CHEMISTRY COMMUNITY

Nathan Lao 2I

I think the only time we really use the integrated rate law, ln[A]=-kt+ln[A], is if we are given a problem that asks us to find the concentration at a certain time. Time is the main thing we solve with that equation.

Nathan Lao 2I

I like to write the value of K in comparison to Q. The reason is that you can essentially visualize the greater than/less than sign as an arrow that points to which side it will shift. K < Q means it will shift towards reactant (I visualize it as K <-- Q, arrow points left so reaction shifts to the ...

Nathan Lao 2I

I think emailing them would be your best bet!

Nathan Lao 2I

I'm pretty sure the question would also clarify if they want the rate of the overall reaction as opposed to an elementary reaction. After going to the review sessions, there were a lot of practice problems that said to write the rate law of step 1, 2, etc and then another question that said to write...

Nathan Lao 2I

The short answer to your question is no. The long answer is that the order of a reaction is dependent on the orders of the reactants. The order of a reactant indicates how much the reactant affects the rate of the reaction. You would need to add up the orders of each reactant to get the overall orde...

Nathan Lao 2I

I would definitely take advantage of the UA and TA review sessions that are left! They're really helpful for explaining confusing topics and some of them even go through past exam questions. Other than that, studying with friends is pretty helpful (as long as you stay focused).

Nathan Lao 2I

For this one I looked at how the half life changed based on the change in reactant concentration. In the question you have, notice that the half life increases when the reactant concentration decreases(this is an inverse relationship). The only half life equation that shows an inverse relationship f...

Nathan Lao 2I

All you have to do is add the order of all the reactants together!

Nathan Lao 2I

The anode is often on the left and the cathode is usually on the left. If you really want to make sure, you can use the table of standard reduction potentials to determine the reaction that is taking place. From there you would have to determine which metal is which position and whether it is being ...

Nathan Lao 2I

Electrochem is the most interesting to me!

Nathan Lao 2I

I use the homework/syllabus outlines as a study guide and I've been keeping track of the problems that confused me throughout the quarter.

Nathan Lao 2I

I agree with the post above! In general, positive voltages (E°> 0) are spontaneous while negative voltages (E°< 0) are not.

Nathan Lao 2I

Just to clear any confusion, it's a lowercase k and it is equal to 1.38 x 10^-23 J/K (<-this K is for Kelvins)

Nathan Lao 2I

To be honest, I was kinda nervous during the test because I wasn't completely sure of the answer on most of the questions. After getting it back I did better than I expected though.

Nathan Lao 2I

I not entirely sure if they're interchangeable since they do produce different values. If you use logQ in the Nernst equation, you would have to have E = E° - ( (2.303RT)/nF )logQ. But if you use lnQ then its just E = E° - ( RT/nF )lnQ.

Nathan Lao 2I

This problem involves the Nernst equation. The first step is to find the standard reduction potential of this cell. Then, using the given concentrations, we can solve for Q and plug everything else(such as temp, R, F, etc) into the equation. It''s important to note that the variable n in the questio...

Nathan Lao 2I

The anode is where oxidation happens. Typically, this is where the anode loses its electrons and forms ions in solution. The anode becomes those ions in solution, so the mass of the anode would decrease over time. For the cathode, a general reaction is an ion in solution gaining electrons. This lead...

Nathan Lao 2I

I was hoping to take Chem 14C but its all taken ;-;

Nathan Lao 2I

I just watched Netflix/Wandavision and treated myself to good food LOL. And slept. A lot.

Nathan Lao 2I

Something that really helps with this class if forming a study group with some friends and teaching them about a topic we're discussing in class. Being able to explain something to someone or teach them is a sign that you yourself know the concepts.

Nathan Lao 2I

Like most questions, it really depends on the units given. Technically they are all interchangeable but it's best to use the R value that corresponds with the units given in the problem. This would make it the easiest in terms of calculations. For example, if you're trying to find the entropy change...

Nathan Lao 2I

I feel like it's more for convenience. Maybe because ΔH°usually deals with really high energy changes whereas ΔS° isn't as extreme.

Nathan Lao 2I

Wandavision is literally so good. I just don't like how each episode is a cliffhanger. They answer our questions about the previous episode but then introduce way more at the end of each one. The suspense kills me.

Nathan Lao 2I

The qrev just refers to the way that specific equation is derived. In general, I'm pretty sure the given ΔH (or ΔH that we might have to calculate) is synonymous the qrev that we use to find ΔS.

Nathan Lao 2I

I believe both of those methods work since ΔG, ΔH, and ΔS are all state functions.

Nathan Lao 2I

^^ That's correct for the third condition but for the very last one where ΔH and ΔS are negative, the (T)(ΔS) would need to be smaller. Otherwise, you would get a positive ΔG and the reaction would not be spontaneous.

Nathan Lao 2I

I converted the kPa to atm for this question. As long as your units cancel out correctly in the end, you should be fine. I hope this helps!

Nathan Lao 2I

To solve, we use the equation ΔU = q + w. We know that w = -PΔV but when there is constant volume, there is no change in volume. This means w = 0 and ΔU would just be equal to q. The q value in this problem refers to the energy/heat released by the reaction, which is negative.

Nathan Lao 2I

It really depends on each question and the units given. From there, you'd just have to choose the R value that corresponds with units!

Nathan Lao 2I

Yeah I think your set up is fine. It might be the values you plugged into your calculator. I tend to mess those up sometimes too lol.

Nathan Lao 2I

This problem uses the equation mcΔT = -mcΔT since the energy released by the metal is the same amount absorbed by the water. The problem is essentially q = -q. In this case, you would change ΔT to Tfinal-Tinitial, where initial is the initial temperature of the different species. So one side would b...

Nathan Lao 2I

Hi! I'm pretty sure it's safe to say that the information would be given in the question.

Nathan Lao 2I

It's really subtle but I overlooked the actual type of reaction this is when I first did this question. It's a combustion reaction, so I went with "enthalpy of combustion of CH4." :)

Nathan Lao 2I

For the most part and in this class specifically, a gas and a vapor are synonymous. I think the technical difference is that vapors are the gas phase of a substance that would usually be liquid at room temperature/standard conditions.

Nathan Lao 2I

Generally, yes it is a strong acid. This is because the 3 oxygen atoms pull the electrons away from oxygen atom engaged in the O-H bond, making it a pretty weak bond. Since the bond between the oxygen and hydrogen atom is weak, it is easily broken and will, for the most part, dissociate completely. ...

Nathan Lao 2I

I like to listen to mellow/chill music whenever I study. I reccomend LANY, Lauv, Jeremy Zucker... the list goes on but these are a few of my favorite artists!

Nathan Lao 2I

I'm not sure if this is entirely correct but I believe the hotter one would have the negative enthalpy change since it's the "colder" substance that absorbs the energy of the "hotter" one. One substance will always transfer its energy to the other, and hotter substances have high...

Nathan Lao 2I

You would first need to set up an ICE box to find the equilibrium concentration of H3O+. This is calculated by setting the given Ka to the equilibrium constant equation, which is [H3O+][A-]/[HA]. I just use HA and A- to represent a generic acid and its conjugate by the way. I believe you can use the...

Nathan Lao 2I

I agree with the post above! Temperature changes can be calculated regardless of what happened to make that change. It's just a matter of finding the difference between its original temperature and the final temperature. Heat, however, depends on what happened in a reaction. For example, you'd have ...

Nathan Lao 2I

I believe so. Ka stands for the "acid dissociation equilibrium constant" and will always be the concentrations of [H3O+][A-]/[HA]
(HA just represents an acid with that H proton and A- is its conjugate).

Nathan Lao 2I

the homework problems are your best bet. It's best to expose yourselves to different types of problems so that the first time you see that type of question isn't on the exam.

Nathan Lao 2I

Hi! You would start by creating an ICE box using the given concentrations. As a generic acid dissociation equation I usually write HA + H2O -> (A-) + (H3O+). You know the forward reaction is favored in this case, so the concentrations of (A-) and (H3O+) would be positive, or +x. Set the Ka equation ...

Nathan Lao 2I

For this specific K equation, you can actually take the square root of both sides. It would simplify to square root of 53.3 = 2x/(0.7-x) and then you would solve for x from there by rearranging the equation.

Nathan Lao 2I

I agree with the post above. The autoprotolysis of water is really small so it's almost negligible, and the H2O molecules remain in excess.

Nathan Lao 2I

For this problem, you would calculate Q and see that when the concentration of NO is changed to 0.700M, the reverse reaction would take place. This means that the equilibrium concentrations for N2 and O2 would be (0.100-x) and for NO it would be (0.700-2x). Once you plug these into the K equation, y...

Nathan Lao 2I

I think you're correct that compressing the system means a change in volume. However, pressure and volume changes relate to gases. It's true that when volume decreases the system will shift to the side with less moles, but since there are an equal amount of moles of gases on each side of the reactio...

Nathan Lao 2I

The first thing to do it set up an ICE box for the equation and solve for the equilibrium pressures. It's always best to solve for Q just to find out which way the reaction is proceeding. After setting up the equilibrium constant equation, you would solve for x (using the quadratic equation). Since ...

Nathan Lao 2I

They are inversely proportional, meaning if you increase one the other will decrease. Let's say there's a box(of some volume) full of gas particles that are constantly moving and hitting the walls of the box. If you were to shrink the size of the box, the gas particles would hit the box more frequen...

Nathan Lao 2I

We usually emit water in a liquid state from the K equation. So in this case, I'm pretty sure gaseous H2O would be considered in the calculating this K value.

Nathan Lao 2I

Basically, an inert gas is any of the noble gases. It's considered "inert" because most of the noble gases do not react.

Nathan Lao 2I

It depends on the initial concentrations of the reactants and products. There's no such thing as negative concentration/partial pressure; the lowest it would be is zero. When finding the x value using the quadratic formula, it's important to plug the values into the equilibrium equations for each re...

Nathan Lao 2I

I agree with the posts above, assuming that the temperature stays the same. If the temperature were to change, then the value of K would also change.

Nathan Lao 2I

Coordination number is essentially the number of bonding sites that a TM will form with ligands. It's important to know the denticity of the ligands since they will affect the coordination number. For example, a bidentate ligand counts as 2 bonds, a tridentate ligand forms 3, and so on. I'm not comp...

Nathan Lao 2I

Hi! En refers to the ethylenediamine ligand, C2H4(NH2)2, and is a bidentate ligand. This means that it will bond to the cobalt ion at two sites. Using this information, the two en ligands will make 4 bonds. Add the two CO bonds and you'll get a total of 6 bonds, or a coordination number of 6.

Nathan Lao 2I

I'm pretty sure you would still write the (aq). The reaction is still occurring in water, so the H3O+ would be denoted as aqueous.
When in doubt, I don't think writing a state of matter in reaction equations will do any harm (unless its incorrectly identified).

Nathan Lao 2I

I got this question wrong at first but then I realized it is super important to look at the denticity of the ligand, not just the number of ligands! Because en, or ethylenediamine, is a bidentate ligand, it will bond to the cobalt cation at two sites. For the two en ligands, that's a total of 4 bond...

Nathan Lao 2I

I agree with the statement above! To kind of explain why this happens though, the reason we keep the weak acid/base in its molecular formula is that it has very little dissociation. It dissociates so little that we essentially say "it doesn't happen," and we continue with the net ionic equ...

Nathan Lao 2I

A non polar bond would have to be a bond between atoms whose electronegativity difference is essentially zero. There wouldn't be any uneven distribution of electron density. This is why non polar bonds are usually between two atoms of the same element.

Nathan Lao 2I

It depends on the atoms you're comparing. You can think of the equatorial atoms as a circle divided by the three atoms, so every angle between the equatorial atoms would be 120 degrees. For the axial and equatorial atoms, when I see a picture of them I usually envision a right angle between them. Th...

Nathan Lao 2I

Phosphorus is a period 3 element and has d orbitals that electrons can fill, which allows them to have expanded octets.

Nathan Lao 2I

I don't think we'll have to know the specific angles. Those are usually determined experimentally. It's more about predicting a smaller/bigger bond angle relative to its VSEPR structure. (For example, NH3 has slightly smaller bond angles than 109.5 since the lone pair exerts a higher repulsion than ...

Nathan Lao 2I

I think it has to do with the fact that the regions of electron density create a tetrahedral structure (the molecule is bent but the electron geometry is tetrahedral). If you switch any of the positions of the atoms/lone pairs, the structure will essentially be the same thing again. Dr. Lavelle desc...

Nathan Lao 2I

The deciding factor between the two structures is a lone pair! Keep in mind that a trigonal planar structure has 3 regions of electron density around the central atom while a trigonal pyramidal structure has 4. The extra region of electron density on a trigonal pyramidal structure is a lone pair, no...

Nathan Lao 2I

The lone pair has to be one of equatorial positions because it creates a lower energy/more stable structure for the molecule. Essentially, the regions of electron density would be farther apart if the lone pair is in one of the equatorial positions, which would minimize repulsion and create a favora...

Nathan Lao 2I

If the element is in energy levels n = 3 or higher, it can have expanded valence shells. This is because they have other orbitals.
A period 3 element in the p-block, for example, has d-orbitals that can accommodate for the extra electrons.

Nathan Lao 2I

The periodic trend is that effective nuclear charge increases across a period (left to right) and up a group. So generally the elements towards the top right have the most effective nuclear charge.

Nathan Lao 2I

We would look at intermolecular forces since those are the attractions between the molecules. For example, if the IMFs are stronger in one compound, they would "stick together" more than the other compound and have a higher boiling point(more energy would be needed to break those intermole...

Nathan Lao 2I

I think a difference of 0.4 or less leads to nonpolar covalent bonding. Following that rule, a C-H bond would be nonpolar.

Nathan Lao 2I

An ionic bond essentially involves the transfer of an electron, usually from a metal to nonmetal, to achieve a lower energy. A dipole moment is a separation of charge due to differences in electronegativity. There is a momentary unequal distribution of electrons that leads to a dipole moment.

Nathan Lao 2I

Intramolecular forces are attractive forces that occur within a molecule. For example, atoms within a molecule are attracted to one another by the sharing of electrons(bonding). Intermolecular forces are attractive forces between molecules. Hydrogen bonding would be an example of this since the hydr...

Nathan Lao 2I

The periodic trend for atomic radii is that it increases down a group! Since S is one period below O on the periodic table, it has a larger atomic radius and that's why the bond length would be larger.

Nathan Lao 2I

Yeah I don't think its required but its really useful to know some polyatomic ions. For the question above, there are 18 electrons to account for in NO2-. When you go through the general steps of drawing a lewis structure for NO2-, you'll see that the oxygens single bonded to the nitrogen will fill ...

Nathan Lao 2I

Hydrogen bonds usually only form between H and fluorine, oxygen, or nitrogen. In the context of the problem, that's why there must be an O on either side of the H in the H-bond.

Nathan Lao 2I

A Lewis acid is a substance that will accept electrons whereas a Lewis base is one that will donate electrons. So using the example that Professor Lavelle gave in lecture, BF3 would be a Lewis acid since the boron atom, whose octet is not filled, can accept more electrons from an F- atom. The F- ato...

Nathan Lao 2I

We can usually look at the periodic table to find the ionic charges of elements. Elements in group 1 and 2 have a charge of +1 and +2, respectively, and group 13 has a charge of +3. The turning point is group 14, which has a charge of +4 or -4, and from there on, as you move to the right the ionic c...

Nathan Lao 2I

Formal charge is essentially how many electrons an atom in a molecule "holds." I'm pretty sure it's the amount of electrons that are associated with the atom in terms of its bonding characteristics within the molecule.

Nathan Lao 2I

The way you wrote it is correct in terms of which levels are filled first (electrons fill the 4s orbital before the 3d orbital) but electron configurations are often written in order of increasing energy levels. I think that's just a general rule that we follow. That's why the answer in the book is ...

Nathan Lao 2I

The amount of electrons is the same as the atomic number. If you want to find valence electrons, it would be the number in the ones place for elements in group 1,2, and 13-17. For example, oxygen is in group 16 and it has 6 valence electrons.

Nathan Lao 2I

h is Planck's constant, which is defined as 6.626x10^-34 J s. ħ is equivalent to Planck's constant(h)/2pi. I'm assuming the second formula should have a ≥ instead of an = sign but the formulas should represent the same thing, just in different notations. The textbook also explains this in section 1B...

Nathan Lao 2I

I like to think of effective nuclear charge as " the more protons in an atom, the stronger the pull," where "the pull" is the electrostatic attraction between the nucleus and electrons that Professor Lavelle mentioned. Essentially, it's the positive charge/attraction of the nucle...

Nathan Lao 2I

I think I remember Professor Lavelle saying something along the lines of "someone is more likely to say 'I need 6.36mL of solution' instead of 'I need 0.00636 L of solution." and that's why we usually convert to mL. It's not that big of a deal as long as you convert units correctly though.

Nathan Lao 2I

I think you switched the n1 and n2 values in the equation. The final level(n2) is 6 and the initial level(n1) is 2 for this electron transition. This is because the electron is going from n=2 to n=6. This should help you get a positive value.

Nathan Lao 2I

It's probably easier to use kg instead of grams since Planck's constant is given as 6.626x10^/34 kg m^2 s^-1. Cancelling units is pretty easy from there.

Nathan Lao 2I

The work function that you found is equivalent to the amount of energy needed to eject an electron. That value is in joules per photon, so to solve for how many electrons are ejected, you would just divide the amount of energy of the photon burst (4.61×10−7 J) by the work function(remember the energ...

Nathan Lao 2I

It really does depend on the question but if you solve for an amount of atoms/photons and you're trying to convert it to moles, you would most likely divide by Avogadro's number.

Nathan Lao 2I

It really depends on the question but the first thing I do when I start a problem is to write down/identify what information is already given to me and what the question is asking me to find. What the question is asking for it the key component because it tells you what equation it relates to. For e...

Nathan Lao 2I

Hi! I actually kept messing this part up too. This is how I ended up finding the mass of a single Helium atom. The tricky part of dimensional analysis is the units and cancelling them out accordingly. To solve for the uncertainty in the helium atom's velocity, we plug that into the equation h/(4pi)(...

Nathan Lao 2I

When the energy from an electron is being emitted, the energy is "taken away" or "subtracted" from the electron, which results in the negative sign. On the other hand, that is the same energy as the photon emitted. The values are equal and opposite in order to satisfy the law of ...

Nathan Lao 2I

It seems a little confusing since diatomic means 2 atoms and a particle refers to a single unit. In this case, the entire molecule of oxygen acts as one particle! The unit itself is the diatomic oxygen molecule. So when calculating the molar mass of this particle, you would say it is 2 times the mol...

Nathan Lao 2I

The enthalpy of fusion is essentially the amount of energy, usually in the form of heat, needed to change a substance from solid to liquid(melting). Specific heat is related to the amount of energy needed to raise the temperature of the substance by 1 degree Celsius. Since the problem deals with mel...

Nathan Lao 2I

The steps you took seem correct to me but I think you may have made an error when entering the values into your calculator. I followed the steps you provided and got 1.74x10^-11 m for the wavelength instead.

Nathan Lao 2I

Only the first part counts, not the 10^x part.

Nathan Lao 2I

My AP chem teacher used notes from the National Math and Science Initiative and I still use them for class today. I think you can just search "NMSI chemistry notes" and they should pop up. Crash course is really helpful too.

Nathan Lao 2I

I don't think intensity necessarily refers to frequency since frequency has to do with the wavelength and the energy of the light. In terms of the photoelectric effect, I'm assuming intensity means using more photons of the same wavelength as opposed to a higher/lower energy light to eject the elect...

Nathan Lao 2I

Hi! Essentially the photoelectric experiment involves shining light at a metal surface to eject electrons from the atoms of the metal. The equation used to calculate energy released is E(photon) - (threshold energy/energy needed to eject the electron) = excess kinetic energy. If the energy of the ph...

Nathan Lao 2I

A balanced equation has the same number of atoms of each element on both the reactant side and product side of the equation. So 2Na + 2H2O -> 2NaOH + H2 is the balanced equation. I think the 2 moles + 2 moles -> 2 moles + 1 mole equation is just meant to show the stoichiometric coefficients in the b...

Nathan Lao 2I

One kilogram is 1000 grams, so if you're trying to convert grams to kilograms, divide by 1000. If you're converting kilograms to grams multiply by 1000!

Nathan Lao 2I

A mole is a unit of measurement and a molecule is a group of atoms bonded together. Avogadro's number plays into this since a mole is equivalent to 6.022x10^23 particles(or molecules) of an element or compound.

Nathan Lao 2I

On Dr. Lavelle's website under the Chem 14A Class Website, there is a tab called "Weekly Online Discussion" that outlines how we get our participation points. It says that "Post can be a question, an answer, a comment on a question or answer, general discussion, assisting other studen...

Nathan Lao 2I

I'm not sure if it's absolutely necessary but I think it's a lot easier to find the molecular formula in the end using whole number integers in the empirical formula. From there it's just a matter of comparing the molar mass of the empirical formula to the given molar mass to determine the molecular...

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