CHEMISTRY COMMUNITY

Juliet Stephenson 4E

E₀ represents E_cell at standard conditions.

Juliet Stephenson 4E

So what is the difference between oxidation number and the charge the atom carries? Charge says something about the relationship between the number of protons and electrons. Charge equals [Proton # - Electron #]. Oxidation numbers are more representations of the total number of electrons that an at...

Juliet Stephenson 4E

Units of k also change for each order. Rate will always equal Moles/liter.seconds but since we are sometimes squaring or cubing concentration, K will have different units for each order. Most charts contain some reference to this.

Juliet Stephenson 4E

Siddiq 1E wrote:does this mean molecularity corresponds with rate order?

I believe this is true. So reactions can be unimolecular (1st order), bimolecular (2nd order), or termolecular (3rd order).

Juliet Stephenson 4E

My friend's TA emailed and asked for students to email pictures of their week 10 homework, but I would wait for your TA to email you to be sure that they don't get flooded with emails.

Juliet Stephenson 4E

At the lower temperature, when kinetics dominates, CH 3 CHBrCH=CH 2 is in a higher quantity. At the higher temperature, when thermodynamics dominates, CH 3 CH=CHCH 2 Br is in a higher quantity. The product formed at lower temperatures will have the lower activation energy, since it is forming as a r...

Juliet Stephenson 4E

What defines a solid conductor?

Juliet Stephenson 4E

Yes. I find it useful to remember that Reduction occurs on the Right in the Cathode (all consonants, two Rs) while Oxidation occurs in the Anode (vowels) on the left.

Juliet Stephenson 4E

You need to multiply the reactions so that the number of moles of e- are the same and they will cancel out correctly.

Juliet Stephenson 4E

I don't think this is the easiest way, but one important distinction is that different orders have differently shaped graphs of concentration vs time. You can tell what the order is by observing which concentration vs. time graph is linear. For first order reactions, ln[A] vs. time is linear. For se...

Juliet Stephenson 4E

Galvanic/Voltaic cells include spontaneous reactions and they release energy. I found this image helpful.

Juliet Stephenson 4E

Which equation are you referring to?

Juliet Stephenson 4E

I think the electrochemical series is a sort of list of reactions that allows us to classify elements or molecules in order of their reduction potentials, but outside of this I am not really sure what we need to know.

Juliet Stephenson 4E

What exactly does the order of the reaction mean and how do we determine it? I read in my notes that it is equivalent to the number of molecules of the reactant which are colliding during the slowest step of the reaction, but I feel like I'm missing some context.

Juliet Stephenson 4E

Yes, the reaction will be electrolyzed to allow it to proceed when it cannot otherwise proceed because it is non-spontaneous.

Juliet Stephenson 4E

ng1D wrote:When balancing redox reactions, are we balancing the number of atoms or electrons?

Both!

Juliet Stephenson 4E

Yes! After doing a little internet research I found that, generally, a substance containing both hydrogen bonds and lone pairs can potentially experience autoprotolysis. For example, ammonia and acetic acid can both experience autoprotolysis.

Juliet Stephenson 4E

Delta E and Delta U both represent the change in internal energy in a system. Both values are equal to the Final Energy of the System-Initial Energy of the System.

Juliet Stephenson 4E

Q: What is the chemical formula for a banana?
A: BaNa₂

Juliet Stephenson 4E

Yes, I would say this is a correct assumption.

Juliet Stephenson 4E

I agree! I believe isothermal expansions are irreversible because they progress towards a uniform state, which is an irreversible change.

Juliet Stephenson 4E

It looks like you're using 25 kJ as the value for qcal but it should be 22.5 kJ.

Juliet Stephenson 4E

For these questions, I find it really helpful to imagine a glass half-full of water. For this glass, as for any system, you would be able to assign specific values to different properties. If, by filling the glass up all the way, one of these values would change, then that corresponding property is ...

Juliet Stephenson 4E

A process is reversible if the system maintains thermodynamic equilibrium with its surroundings throughout the whole process. Reversible processes are less common than irreversible processes, and a perfectly reversible process is not possible.

Juliet Stephenson 4E

I am at Lyndon's review session and he said we won't be provided the equation for degeneracy (W=# of microstates^{# of particles}), so be sure to memorize that!

Juliet Stephenson 4E

This is kind of irrelevant to the question but will the question always say if it is reversible or irreversible? Or is there another way we will know which equation to use? I am pretty sure the question will always say whether the expansion is reversible or irreversible. I am not aware of a way we ...

Juliet Stephenson 4E

I don't think there are any values we need to memorize for this midterm. It is always very helpful to me to go over the constants and equations sheet before exams as a little study guide of sorts. We don't know how to use all of the equations yet, but it is still a helpful review of some of the topi...

Juliet Stephenson 4E

Parts of it seemed familiar to me from lecture, so I would guess we are responsible for knowing the fundamentals of that section. It is definitely included in the reading assignment as well.

Juliet Stephenson 4E

Hi Amy! I found example 4A.4 in the book on page 253 to be super helpful in walking me through this problem. I would highly recommend looking at it. In summary, first use C_cal=q_cal/deltaT and then use your C_cal value to find q using q=-C_cal * deltaT.

Juliet Stephenson 4E

The first time I did this problem is ended up with -.86 kJ but it was because I made an error (multiplied mass times density instead of dividing). I am thinking the answer key is wrong. I don't think such a large discrepancy could be produced by premature rounding.

Juliet Stephenson 4E

Since O₂ and other diatomics are so stable, they tend to form in nature. As a result, there is no enthalpy of formation, as oxygen will already be in this form.

Juliet Stephenson 4E

An extensive property is a property that depends on the amount of a substance that is present in a sample. For example, the mass of a substance changes depending on how much of the substance you are measuring, so it is an extensive property. An intensive property is a property that stays the same re...

Juliet Stephenson 4E

An open system is not contained, and as a result can lose or gain mass or energy during the course of a reaction. A closed system is contained (and therefore cannot lose or gain mass during the course oof a reaction) but can still transfer or receive energy to/from the environment. An isolated syste...

Juliet Stephenson 4E

I also think it would be possible that we would have to calculate the phase change enthalpy for sublimation based on given values of vaporization and fusion.

Juliet Stephenson 4E

Energy is the ability to do work, and is usually expressed in joules. Enthalpy is more specific - the ability of a certain material to do work, usually expressed in joules per mass unit (gram, kg, etc).

Juliet Stephenson 4E

The method you should use depends on the information provided in the problem.

Juliet Stephenson 4E

Since state functions are based on the end state of a system, they can be added without concern. This is because they represent a specific phase/time after the reaction has reached equilibrium.

Juliet Stephenson 4E

An endothermic reaction can only occur when there is excess free energy in the environment, while an exothermic reaction can occur in the absence of this energy. Therefore, the larger the deltaH value, the less spontaneously the reaction will occur.

Juliet Stephenson 4E

Since the concentration isn't changing, [H₂O] will appear in the numerator and the denominator, so it will be equivalent to one and have no affect on the equilibrium constant. Therefore, it makes the most sense to just exclude it from your calculations altogether.

Juliet Stephenson 4E

I think you would exclude the solid, because it will not be affected by a pressure change. A solid is already in the most compressed state it can be in, so increasing the pressure won't have any effect on the concentration. Increasing pressure will only affect the solid in the way that the solid is ...

Juliet Stephenson 4E

Salts form when an anion (Cl^-, for example) and a cation (Na⁺, for example) form a compound in a substance. Whenever an acid reacts with a base, salts are formed.

Juliet Stephenson 4E

When there is increased pressure, the reaction will favor whichever direction has fewer moles of product. For example, for the reaction Cl 2 <-> 2Cl, if the pressure were increased at equilibrium, the reverse reaction would be favored since there is only one mole of Cl 2 for every two moles of 2Cl.

Juliet Stephenson 4E

In Table 5G.2, a list of equilibrium constants for various reactions are provided. For many reactions, K and Kc have the same value, which I would expect based on the fact that these two constants are calculated in the same way. However, for the bottom half of the table, K and Kc have different valu...

Juliet Stephenson 4E

In regards to the activity of each ion, I believe they are referring to the ion's respective concentration. I hope this helps! That makes sense, in a problem would that value be given or is there a way to calculate it? it would likely be given but i guess if they did want us to calculate the ions' ...

Juliet Stephenson 4E

I think equilibrium composition might mean the concentrations of reactants and products at equilibrium but I am not certain.

Juliet Stephenson 4E

Protons and electrons are attracted to each other because protons are positively charged and electrons are negatively charged. Opposite charges always attract. In the structure of atoms, the presence of neutrons helps to stabilize the system.

Juliet Stephenson 4E

The rule of thumb is that the equilibrium will respond in a way that counteracts the change. However, this doesn't give you an accurate representation, so the best way to do so is to recalculate using equations to determine equilibrium. Can you specify which equations we would use to recalculate? W...

Juliet Stephenson 4E

If one were to manually add products to a system at equilibrium, Q would be greater than K until the system rebalanced itself.

Juliet Stephenson 4E

I thought it was pretty difficult! I wasn't expecting there to be any titration problems because I thought I heard Lavelle said we wouldn't be doing it until 14B, so that really threw me off. In general there were a lot of things I wasn't sure about. I am still proud of how much I prepared for the e...

Juliet Stephenson 4E

HF is a weak acid because the H-F bond is incredibly strong due to the electronegativity of F and the small atomic radius of F. As a result, it has short bonds that aren't equally broken, which translates to low levels of dissociation.

Juliet Stephenson 4E

I feel like I have a pretty good handle on hybridization when there is electron promotion, but I am struggling to understand 2F9 part d. d) Find the hybrid orbitals used by PCl 3 . I know the solutions manual says it is sp 3 , but I am having a little trouble understanding why. The electron configur...

Juliet Stephenson 4E

Amphiprotic (think proton) refers to a molecule that can donate or accept a proton. Of course, this means that it can act as both an acid and a base. Amphoteric means the same thing but the term is a little more general, whereas amphiprotic is referring to the specific idea of protonation/deprotonat...

Juliet Stephenson 4E

Something can form a chelating complex if it has a) more than one lone pair available for bonding (bidentate or polydentate) AND b) has those bonds close enough together to form a ring (i.e. they are on the same side of the molecule). A molecule that can form a chelating complex should have the stru...

Juliet Stephenson 4E

Bronsted acids and bases are actually concerned with the transfer of protons. Lewis acids and bases are concerned with the transfer of electron pairs. Bronsted acids are capable of donating a proton because the products of this reaction are more stable than the reactants. Bronsted acids produce Hydr...

Juliet Stephenson 4E

Each H present in the anion will go on to form one hydronium ion. In HCl, one H begets one hydronium ion per molecule HCl (monoprotic). In H2S04, two H's will beget two hydronium ions per molecule H2So4 (biprotic/polyprotic). This is important to remember in calculating the pH of a substance.

Juliet Stephenson 4E

Given most chemical reactions (consider the dissolving of salt in water) reactions are proceeding in both directions at all times. In our saltwater example, NaCl is both being dissolved and reforming at all times. In most chemical reactions, one direction of the reaction takes more energy, and so no...

Juliet Stephenson 4E

I would start by assigning an arbitrary value to your starting pH and then manipulating the values as the problem suggests. Since pH is a logarithmic scale, differences should be the same given any starting value.

Juliet Stephenson 4E

I can't remember what the full name of the compound is but the drug is called cisplatin, and if you feel like googling that, it should yield more information regarding the full name of the coordination compound.

Juliet Stephenson 4E

It is helpful for me to remember that if I'm writing out the dissociation of an acid, one of the products will usually be hydronium (H3O), and if I'm writing out the dissociation of a base, one of the products will be hydroxide (OH).

Juliet Stephenson 4E

Seesaw shape occurs when you have a molecule with five areas of electron density, one being a lone pair. Normally, such a molecule would have a trigonal bipyramidal shape, but since the lone pair doesn't show up in our visualization of the shape, we can visualize the removal of one atom. This leaves...

Juliet Stephenson 4E

I am confused about how we can identify sigma and pi bonds in a molecule. Will someone please explain? Thank you!

Juliet Stephenson 4E

I think it all has to do with the structure of whatever the molecule is binding to. If the main molecule features groups that are close together, then the attaching molecule would probably need to be cis.

Juliet Stephenson 4E

In Step-Up my UA advised that, while not incorrect to list both, it is only necessary to list hydrogen bonds.

Juliet Stephenson 4E

Yes, because if this type of intermolecular force were occurring between two of the same type of molecule they would either be dipole-dipole (in the case that the molecule is polar) or London dispersion (in the case that the molecule is nonpolar).

Juliet Stephenson 4E

Yes. If we are talking about two different molecules binding to each other, then dipole-induced dipole is also possible.

Juliet Stephenson 4E

This chart from step up was really helpful for me. Besides that, I think if you just understand the shapes, you'll probably be able to figure out the angles just by dividing 360 by the number of bonds.
https://drive.google.com/file/d/1h2MnGaEAe2sEf8yZ96NREj_8hGNLmluw/view?usp=sharing

Juliet Stephenson 4E

When we're discussing two of the same molecules binding with each other, hydrogen bonds are the strongest, followed by dipole-dipole, followed by London dispersion/Van der Waals forces. Dipole-induced dipole interactions will only occur between two different molecules, usually a polar molecule and a...

Juliet Stephenson 4E

I have always been advised by both peers and my department counselor not to take B and BL at the same time. BL is known for being pretty difficult and from what I understand, it depends on concepts which are taught in B, which can be a little tricky if you're figuring them out at the same time that ...

Juliet Stephenson 4E

Induced-dipole-induced-dipole interactions occur when two molecules become dipoles by chance. They are not naturally dipoles, but because of their environment or just random variations in the electron clouds, they become dipoles and experience bonding. An example would be two molecules of the same a...

Juliet Stephenson 4E

The number of hydrogen bonds/induced-dipole-induced-dipole interactions also contribute to boiling points (as they also contribute to bond length/strength). I found Lavelle's example of A-T vs G-C bonding in DNA particularly helpful. Since A-T has dipole-dipole interactions at two sites, it has a lo...

Juliet Stephenson 4E

Daniel Chen 1B wrote:But are we able to turn in homework from the Topic 2 section still?

I feel like we probably can't do problems from topic 2 anymore since all of last week was chemical bonds.

Juliet Stephenson 4E

I don't think we would have to illustrate this very often. Maybe we might have to use partial charges like we do with hydrogen bonds. Hopefully we will get more examples in class.

Juliet Stephenson 4E

Kevin Antony 1J wrote:H, He, Li, Be, B, and Al I believe will never have a full octet. Additionally, some elements like Si, P, S, and Cl can have expanded octets. I'm pretty sure we just have to memorize what the exceptions are.

What is the reasoning behind Al not being able to have a full octet?

Juliet Stephenson 4E

Hydrogen, Helium, Lithium, and Beryllium are exceptions because they only have s orbitals. Elements that are period 3+ can also potentially expand their octet.

Juliet Stephenson 4E

The octet rule states that atoms tend to bond with each other in order to form a complete octet, meaning they have 8 total electrons in their outer shell.

Juliet Stephenson 4E

I feel like Lavelle was pretty firm about us not working with the F block in this course. My TA also told us it would not be featured in the course.

Juliet Stephenson 4E

For the purposes of this class, 10 electrons, because that is how many can fit in the d orbital. But if we were working with the f orbital, I assume it would be more.

Juliet Stephenson 4E

This is a little tricky to type out but this is what I got. c) :O=Si=O: d) :F : : :F : : :Br: :F : : :F : : which is to say a Br surrounded by four F connected with single bonds. 2 sets of loan pairs around Br. This works because Br can have an expanded octet. If you calculate the formal charges, yo...

Juliet Stephenson 4E

Will we be having lecture on the Wednesday before Thanksgiving (Nov. 27th)? Will we be meeting in our discussion sections that day?

Juliet Stephenson 4E

Electronegativity is a measurement of an atom's attraction to electrons - its own as well as outside electrons. Ionization energy is the energy input required to remove an electron from an atom. Electron affinity is a measurement of change in the energy of an atom when an electron is added. Electron...

Juliet Stephenson 4E

I found this table very helpful in understanding this concept.

more info at https://socratic.org/questions/how-can-i-find-valence-electrons-of-transition-metals

Juliet Stephenson 4E

megan3j wrote:for IE, EA, EN: as you move to the right --> these increase; also as you move up the periodic table they increase
for AR: as you move to the left <-- the AR increases; as you move down the periodic table the AR inrcreases

Can you say what all of these stand for? Thanks!

Juliet Stephenson 4E

I always interpret this to mean that the electrons have a less precise location. Because they have multiple "possible" locations, we can't assign them a particular place in the octet or to a particular bond, so they are delocalized. The effect of this is that they create more stable bonds.

Juliet Stephenson 4E

These elements are all in the d block, which means they have access to extra orbitals in which they can store electrons.

Juliet Stephenson 4E

Is there a good way to visualize spin? Are electrons actually spinning in the magnetic field? Or is it some other quality that can't be visualized?

Juliet Stephenson 4E

Yes. Electrons in the S orbital are more effective at shielding, so an electron in the p orbital would experience less Z_effe.

Juliet Stephenson 4E

I just read something that said that violations to the aufbau principle are more common for elements with higher atomic numbers than for those with lower atomic numbers. Why is this? Hi Will, As was mentioned earlier in this thread, full and half-full subshells are rather stable and can have lower ...

Juliet Stephenson 4E

I think its best to think of this as an effect of mass. The mass of a baseball is around .145 kg. The mass of an electron is around 9.109 * 10 -31 kg. Photons are massless, but they do have energy and are moving at very high speeds. A massless particle won't affect the position or momentum of someth...

Juliet Stephenson 4E

Can anyone walk me through this problem using E_n=-hR/n²? I understand how to do this using the Rydberg Equation, but am having trouble figuring it out the other way. Thank you!

Juliet Stephenson 4E

I would also add that, practically, work function is usually in different units than joules, so it usually takes a little bit of finagling to get it in the right units for the problem. Be sure to watch out for that! In my experience so far, when threshold energy is given, it is usually already in jo...

Juliet Stephenson 4E

Constructive interference occurs when the waves line up in-phase, so they stack on top of each other and gain energy (higher frequency, shorter wavelength). Destructive interference occurs when waves line up out of phase. Imagine that the peak of one wave is lining up with the trough of a different ...

Juliet Stephenson 4E

Hi Jessica, The differences between these series are the energy level transitions they are referring to. The Lyman series comes from electrons which are moving from any energy level to the n=1 energy level. The Balmer series comes from electrons moving from any energy level to the n=2 energy level. ...

Juliet Stephenson 4E

Thank you Ashley!

Juliet Stephenson 4E

Hi, I was hoping someone could walk me through Problem 28 on the Photoelectric Module. 28. Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 105 m.s-1. The work function for sodium is 150.6 kJ.mol-1. Answer the following three questions. A. What is the kinetic ener...

Juliet Stephenson 4E

I think the reason why you're getting a different answer is that you're using the incorrect units for velocity. On Friday, Lavelle talked about the units for the DeBroglie equation: h is in joule seconds (kg.m 2 .s -2 seconds). In order for everything to cancel out properly, you need your mass to be...

Juliet Stephenson 4E

Hi Annie! Here's how I did this problem. First, we need to calculate the molarity of the diluted NH4NO3 solution using M1V1=M2V2. These values are supplied by the problem. (0.2 M NH4NO3)(1 L)=(M2)(4 L) M2=.05 M NH4NO3 From here, we need to calculate how many moles of NH4NO3 would be present in 100 m...

Juliet Stephenson 4E

Hi Anokhi, This is what worked for me: We start with NaCl + SO3 + H2O --> Na2SO4 + HCl. Hydrogen and Sodium are unbalanced. Oxygen is already balanced because it has three moles from SO3 and 1 mole from H2O, which corresponds to the 4 moles of Oxygen in Na2SO4. To balance Hydrogen and Sodium, I'll a...

Juliet Stephenson 4E

I believe we're supposed to interpret this image literally, as in there are 9 gallium atoms on the left. I multiplied 9 by the provided molar mass of gallium, which has the same significant figures as the molecular mass aka the mass of a single gallium atom in atomic mass units (AMU). Then I divided...

Juliet Stephenson 4E

I had to google this before doing the problem, but mmol stands for millimoles, aka 1/1000 of a mole. So in order to keep all the units consistent, I converted the volume given in a) (2.15 mmol Na+) into moles by dividing by 1000 --> .00215 M Na+. Hope that makes sense!

Juliet Stephenson 4E

So this problem is asking us to determine the volume of the solution should be transferred into a flask in order to obtain 2.15 mmol of Na+ . Importantly, this is not a dilution problem. Therefore, we won't be using the M1V1=M2V2 equation. The molarity of the solution won't be changing when we trans...

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