CHEMISTRY COMMUNITY

Faith St Amant 3D

I usually make sense of the order of the diagrams by just looking at my half reactions and what order they are written in. This usually corresponds to the order that the compounds come into contact. Hope that helps :)

Faith St Amant 3D

There are two links to PDFs on Lavelle's website underneath the math assistance that are labeled that are guides to balancing redox reactions in both acidic conditions and in basic conditions. They are very helpful!

Faith St Amant 3D

The dot in between the Fe2O3 and 3H2O (Fe2O3∙3H2O) indicates that they form one compound, so the stoichiometric coefficient in front of the iron applies to the entire term. Hope that clears things up:)

Faith St Amant 3D

My problem #9 in Sapling reads: A galvanic (voltaic) cell consists of an electrode composed of aluminum in a 1.0 M aluminum ion solution and another electrode composed of copper in a 1.0 M copper(I) ion solution, connected by a salt bridge. Calculate the standard potential for this cell at 25 °C. Re...

Faith St Amant 3D

Hi everybody! I'm having some trouble with this problem, and the feedback that Sapling gives has not given me any leads as to what I'm doing wrong... Can anybody explain how they approached this one? Thank you!

Faith St Amant 3D

Hi, so we have a table that gives us the \Delta G^{\circ} values for a whole bunch of compounds, but those are measured at room temperature. Since the temperature given in the problem is not room temp, then we have to use the longer process of determining the change in enthalpy of the reaction using...

Faith St Amant 3D

Hi! So for this problem, the total heat that needs to be supplied to the system consists of two parts: the heat required to raise the temperature of the copper kettle, and the heat required to raise the temperature of the water, q(total)= q(copper) + q(water). Using the q=mCdeltaT formula, we find t...

Faith St Amant 3D

The second law of thermodynamics encapsulates the fact that creating order from disorder requires an input of energy. So in this question, the answer is no because theories of biological evolution suggest that order was created from disorder without a known source of energy input, so the theory is n...

Faith St Amant 3D

You can't tell anything from H or S alone, because the spontaneity of the reaction is dependent on the relationship between the two... So like knowing just the entropy doesn't tell you anything about whether the reaction is spontaneous or not because you don't know what the enthalpy value is so it c...

Faith St Amant 3D

If you go to the Sapling homepage, and then click on resources, the link called "Atkins 7e SSM" will download the solution manual onto your computer. This has the final results as well as step by step calculations and explanations.

Faith St Amant 3D

If you think about entropy in the context of a gas, the system will naturally tend towards a state when the gas has the most possible positions, when the particles are most spread out and most disordered. Just think about the air in you room as a system, the molecules are as dispersed as possible an...

Faith St Amant 3D

Hi! So, for part a we can use the equation we derived in class that deltaS T1->T2 = n*C*lnT2/T1, plugging in the Cp value for an ideal gas for C which is 5/2R because its at constant pressure. And then, for part b at constant volume, the same equation for deltaS can be used except with the Cv value ...

Faith St Amant 3D

For this problem, you can use the equation q=mCdeltaT to determine the final temperature of the system. At constant pressure, this means you need to use the Cp value for an ideal gas when solving for q, which is 5/2R, and at constant volume, you need to use the Cv value for an ideal gas when solving...

Faith St Amant 3D

From what I understand, if the atoms are all the same (like the three flourine atoms in BF3) then there is no disorder/degeneracy because changing which flourine is in which position makes no difference because the resulting molecule is identical to the original. So COF2 has more entropy because it ...

Faith St Amant 3D

This is what I got as well!

Faith St Amant 3D

On our constants and equations sheet, it's given that the Cp value for an ideal gas is (5/2)R while the Cv value for an ideal gas is (3/2)R. For this problem, you just need to plug those values into the equations qp=nCpdeltaT for part a, and qv=nCvdeltaT for part b to solve for the final temperature.

Faith St Amant 3D

Both! It just depends on the system.

Faith St Amant 3D

Exactly! Since the system is at constant pressure, the volume does not change, so the change in internal energy is equal to nCv(Tfinal-Tinitial). Once you determine the Cv value given the relationship between Cv and Cp as mentioned above, you should be able to calculate the change in U.

Faith St Amant 3D

Does the term for degeneracy (W) have any units?

Faith St Amant 3D

Exactly! It's easiest for me to think of q as heat supplied to the system, so it's negative if the system is releasing heat. Same thing for w... if w is work supplied/done to the system (compression), then it's negative when the system is doing the work (expansion).

Faith St Amant 3D

Notice that BCl3 is the only gas in the reaction, so it's concentration is the only one included in the calculation of Q. Since Q = [products]/[reactants], and we're only considering BCl3, Q= 1/[BCl3]^2. As for the difference between K and Q: K is the equilibrium constant and denotes the ratio of eq...

Faith St Amant 3D

I am also wondering the same this! It doesn't seem like there's enough spaces available in 14C, but I'm wondering if they'll open up more seats like Lavelle did for this quarter?

Faith St Amant 3D

In this case, it would help to write out the full reaction equation: (Cl-) + (NH4+) + H2O <> (H3O+) + (NH3) +(Cl-). Since there is Cl- ions on both sides, they cancel and we can ignore them. Then you can use the Kb value for NH3 and the relation Ka x Kb = 10^-14 to calculate the Ka of NH4+, and then...

Faith St Amant 3D

Kc is simply a more specific way of denoting the equilibrium constant, K, when we use equilibrium concentrations to calculate it.

Faith St Amant 3D

As far as I know we've been considering it a strong acid! I'd assume that we'd be given more context in a problem though if we needed to distinguish its strength.

Faith St Amant 3D

Hi! So in this case, water is the solvent on the reactant side, and since the concentrations of solvents exhibit very little change during a reaction we don't really consider it when we are analyzing the equilibrium composition for a reaction. Hope that makes sense!

Faith St Amant 3D

How did ya'll work through this one? I'm having trouble figuring out where to start.

For a certain reaction at constant pressure, ΔH=−15 kJ, and 22 kJ of expansion work is done on the system by compressing it into a smaller volume. What is ΔU for this process?

Thanks in advance!

Faith St Amant 3D

I think we're just supposed to use Lewis structures to determine the bonds in compounds, and then use given tables/given values to determine the enthalpies of the bonds, if that's what you're asking.

Faith St Amant 3D

Hi ya'll! I'm having trouble with this textbook problem: The reaction of 1.40 g of carbon monoxide with excess water vapor to produce carbon dioxide and hydrogen gases in a bomb calorimeter causes the temperature of the calorimeter assembly to rise from 22.113°C to 22.799°C. The calorimeter assembly...

Faith St Amant 3D

Sometimes, the question will specify whether the acid or base in the problem is strong or weak, but I've had both of my TAs so far tell us that we should commit the strong acids and bases to memory.

Faith St Amant 3D

Hi! So for each of these, I started by using an ICE box and the given K values for each compound (in Table 6C.1 and 6C.2) to determine the x-value, which represents the equilibrium concentration of OH- or H3O+ for each equation. Using this value, you can then calculate pH and subsequently pOH or in...

Faith St Amant 3D

Just a general guideline to follow with percent protonation questions is to just divide the equilibrium concentration of the conjugate acid/base by the initial concentration of the acid/base, and multiply by 100.

Faith St Amant 3D

Hi! So for each of these, I started by using an ICE box and the given K values for each compound (in Table 6C.1 and 6C.2) to determine the x-value, which represents the equilibrium concentration of OH- or H3O+ for each equation. Using this value, you can then calculate pH and subsequently pOH or in ...

Faith St Amant 3D

Hi! It looks to me like the exponent in your expression for Q for [H2] should be 3 instead of 2, since H2 has a stoichiometric coefficient of 3 in the balanced equation.

Faith St Amant 3D

Hi everyone! I'm having trouble determining the pH of a few of the compounds in textbook problem 6D. 13: Rank the following solutions in order of increasing pH: (a) 1.0×10−5M HCl(aq); (b) 0.20M CH3NH3Cl(aq); (c) 0.20M CH3COOH(aq); (d) 0.20M C6H5NH2(aq). Justify your ranking. How did you guys work th...

Faith St Amant 3D

Yes, since Ba(OH)2 is a strong base, we think of it as being 100% dissociated in solution, which means that for every 1 molecule of Ba(OH)2 introduced as a reactant, there will be 1 molecule of Ba2+. Hope this helps.

Faith St Amant 3D

Hi everyone! I'm having trouble piecing together this problem from section 6C in the textbook: Use data from Tables 6C.1 and 6C.2 to place the following acids in order of increasing strength: HNO 2 , HClO 2 , + NH 3 OH , (CH 3 ) 2 NH 2 + . Where should I start? How did ya'll work through this one? T...

Faith St Amant 3D

Yes^ exactly. I think most of the time, though, we'll be given problems that have larger K values and that require the quadratic equation in order to avoid any confusion.

Faith St Amant 3D

Here's how my TA worked through it. You just need to add the two reactions shown from the table together, and then multiply their K values.

Faith St Amant 3D

Hi everybody! Going through the textbook problems for this section, I'm a bit confused as to when to use the partial pressure as opposed to concentration when calculating K. Are there specific instances to use either? Thank you!

Faith St Amant 3D

Reactions with halogens can be endothermic or exothermic, it depends on how many electrons are being transferred during the reaction.

Faith St Amant 3D

yes, so when you see that a solution is 4M, that means that it has 4 moles of solute per liter of solvent and you would call the solution a 4 molar solution.

Faith St Amant 3D

I think they're equal because each bond is the same length, even though there are is one more double bond in SO3 than there is in SO2. So, when they say to compare bond lengths, since they both have the same types of bonds in there and no other types are present, they have equal bond length.

Faith St Amant 3D

The most favorable resonance structure is the structure with the smallest magnitude of formal charge, and with the negative charge on the most electronegative atom. Sometimes, like in 2.7, all of the resonance structures for the molecule have different formal charges within the molecule. To choose t...

Faith St Amant 3D

I was wondering the same thing on that problem! I was thinking of H2O, and how the molecule would be linear with 180 degree bond angles if there were no lone pairs on the O, but since there's two lone pairs its shape is always bent/angular. Maybe it's a typo in the book? Or maybe there's a situation...

Faith St Amant 3D

When I did this problem, I just assumed that the oxygens wouldn't accept a proton because they already have two bonds and are more stable that way, but you do raise a very good point. I guess the only times I can think of an oxygen accepting a proton is when the O has a negative charge (one bond &am...

Faith St Amant 3D

When you draw out the Lewis structure for both acids, you'll notice that the bond being broken when the acid is in the presence of water is not the Cl--O and Br--O bonds, it's the H--Cl and H--Br bond being broken. In this case, the higher electronegativity of Cl is more influential in the acidic ch...

Faith St Amant 3D

Exactly^! As we did in 2F.5, we should start by drawing out the Lewis structure of each compound, and then identify the regions of electron density, and then identify the hybridized orbitals present. So for part b, for example, AsF3 has a Lewis structure with 4 regions of electron density (and a tri...

Faith St Amant 3D

Exactly as Marcus put it^. It is still important to include the Roman numerals to indicate the oxidation state of the metal in a coordination compound, regardless of the charge of the entire complex. Without it, there would be no way of knowing what the charge of each ligand is, and the name on the ...

Faith St Amant 3D

Exactly^ the two definitions just emphasize two different aspects of acidic/basic character. The Bronsted definitions emphasize the transfer of protons, while Lewis definitions emphasize the transfer of electrons. Both lead to the same conclusion.

Faith St Amant 3D

Exactly^! Dr. Lavelle has mentioned, though, that we will not be expected to know these exact angles--we just need to know which normal bond angle (90, 109.5, 120, etc.) the angle will be slightly less than due to repulsion from lone pairs.

Faith St Amant 3D

Another great resource that my TA showed us for looking at molecular/electron geometry in 3-D to get a better understanding of what is most stable is interactive simulation on the University of Colorado, Boulder website. Here's a link if you want to check it out: https://phet.colorado.edu/sims/html/...

Faith St Amant 3D

Yes I wouldn't worry about it too much! If we're expected to do it at some point, I'm sure that Dr. Lavelle will go over it in lecture.

Faith St Amant 3D

Another way of thinking about the relationship between radius and charge is to remember that electrons are what make up the majority of the size of an atom, so when an ion loses electrons and becomes more positively charged, the overall size of the atom decreases as a result.

Faith St Amant 3D

I'm not totally sure, but just thinking about it I would assume that the molecule is most stable when it is nonpolar, so I wouldn't expect a molecule that has the ability to be nonpolar to rotate into a less stable position. There could be other factors, and I could be totally wrong on this but that...

Faith St Amant 3D

I think you're right on this one!

Faith St Amant 3D

For me, it's easiest to just remember what the roots of the words mean. For example, tetrahedral molecules have 4 atoms, hence the "tetra" prefix. Trigonal implies three atoms in the same plane, hence the "tri," etc. Hope this helps!

Faith St Amant 3D

Yes, and since electronegativity is generally periodic, it's safe to assume that the farther apart two elements are on the periodic table, the bigger the difference in their electronegativity. This helps when you're trying to figure out, for example, which of a group of bonds has more covalent chara...

Faith St Amant 3D

The easiest way for me to go about identifying hydrogen bonds is to first draw out the lewis structure that is most electronically stable. Then, it's just a matter of remembering that hydrogen bonds can only occur between a hydrogen that is covalently bonded to a N, O, or F atom and another N, O, or...

Faith St Amant 3D

That's an interesting observation, but I think it's just a coincidence. Bond lengths vary depending on the size of the atoms that are bonded together, so I wouldn't expect there to be any concrete pattern to the difference in bond lengths.

Faith St Amant 3D

Has any TA announced when we should expect to get our midterm 2 grades back? I know that a mistake in one of the test versions last time made the scores get delayed, but I was wondering if we might know our scores once everybody finished on Friday?

Faith St Amant 3D

I'm not sure if we'll be tested on the exceptions of the periodicity of ionization energies, but, from what I understand, the electron configuration of oxygen is the first of the p-block elements to require an electron to be paired in a p-subshell (nitrogen, for example, has 3 electrons all in separ...

Faith St Amant 3D

A resonance hybrid is the blending together of all the possible resonance structures for a molecule, so each possible resonance structure "contributes" to the blended version to a certain degree. We can look at the observed bond lengths and the expected bond lengths to get a rough idea of ...

Faith St Amant 3D

From my understanding, a dipole moment is a permanent dipole that arises from a molecule's structure, whereas an induced-dipole is a fluctuation in charge distribution that arises from interactions between molecules. Hope this helps!

Faith St Amant 3D

I don't think that whatever platform the bruincast lectures are uploaded through provides subtitles, the only feature I've used is the one that allows you to change the playback speed. Subtitles would be nice though, agreed!

Faith St Amant 3D

Hi all! Going through the textbook problems, I had trouble understanding how to start with drawing the Lewis structures for ionic compounds. The specific question asks "Draw the complete Lewis structure for each of the following compounds: (a) ammonium chloride; (b) potassium phosphide; (c) sod...

Faith St Amant 3D

First you want to determine the electron configuration for each ion. In+ for example has an electron configuration of [Kr]4d105s2. Looking at the valence configuration, you then can determine how many electrons are left without a pair. In this example with indium, there are no unpaired electrons bec...

Faith St Amant 3D

First, you want to start by identifying the electron configuration for each element. For example, the first one, Zn, has the electron configuration [Ar]3d104s2. Looking at the subshells that the valence electrons occupy, you then want to determine which subshell is more favorable to remove an electr...

Faith St Amant 3D

Wow @Gillian, your explanation is so helpful! Just adding onto that, when it comes to atoms have electrons that are highly polarizable, I like to think of the electrons as being arranged in a negatively charged cloud around the nucleus. So, when an atom has more electrons, it has a bigger "clou...

Faith St Amant 3D

And just following up with my previous question, if you are allowed to express the p-orbital electron configuration as px, py and pz, how would you express an element like F using that format? Would it be something like this? [He] 2s^2 2px^2 2py^2 2pz^1 In this case, it would need to be noted in th...

Faith St Amant 3D

Yes, they mean the same thing but the second one is just more specific. I think Dr. Lavelle prefers the second iteration, but I'm sure he would specifically ask for the configuration in that form if that came up on a test or something. Hope this helps!

Faith St Amant 3D

Hi! I know everybody says it, but Khan Academy has honestly helped me so much with gaining conceptual understanding of the topics we go over in this course. They have lots of practice problems, videos, articles, etc. that go over a whole bunch of chemistry topics so odds are you'll find some useful ...

Faith St Amant 3D

Yes that's right! The 3d subshell starts to fill up after the 4s, and the 4d after the 5s, and so on.

Faith St Amant 3D

Exactly^! In general, we can only really assume the correct number of valence electrons for non-transition metals in the s and p-block. I don't think that this should be an issue when we draw Lewis structures (in this course at least) because what I've seen so far has only really been asking for str...

Faith St Amant 3D

Yea, Dr. Lavelle commented a few days ago on someone's post saying that one of the questions had an error, so each test will need to be looked over by a TA this week. He said "it will take a while," so they'll probably be out in a few days.

Faith St Amant 3D

I think in that part of the lecture, Dr. Lavelle was making the distinction that the circular standing wave that an electron follows around the nucleus must be in phase. These wavelengths at which the circular standing waves are in phase are quantized, and support the notion that electrons have only...

Faith St Amant 3D

I'm pretty sure that just in general atoms won't absorb EM radiation unless it has an energy that is equal to the energy difference between the different energy levels (n). So if the radiation has the right wavelength to match the sprectra of the metal it is being directed at, it will be absorbed an...

Faith St Amant 3D

On our constants and equations sheet, the italicized v stands for the greek letter nu, which is the variable for frequency. The regular v stands for velocity. On that sheet specifically, c = λ v and E = hv are the only equations that use the greek letter nu (frequency). Hope this helps!

Faith St Amant 3D

Although I don't have the most concrete understanding of quantum numbers and electron configurations yet, I think also that we are supposed to generally look at the grouping of the subshells as being based of the angular momentum quantum number (l), and not necessarily on the shape of the modeled or...

Faith St Amant 3D

Oh ok! Thanks.

Faith St Amant 3D

In today's lecture, it's noted that Pauli's exclusion principle prohibits more than two electrons from having the same first three quantum numbers (orbitals). I was wondering though, what happens when electrons are excited to different states? Does there need to exist only one other electron in that...

Faith St Amant 3D

In his office hours a few meetings ago, Dr. Lavelle assured us that he generally avoids giving us a compound in a problem that requires us to know their molecular formula without giving the molecular formula in the question. He did specify that there are some simple compounds he expects us to know b...

Faith St Amant 3D

For part (a), the textbook actually asks for the wavelength of the ejected electron! For this, I used the DeBroglie equation plugging in Planck's constant (h), 9.109 x 10^-31 kg for m, and 3.6 x 10^6 m/s (which is converted from the given value in km/s to m/s) for v. Hope this clears things up!

Faith St Amant 3D

I'm pretty sure that the only materials we'll be allowed access to will be the formula sheet and periodic table that are on Lavelle's website! I do think Lavelle is going to send us an announcement about the specifics sometime before next week though.

Faith St Amant 3D

Makes perfect sense! Thank you.

Faith St Amant 3D

Hi! For questions 12 and 14 in the Sapling homework, the problems ask us to calculate the wavelength of the spectral line produced when an electron in a hydrogen atom undergoes a transition between specific energy levels. Immediately, I knew I'd probably need to use Rydberg's equation to start my wo...

Faith St Amant 3D

What I understood from the lecture's car example was that the "cutoff" for whether objects have measurable wavelike properties is not necessarily defined in terms of a specific mass, but rather wavelengths with values smaller than 10^-15m are not detectable and are therefore not significan...

Faith St Amant 3D

Perfect--thank you professor!

Faith St Amant 3D

Hi! I know that in the announcement on Dr. Lavelle's website regarding the content covered on our first midterm next week, he only mentioned the topics within the Quantum outline that will be covered. Does this mean that we won't see fundamentals problems on the exam? Thanks!

Faith St Amant 3D

@Edward thank you! Makes much more sense now. And @Xinying, R is the Rydberg constant: 3.29 x 10^15 Hz. Also, yes, c = 3.0 x 10^8 m/s.

Faith St Amant 3D

Hi everyone! I'm working on the Week 2 & 3 homework and am stuck on problem 5. The problem is: The electron in a hydrogen atom is excited to the n=7 shell and emits electromagnetic radiation when returning to lower energy levels. Determine the number of spectral lines that could appear when this...

Faith St Amant 3D

I attempted this problem using the equation Ek=1/2mv^2 to get the kinetic energy value for the ejected electron, and then tried to get the wavelength by using the E=hc/λ equation. I tripled checked my work and still never managed to get the same wavelength that the answer key states. I don't know wh...

Faith St Amant 3D

I was stuck on the same problem! I'm wondering if Dr. Lavelle will get into more information about spectral lines in his next lecture tomorrow morning?

Faith St Amant 3D

Since diluting the original solution will not change the number of molecules of solute in the solution, the number of X molecules that are present in the .1M solution will be equal to the number of X molecules present after it's been diluted. As for the health benefits, diluting the solution 90 time...

Faith St Amant 3D

Yes! In office hours yesterday, when someone asked this same question, Dr. Lavelle assured us that he will always provide both the compound name and its molecular formula on exams. I believe he said that he likes to provide us with the compound name and formula in order to test on what the problem i...

Faith St Amant 3D

Yes, it says on the syllabus that midterms will be 50 minutes closed note and TA proctored.

Faith St Amant 3D

I'm not sure if we will be having any quizzes... they aren't mentioned on the syllabus. Or are there quizzes that I haven't heard about?

Faith St Amant 3D

The way that I usually approach problem solving examples like yours is I'll find how many moles of each reactant are present, and then I'll multiply whichever reactant that has the smallest stoichiometric coefficient in the balanced equation by the other coefficients to determine which is the limiti...

Faith St Amant 3D

Ok, makes perfect sense! Thank you both.

Faith St Amant 3D

Thanks for the info! I'm still confused though, if it takes 1 Al atom with a +3 charge to cancel out the -3 charge of 1 Cl then shouldn't the molecular formula of aluminum chloride be just AlCl? Why are there 3 Al atoms in AlCl3? Also, the molecular formula of chlorine is Cl2--how do I determine tha...

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