CHEMISTRY COMMUNITY

Alexandra Ahlschlager 1L

If you’re using Q with the Nernst equation, it means that your reaction is not at equilibrium, but by using K (the equilibrium constant), it means your reaction is at equilibrium.

Alexandra Ahlschlager 1L

The oxidizing agent basically causes another molecule to be oxidized, which means it itself is reduced. For example, in order for another molecule to be oxidized and lose a negative charge, the oxidizing agent must be the one to accept that negative charge, so it becomes reduced.

Alexandra Ahlschlager 1L

The salt bridge prevents a buildup of charge on one side of the galvanic cell and essentially maintains a neutral environment to allow the reaction to continue.

Alexandra Ahlschlager 1L

Your work looks right to me! Since the other molecules are solids, you don’t include them in your value for Q, so you have all the information you need to solve the problem.

Alexandra Ahlschlager 1L

I definitely agree that doing the textbook problems again is a great way to study, especially for the math based problems; for the conceptual problems, I also usually read the textbook skimming for any concepts that I hadn’t come across anywhere else. If there’s a topic I really can’t seem to unders...

Alexandra Ahlschlager 1L

Since degeneracy is the amount of possible ways that the molecule can exist at a certain energy state, the randomness increases with degeneracy because those molecules can exist in any one of the various possible states within a system. On the other hand, if a molecule can only exist in one orientat...

Alexandra Ahlschlager 1L

Whenever you use this work equation, you’ll get units of liters times atmospheres. However, if the answer is in Joules or you need to combine this equation with another value, like with heat to find delta U, you’ll need to convert the units to Joules. You can do this by multiplying your answer for w...

Alexandra Ahlschlager 1L

If both delta H and delta S are positive, then the spontaneity will depend on temperature. Looking at the deltaG = deltaH - TdeltaS equation, having a positive enthalpy and entropy means that if temperature is large enough, it will make delta G negative, making the reaction spontaneous. However, if ...

Alexandra Ahlschlager 1L

I think it's most helpful for finding the equilibrium constant for a reaction at a different temperature, but you can basically use it to solve for whatever term you want that’s in the equation, as long as you know all the other variables. For example, if you already know both equilibrium constants,...

Alexandra Ahlschlager 1L

I’m planning on de-stressing by making myself some hot chocolate and catching up on all the tv shows I haven’t had time to watch!

Alexandra Ahlschlager 1L

Hi Mrudula! So the way I solved this problem was basically by looking at the units of each answer and working backwards. The first thing you have to do to determine enthalpy is find out how many kilojoules of energy are being supplied to the system. Using 550 W (which is equal to 550 J/s) you can co...

Alexandra Ahlschlager 1L

You shouldn’t need to use temperature in your calculations, since it’s basically just there to let you know that the reaction is occurring at standard conditions, which is consistent with the values from the chart given to you. To solve the problem, you can just add up the sum of the deltaG of forma...

Alexandra Ahlschlager 1L

Some general guidelines for how to determine what has higher entropy is by looking at the states of the molecules. So liquids will have higher entropy than solids, and gases - which are very disordered - will have higher entropy than liquids. You can also look at degeneracy, so a compound with a lot...

Alexandra Ahlschlager 1L

The way I solved this problem was by using deltaS = nRln(V2/V1). To find the values for V2 and V1, I basically wrote V2 in terms of V1, so V2 = (V1*⅙). When you plug these expressions into the ln function, the V1 variables cancel out and leave you with a number. Then, by using n=1, you should be abl...

Alexandra Ahlschlager 1L

Like others have said, I’m pretty sure all the values/formulas we’ll need are on the equation sheet, so you shouldn't have to worry too much about memorization. I would also add that the expressions with coefficients in front of R on the equation sheet are for monatomic ideal gases, so we might have...

Alexandra Ahlschlager 1L

Work is negative when the system itself is doing work (because the system requires energy and therefore loses it); conversely, work is positive when the surroundings are doing work on the system (because energy is being applied to the system).

Alexandra Ahlschlager 1L

Like Steph said, the equation you would use to solve this problem would be w = -nRTln(Vf/Vi) where Vf is the final volume and Vi is the initial volume. You are given all the values in this equation except for moles (n). To find this, you would use the equation PV = nRT and plug in the values given t...

Alexandra Ahlschlager 1L

Cubic equations are not the only instance where you can make X negligible. You can use this process anytime the equilibrium constant is less than 10^-4. You can also use the 5% rule by making sure that your final concentration is less than 5% than the initial concentration. If it is, then you can co...

Alexandra Ahlschlager 1L

The right side of your equation looks correct but the left side should actually have two terms: one to represent the ice melting and the other to represent the melted ice heating up. The melting of ice can be represented as moles times enthalpy of fusion. (we use moles instead of mass because enthal...

Alexandra Ahlschlager 1L

An isothermal reaction means that temperature remains constant throughout the reaction; however, there is still a transfer of heat, energy, etc. Hope this helps!

Alexandra Ahlschlager 1L

Like others have said, the reason they calculate 6 bonds using the same bond enthalpy is because of resonance, as the electrons are shared among the carbons. The answers have also disregarded the 6 C-H bonds because they are not being broken; however, I would also add that you can still get the same...

Alexandra Ahlschlager 1L

% deprotonation refers to when acids donate an H+ ion to water, as they are losing that proton. % protonation is the opposite and refers to bases when they are gaining a proton. You can calculate these two values by dividing the H3O+ or OH- concentrations at equilibrium by the initial concentration ...

Alexandra Ahlschlager 1L

If the reaction is exothermic and heat is being released, this means that the reaction will shift towards the reactants to account for this change. Since reactants are in the denominator of the K expression, increasing temperature will cause K to decrease. The opposite is true for exothermic reactio...

Alexandra Ahlschlager 1L

The reason steam causes higher burns is because it undergoes condensation when it hits our skin. This is represented by the second horizontal line in the phase change diagram, which covers a significant amount of heat released, specifically 46kJ. Since condensation releases this much energy, it caus...

Alexandra Ahlschlager 1L

Yes, most of the diatomic molecules will be equal to 0 if they are in the gas phase. This is because they are in their standard state. However, I think a diatomic bromine molecule would be zero when it is in its liquid state whereas the diatomic iodine molecule has to be in a solid state, as these m...

Alexandra Ahlschlager 1L

In addition to what Hila already mentioned, there are also some common signals that you can recognize to determine whether the compound is an acid or base. For example, if a compound ends in COOH, it is most likely a weak acid, and if a compound has a Nitrogen with a lone pair, it is most likely a w...

Alexandra Ahlschlager 1L

If the value of the equilibrium constant is less than 10^-4 you can assume that it will be small enough to approximate. You can also use the 5% rule, so if your x value is less than 5% of the initial concentration, you should be able to approximate. Hope this helps!

Alexandra Ahlschlager 1L

Yes, we only exclude H2O in equilibrium constants if it is in solid or liquid form since there is so much present before and after the reaction that the change in H2O is negligible. This applies to substances other than water as well, so any substance in the form of a liquid or solid should not be i...

Alexandra Ahlschlager 1L

I think the Kb value should actually be 1.8x10^-5 with a negative exponent instead of a positive one. If you use this number you should get the right answer, so I don’t think you’re doing anything wrong with your calculator. Hope this helps!

Alexandra Ahlschlager 1L

A general trend is that a stronger acid has a lower pKa and a higher Ka. This is because strong acids dissociate completely, so a strong acid should have a high dissociation constant. The same trend can be said for bases, so stronger bases would have a lower pkb and higher Kb.

Alexandra Ahlschlager 1L

Yes, like others have said, you can still get credit for answering questions that already have multiple answers. I actually find it helpful since everyone learns differently, so certain explanations might make more sense than others to different people. Hope this helps!

Alexandra Ahlschlager 1L

The question will likely tell you what delta H is and you can use this to determine whether the reaction is endothermic or exothermic and how it will react to changes in temperature. If the question doesn’t give you a value for delta H, a general trend is that when you’re forming bonds, you release ...

Alexandra Ahlschlager 1L

Yes! For your first question you're right, [H+] and [H3O+] mean the same thing. Your second question is right as well, you only include H20 in the equilibrium equation when it is in gas form, not when it is in solid or liquid form. Hope this helps!

Alexandra Ahlschlager 1L

K is the ratio of products to reactants only when the reaction is at equilibrium, while Q is the ratio of products to reactants at a random point in time. By comparing Q and K, we can find out what direction a given reaction (Q) shifts towards in order to reach equilibrium (K). For example, if Q is ...

Alexandra Ahlschlager 1L

When looking at a reaction’s response to an increase in pressure or decrease in volume, you’ll want to look at the moles of the equation. The reaction will shift towards the side with less moles, which in this case would be towards the left. This is because there are two moles of gas on the right an...

Alexandra Ahlschlager 1L

Like others have said, only some UAs give out an answer key for their worksheets, but if you can’t find them anywhere you could always ask about some specific questions on Chem Community for some help and easy points! Hope this helps!

Alexandra Ahlschlager 1L

Like others have said, you wouldn’t have to convert any values in this case since they are already given in the units of concentration (mol/L). If the units they gave you were in terms of atm or bars, this would indicate partial pressure and you would use PV = nRT in order to convert these values. H...

Alexandra Ahlschlager 1L

If Q is less than K, this means that the value of the denominator (reactants) is too big and the numerator (products) is too small. In other words, there are too many reactants in the reaction compared to how much there needs to be in order for the reaction to be at equilibrium. Therefore, in order ...

Alexandra Ahlschlager 1L

when do you know you are allowed to use this formula? You would be using this formula if you want to convert between partial pressures and concentrations. For example, if the problem asks you to solve for Kc but you are only given partial pressures, you can plug these values into the equation to ge...

Alexandra Ahlschlager 1L

Like others have said, both Kp and Kc can be used when calculating an equilibrium constant involving a reaction of gases. However, I think Kp is typically used for calculating the equilibrium constant for these reactions with only gases as opposed to aqueous solutions. Hope this helps a little!

Alexandra Ahlschlager 1L

Whenever I'm feeling really anxious about something, I like to take a break by going on a walk and just listening to music. It really helps me take my mind off of what is stressing me out and just overall helps me feel more refreshed.

Alexandra Ahlschlager 1L

In the photoelectric effect, an electron will be released if the incoming photon is higher in energy than the threshold energy. If you’re talking about atomic spectra, then an incoming photon must have energy that exactly matches the difference in energy between two energy levels in order for the el...

Alexandra Ahlschlager 1L

The more ionic character a molecule has, the more soluble it will be. I’m not too sure why that is though so maybe someone else can elaborate on this!

Alexandra Ahlschlager 1L

I think it helps to understand if you do an example; for instance, cisplatin: [Pt(Cl)2(NH3)2]. Since the charge of the compound is neutral (or 0), you know that when you add all the charges of the molecules inside the brackets, they should also equal 0. We know that the charge of Cl is -1, and since...

Alexandra Ahlschlager 1L

What confused me at first was the vocabulary of “first excited state and second excited state” so maybe this is your problem too? The first excited state would be n=2, (not 1 since n=1 is the ground state), and the second excited state would be n=3. So you would use the equation -deltaE = -hR/nf^2 -...

Alexandra Ahlschlager 1L

In the Week 10 review session pdf it said the final would be weighted based on how much time we spent on each unit, so I don’t think there will be a super large focus only on the last units. I think he said that the quantum unit was the longest so we should expect more questions from that section. H...

Alexandra Ahlschlager 1L

I don’t think it matters where you place them, since either way, you’ll end up with a polar molecule. Although it doesn’t look like it in the Lewis structure, none of the atoms in a tetrahedral shape are located directly across from each other (every bond angle is 109 not 180), so no matter where yo...

Alexandra Ahlschlager 1L

In the pdf about Week 10 final review sessions, it says the only resources we can have on the final are a calculator, the periodic table, and the constants/equations sheet, so I think we’ll have to memorize the ligand names since we won’t have it for the test. Hope this helps!

Alexandra Ahlschlager 1L

Yes, you would use “ferrate” instead of “ironate” in coordination compounds when the overall charge of the compound is negative. However, if there is no charge or the compound is positive, you would just use “iron.” Hope this helps!

Alexandra Ahlschlager 1L

In the answer key that explains how to solve each problem it says the answer is AX2E3, so maybe there’s a typo in your book? The middle iodine would be the central atom A, leaving the two other iodine atoms to be X2. There are also three lone pairs on the central iodine, so E would be three. Hope th...

Alexandra Ahlschlager 1L

Is there a rule to these exceptions or do we just need to memorize them? Yes, only the atoms that are one electron away from the 3d5 or 3d10 orbitals would take away an electron from the 4s subshell to become more stable, which is why chromium and copper are exceptions. I’m pretty sure this same tr...

Alexandra Ahlschlager 1L

Maddie Turk Disc 2C wrote:So just to clarify, AX4 will always signify a tetrahedral shape, alone with AX3E always being trigonal pyramidal, correct? almost as if they are code for that shape?

You're right! Molecules with the same VSEPR notation will always have the same shape no matter what the atoms are.

Alexandra Ahlschlager 1L

I'm pretty sure 465/500 points would be an A, if 93% is the cutoff. For an A- I think 450/500 would get you a 90% and 480/500 would get you an A+ if 96% is the cutoff. This could change if the cutoffs are different or a curve happens, so just keep that in mind! Hope this helps!

Alexandra Ahlschlager 1L

A polar bond refers to the difference in electronegativity between two elements, while a polar molecule refers to whether a molecule overall carries a charge. For example, in CH3Cl, there is a polar bond between the C and Cl due to the large difference in electronegativity; this dipole causes the en...

Alexandra Ahlschlager 1L

Although both the tetrahedral structure and SO32- have five areas of electron density, SO32- has a lone pair where a tetrahedral has its fifth element. Although lone pairs can influence shape, only atom positions determine the actual naming of the molecule’s shape, so SO32- creates a trigonal pyrami...

Alexandra Ahlschlager 1L

Like Ria said, there shouldn’t be any discussions after Wednesday this week. I don't think those discussions will have to make up anything since the Thursday and Friday discussions are actually ahead right now because we started this fall quarter on a Thursday. After this short week though, we shoul...

Alexandra Ahlschlager 1L

Yes! To build off of what others have said, molecules with the same VSEPR notation will have the same shape. For example, all molecules with the AX3E notation will have a trigonal pyramidal structure. Hope this helps!

Alexandra Ahlschlager 1L

You can determine this by looking at the differences in electronegativity between the atoms; atoms with a large difference in electronegativity will create a dipole and cause the molecule to be polar. For example, in an HF molecule, the large difference in electronegativity would cause electrons to ...

Alexandra Ahlschlager 1L

Oh okay. For V; Vanadium does the 4s2 give electrons to the 3d3 to make a half full 3d orbital then? Vanadium’s electron configuration would actually still be [Ar] 3d3 4s2. Only the atoms that are one electron away from 3d5 or 3d10 would take away an electron from 4s, so only the groups of chromium...

Alexandra Ahlschlager 1L

Like others have said, the quantum number l determines the shape of the orbital, which is how s, p, or d are determined. The ml quantum number tells you the orientation that these shapes are in; for example, in the p orbital, there would be 3 orientations where the two lobes lie on either the x axis...

Alexandra Ahlschlager 1L

I’m pretty sure you’re right about Nickel having the electron configuration [Ar]3d8 4s2, so I’m not too sure why it would be 3d10. Based on the electron configuration of [Ar]3d8 4s2 though, Ni 2+ would be [Ar] 3d8, since you would be removing 2 electrons from the outermost orbital 4s. Also the elect...

Alexandra Ahlschlager 1L

Copper’s ground state electron configuration is actually an exception; instead of leaving only one unpaired electron in the 3d subshell, it fills up the entire subshell and leaves one 4s orbital unfilled. So the normal electron configuration for copper is actually [Ar]3d^10 4s^1. Since the problem a...

Alexandra Ahlschlager 1L

Between two anions in the same row, would an anion with a -2 charge be more or less polarizable than an anion with a -1 charge? I think the anion with the -2 charge would be more polarizable because it would be the larger atom. For example, O^2- would be more polarizable than F^- because atomic rad...

Alexandra Ahlschlager 1L

An element can have an expanded octet if its energy level has access to a d orbital. For example, the elements in the 3rd period have access to the 3d subshell, so elements like Phosphorus and Sulfur can have more than the traditional octet. However, elements in the second period like Nitrogen or Ox...

Alexandra Ahlschlager 1L

Polarizability is how easily an anion’s electrons can be pulled towards a cation. The more polarizable an anion is, the more covalent character the ionic bond has. The main factor that determines polarizability is size; larger atoms like iodine are more polarizable because their electrons are farthe...

Alexandra Ahlschlager 1L

It has a lower ionization energy because of electron repulsion. In an Oxygen atom, the 2p subshell has four electrons: 2px^2, 2py^1, 2pz^1. Since the 2px orbital has 2 electrons, these electrons repel each other, making it easier for one of them to be removed. This is why oxygen and the other elemen...

Alexandra Ahlschlager 1L

Like Justin said, the formula for formal charge equals valence electrons - (shared electrons/2 + lone pairs). To symbolize the +1 formal charge of Nitrogen on the actual Sapling problem, you have to put the green positive symbol under “More” onto the Nitrogen atom. Hope this helps!

Alexandra Ahlschlager 1L

You’re not alone - the conceptual questions really threw me off on the midterm too! Like everyone else said, taking notes is a solid way to study and absorb the material. I think something else that might be helpful is to look up explanations of certain concepts on other websites and sources - they ...

Alexandra Ahlschlager 1L

When I went to one of the workshops, the UA said to prioritize making a full octet over making all formal charges zero. So I don’t think you should create an expanded octet just to get all formal charges equal to zero. I’m still not 100% confident on this concept though, so hopefully someone else ca...

Alexandra Ahlschlager 1L

Yeah, I don’t think the class is curved. The class is out of a total of 500 points, so I think your percentage is based purely on how many points you get out of 500.

Alexandra Ahlschlager 1L

The atomic radius decreases as you go across a period because more protons are being added to the nucleus as you go across. With every proton added, the attraction between the electrons and protons grows, bringing the electrons closer and closer to the nucleus. Therefore, the atomic radius gets smal...

Alexandra Ahlschlager 1L

I heard that they'll be released when the last discussion section is done taking it. I didn't hear it from Dr. Lavelle though so I could be wrong! My guess would be that you could find the score on CCLE under the midterm section.

Alexandra Ahlschlager 1L

I think the h-bar symbol represents h/2pi, so (½)h-bar would be equal to h/4pi. Since both equations are basically equivalent to each other, I’m pretty sure you can use whichever form you’re most comfortable with. Hope this helps!

Alexandra Ahlschlager 1L

The first thing I did was figure out the change in energy using the equation deltaE = - (hc)/wavelength. (The value is negative since it is an emission spectrum, so it releases energy). We know that delta E = E final - E initial, or (-hR/nf^2) - (-hR/ni^2), where nf stands for the final energy level...

Alexandra Ahlschlager 1L

I'm pretty sure the password is LL14A20. Hope this works!

Alexandra Ahlschlager 1L

You’re right in that the electron does absorb energy from the photon, which causes the electron to go up an energy level. However, for the electron to come back down, it has to release the energy of the photon that it absorbed (which is recorded to get the emission spectrum). Hope this helps!

Alexandra Ahlschlager 1L

I think this is because light on the electromagnetic spectrum does not have a mass, so you wouldn’t be able to apply the de broglie equation to it. However, for objects with a small mass, like electrons, you can calculate the de broglie wavelength. Hope this helps!

Alexandra Ahlschlager 1L

It would actually be more helpful to use the de Broglie equation (wavelength = h/mv)! Since you already have the values for h and v (velocity), the only thing you need to solve for is the mass of an individual diatomic chlorine molecule. Hope this helps!

Alexandra Ahlschlager 1L

The equation I used for this was the De Broglie equation: wavelength = h/mv. We already know the values of h and v (velocity) so the only thing we need to calculate is the mass. Since it’s a diatomic fluorine molecule, we can multiply the molar mass of fluorine by two to get the molar mass of the mo...

Alexandra Ahlschlager 1L

The photoelectric experiment was originally designed to test how much energy it would take to eject an electron from a piece of metal. However, they ended up finding out that in the experiment, light acted as a photon rather than a wave.

Alexandra Ahlschlager 1L

The surprising factor of the photoelectric experiment was that light did not act like a wave, but actually a particle. The reason they could conclude this was because increasing the intensity of the light in the experiment did not cause any more electrons to be ejected from the metal. However, when ...

Alexandra Ahlschlager 1L

Two waves are in phase with each other if their peaks line up together and their troughs line up together. This results in constructive interference where when combined, the amplitude of the new wave is larger. Out of phase is the opposite and means that the peaks and troughs of the two waves are mi...

Alexandra Ahlschlager 1L

The way I would start is by writing down the de Broglie equation: wavelength = h / mv, where m is mass of the electron and v is velocity. The problem gives you both these values, so after plugging them in, you should get your answer. For the second part, I would say yes, since objects with a high ve...

Alexandra Ahlschlager 1L

The equation he used in the problem was En = (-hR)/n^2, where n is equal to the electron's energy level. Essentially, the fractions of 1/16 and 1/4 come from dividing by n squared (in this case, dividing by 4 squared and 2 squared. Hope this helps!

Alexandra Ahlschlager 1L

The equation I used to solve this problem was the de Broglie equation: wavelength = h/p or wavelength = h/(mv). For the value of m, I used 9.1 x 10^-31 kg since this is the mass of an electron, and for the value of velocity, I converted the number given in the equation to m/sec by multiplying by 100...

Alexandra Ahlschlager 1L

So to start off you have to use the formula for kinetic energy which is Ek = (1/2)mv^2 where m is mass and v is velocity. The problem gives you the value of velocity, so you then need to know what value to use for m. In this case, m would be the mass of an electron which is 9.1 x 10^-31. (This is ju...

Alexandra Ahlschlager 1L

You’re right, whenever a problem talks about combustion, it implies that you’re adding O2 as a reactant. The same thing applies if a problem uses the term burning.

Alexandra Ahlschlager 1L

You’re right! I just took 5.45% of 510 grams to find the mass of KNO3 and subtracted that value from 510 to find the mass of water. Don’t worry, I don’t think there’s any secret trick or formula you missed!

Alexandra Ahlschlager 1L

The first thing you’ll want to do is find the diameter of one atom by multiplying the radius of 144pm by 2. To find the total length of the fiber, you’ll want to multiply this diameter by how many total atoms there are in fiber. In this case, there is one mol, so the number of atoms in the fiber is ...

Alexandra Ahlschlager 1L

In problem L. 39 it says "A 1.50-g sample of metallic tin was placed in a 26.45-g crucible and heated until all the tin had reacted with the oxygen in air to form an oxide. The crucible and product together were found to weigh 28.35 g. (a) What is the empirical formula of the oxide?" Could...

Alexandra Ahlschlager 1L

You're right, the molar mass of the molecule O2 would be 32 because it would be the molar mass of two oxygen atoms. Hope this helps!

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