entropy for an oxygen molecule (O2) is zero. Can someone explain to me why this is and how we can know if another molecule has a residual entropy of zero? Thanks!

Elena Chen 2E

This is definitely super basic but I love Taylor Swift's two newest albums!

Elena Chen 2E

When water is below 0ºC, the heating curve is at a slant and water is a solid. At 0ºC, it turns into a horizontal line, which represents the process of melting/freezing, and water exists as a solid and liquid. After a while, the horizontal line again turns into a slanted line and reaches from 0ºC to...

Elena Chen 2E

I'm having trouble understanding what work (w) actually is. Is it just a transfer of energy between a system and its surroundings, or is there more to the definition?

Thanks!

Elena Chen 2E

Hi, can someone please explain why we use an integral in the work (w) equation? What is it representing conceptually?

Thanks!

Elena Chen 2E

Hi can someone please explain what exactly a reversible process is? Is a process considered reversible just when the systems and surroundings are in equilibrium or is there another part of the definition I'm missing?

Thank!

Elena Chen 2E

I like to do the textbook problems and go over the outline points to make sure I understand them. Also, making sure you understand the sapling homework explanations really helps. :)

Elena Chen 2E

Just adding on to the discussion that will hopefully strengthen your understanding of a monoprotic acid: The opposite of a monoprotic acid would be a polyprotic acid, where a compound is able to give off multiple protons (H+) in a solution. Some examples include carbonic acid (H2CO3) and phosphoric ...

Elena Chen 2E

pH measures the concentration of hydronium (H30+) in a solution, so anything that adds or takes away from the hydronium concentration will affect the pH. I believe when Prof Lavelle talks about certain elements that don't affect a solution's pH, he's referring to a salt that has either the cation of...

Elena Chen 2E

Could someone please explain how enthalpy and heat differ and why enthalpy is a state property while heat isn't? I understand the definition of state property but I'm not sure why it doesn't apply to the concept of heat (q).

Elena Chen 2E

In problems 9 and 10 of sapling week 2, it asks us to determine the predominant species for a weak acid and weak base by comparing the pH and pKa of the solutions. Can someone please explain how to do this?

Thanks!

Elena Chen 2E

Inert gas is not chemically reactive. It will not react with any of the reactants or products and therefore not change their concentrations.

Elena Chen 2E

The reaction will shift to the right towards the products because it has fallen out of equilibrium and will form more product until the original product to reactant ratio is reached again.

Elena Chen 2E

That's not what I got, but we might have different numbers. Could you give more information?

Elena Chen 2E

In one of the lectures, Prof Lavelle said that as Ka increases, Kb decreases. Can someone please explain why exactly this is and how this applies conceptually?

Elena Chen 2E

Definitely old Disney and Disney Channel Original movies! Also, I love chick flicks like Clueless or Legally Blonde.

Elena Chen 2E

Elena Chen 2E

I still don't quite understand how to determine which way a reaction is going to go from calculating the Q. Can someone please explain to me why when Q<K, we know that the forward reaction is favored and when Q>K, the reverse reaction is favored?

Thanks!

Elena Chen 2E

In a lecture example, Dr. Lavelle asked how to increase the yield of a product without adding more reactants, and the answer was to remove some product. Can someone please explain to me why this is?

Elena Chen 2E

I believe adding helium does have an effect on pressure, but has no effect on the volume or the concentration of reactants/products because it does not engage in the reaction.

Elena Chen 2E

When a reaction favors the left side, that means it's creating more reactant (ie "going" towards left). Same is true for vice versa.

Elena Chen 2E

"Noting that the bond angle of an sp3 hybridized atom is 109.58 and that of an sp2 hybridized atom is 1208, do you expect the bond angle between two hybrid orbitals to increase or decrease as the s-character of the hybrids is increased?" The answer key says that the bond angle increases as...

Elena Chen 2E

Hybridization isn't determined by the number of valence electrons that each atom has but by the number of regions of electron density. So for example, a double bond, like the one in C=C, would be considered one region of electron density. If the atom has two single bonds and one double bond, then it...

Elena Chen 2E

If the salt contains the conjugate base/acid of a weak acid/base, then the salt will be considered acidic or basic. For example, consider ammonium chloride, NH4Cl. Cl- doesn't affect the pH of the solution because it is the conjugate base of a strong acid, but NH4+ is the conjugate acid of a weak ba...

Elena Chen 2E

Hi, can someone explain the conceptual difference between pH and pKa? I understand how to calculate both, but I'm not sure how they differ conceptually. Are they considered synonymous with each other?

Thanks!

Elena Chen 2E

Elena Chen 2E

Another good thing to remember when determining coordination number is to only look at the atoms/compounds included in the brackets. Anything outside the brackets is outside the coordination sphere.

Elena Chen 2E

How do we determine whether a complex is a chelate or not? Is there a certain way to figure that out or is there a list of chelates to memorize?

Elena Chen 2E

Hi, I still don't really understand what it means when a ligand is monodentate, bidentate, tridentate, etc. How do we determine the -dentate state of a ligand?

Elena Chen 2E

We have to put the oxidation number at the end of the TM in parentheses using Roman numerals, so for example, cobalt with a charge of 2+ would be cobalt(II). Also, if the entire complex has a negative charge, we add -ate to the end of the TM name, but this occurrence is rarer. That's all I'm aware o...

Elena Chen 2E

Do transition metals only form coordinate covalent bonds? Or can they also form normal covalent bonds or ionic bonds?

Elena Chen 2E

The negative pole on a molecule is on the atom with higher electronegativity.

Elena Chen 2E

Hybridization depends on the number of regions of electron density of the atom. So for example, an atom with 4 regions of electron density would have an sp3 hybridization because it has 1 s-orbital and 3 p-orbitals.

Elena Chen 2E

A single bond consists of one sigma bond, a double bond consists of one sigma and one pi, and a triple bond consists of one sigma and two pi bonds.

Elena Chen 2E

A greater difference in electronegativity between two atoms leads to a higher dipole moment between them.

Elena Chen 2E

Elena Chen 2E

It's helpful to draw out the dipole moments using arrows, with the arrowhead pointing to the more electronegative atom. If the dipole moments cancel each other out, then the molecule is nonpolar. If they do not cancel, then it's polar.

Elena Chen 2E

Molecules that are polar contain dipole moments, which means atoms within the molecule have significant differences in electronegativity. If two molecules interacting are both polar, then it is likely that there is a dipole-dipole interaction.

Elena Chen 2E

Could someone please clarify how the seesaw shape gets created? Why is the lone pair taken from one of the equatorial positions?

Elena Chen 2E

The two lone pairs cause repulsion on the two bonding pairs, which affect the shape of the molecule and make it nonlinear.

Elena Chen 2E

I was wondering what we'll need to know about the interaction potential energy proportionality (Ep proportional to (a1*a2)/r^6). I think I remember Prof. Lavelle saying we won't need to do any calculations with it, but what sort of questions will we need to use it for? Is it only conceptual?

Elena Chen 2E

Hydrogen bonding only occurs between H and N, O, or F atoms. I believe it's because the dipole moment between H and S is not large enough to reach hydrogen bonding.

Elena Chen 2E

Ga is in Group 13 and therefore has 3 valence electrons. Due to the octet guideline, it can either lose 3 electrons or gain 5 electrons to reach desired 8 total. It's easier for an atom to lose 3 electrons than to gain 5, therefore causing it to have an ionic charge of 3+.

Elena Chen 2E

My TA told me that yes we will be using respondus on our laptops to take the test while logging into the zoom on a separate device. As for the questions, my TA said the amount will be different since there aren't really any calculations to do. She also said there won't be as much/any partial credit ...

Elena Chen 2E

Hi, can someone please explain what exactly is the difference between an ionic bond and a dipole moment, if they both have differences in charge/electronegativity between the atoms involved? What makes a molecule an ion instead of a dipole?

Elena Chen 2E

Hi, I believe electron-electron repulsion occurs between all electrons. If there are any differences in the amount of repulsion in different energy levels, subshells, etc, I don't think they are significant enough to make a difference.

Elena Chen 2E

Atoms in the third row or below of the periodic table can hold more than an octet of electrons since they can utilize the d-orbital to hold more electrons. Since P is in the third row, it is part of this exception to the octet rule. Other common atoms with expanded valence shells include S and Cl.

Elena Chen 2E

Effective nuclear charge is the pull on electrons by the nucleus due to protons in the nucleus (positive-negative attraction). So if something has stronger effective nuclear charge, its electrons are more pulled more tightly to the atom. Effective nuclear charge increases across a period, since the ...

Elena Chen 2E

I believe 3d always comes before 4s because electron configurations are labelled from lower energy to higher energy, and a lower shell number indicates lower energy. As for Ca, I'm pretty sure the electron configuration is [Ar]4s^2 because the d-orbital isn't reached yet in terms of the periodic tab...

Elena Chen 2E

I believe formal charge is allocated per atom, but the collective formal charges of all atoms in a molecule add up to the total formal charge of the molecule.

Elena Chen 2E

Since the work function is given and they are asking for the wavelength, use the equation E=(hc)/lambda to convert energy to wavelength. Then convert the wavelength units from meters (SI unit) to angstroms, which is given on the constants and equations sheet. I believe they give you the radiation in...

Elena Chen 2E

Yes, I believe that the nuclear charge is simply the pull of the nucleus. This is used in the context of periodic trends and ionization energy because protons and electrons are attracted to each other (positive and negative attract). When there are more protons in the nucleus, there is more nuclear ...

Elena Chen 2E

No, gamma rays have the shortest wavelength and therefore the highest energy.

Elena Chen 2E

Ionization energy is defined as energy need to remove an electron from an atom. The further away an electron is from the nucleus, the easier it is to remove because of the lessened nuclear attraction. As you move up the periodic table, the shell number n decreases, meaning the electrons are closer t...

Elena Chen 2E

Jamie2002 wrote:Wouldn't the uncertainty be 10 m/s, since it is (5 m/s + 5 m/s) - (5 m/s - 5 m/s)?

I have this same question too. Is there a specific way to know when to multiply the +/- value by 2 and when to not when finding uncertainty?

Elena Chen 2E

I don't think so because I asked my TA and a couple UAs and they also don't know the exact number of questions (I could be wrong). But it should be completable within our 50 min discussion sections and it's all multiple choice :)

Elena Chen 2E

Yes you would have to convert nm to m since meters is the SI unit for length. Also, yes you would use the Bohr equation, though I find it easier to use E = (hc)/wavelength when needing to convert wavelength to energy (or vice versa) just because it's all in one step. You would also have to use the a...

Elena Chen 2E

Hi, I was just wondering if anyone knew how much of Schrodinger's wave function and equation we will have to know conceptually and applicably for exams? Will we have to use the equation to calculate anything and if so, what might a question requiring us to do that look like?

Thanks!

Elena Chen 2E

Hi! I believe he was showing us the email that he sent Sapling to disable question randomization, not instructing us to disable it ourselves. So we don't have to worry about that :)

Elena Chen 2E

Personally, I've been making sure to keep up with the lectures on the day they are posted. That way I don't feel too overwhelmed by new information on any given day. I also remember hearing that many of Dr. Lavelle's exam questions are derived from the textbook problems so I suggest doing them even ...

Elena Chen 2E

Elena Chen 2E

Yes, the model of the photoelectric effect refers to the photon model. If light acted as a wave in the photoelectric effect, then the energy of the light would increase as the intensity (amplitude) increased, but as we know now, that does not happen. Therefore, there must be an alternative model for...

Elena Chen 2E

Hi, I'm having some trouble with part C from the problem below. The velocity of an electron that is emitted from a metallic surface by a photon is 3.6×103 km⋅s−1. (a) What is the wavelength of the ejected electron? (b) No electrons are emitted from the surface of the metal until the frequency of the...

Elena Chen 2E

Hi can someone help me out with this problem from discussion section 2B week 2 practice? I don't understand how to turn the given joules into a number of photons. A lamp rated at 32 W (1 W=1 J⋅s−1) emits violet light of wavelength 420 nm. How many photons of violet light can the lamp generate in 2.0...

Elena Chen 2E

Atomic spectroscopy supports the idea of light, not electrons, as particles (though electrons are also particles) because photons must match the specific energy levels of the atom (or molecule) in order to be absorbed/emitted. Otherwise, the light simply passes through.

Elena Chen 2E

Hey! Here's how I solved it. First, find the number of moles that was dissolved in the first beaker (converting grams to moles) and find the concentration of that beaker by dividing the moles you found by the volume they gave. Then, using that concentration, find the number of moles in the amount of...

Elena Chen 2E

My high school chem teacher showed us this meme and this is genuinely still how I remember which ion is which.

Elena Chen 2E

Hey guys, I just wanted to start this topic so that all GroupMe links for Chem 14A fall quarter 2020 would be consolidated in one place and it would be easier for people to find them. Drop any groupchat links you have for your lecture or discussion! Whole Chem 14A groupchat: https://groupme.com/join...

Elena Chen 2E

In order to find the grams of the excess reactant, you convert the limiting reactant from grams to moles, then use the stoichiometric coefficients of the balanced chemical equation to convert from moles of limiting reactant to moles of excess reactant. This shows you how much of the excess reactant ...

Elena Chen 2E

Something that helps me remember the concept of Avogadro's number is thinking of it like the number pi. We know that whenever someone says pi, they are always referring to 3.1415... and so on. Likewise, when someone mentions Avogadro's number, they are always referring to 6.022 x 10^23 particles. Av...

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