CHEMISTRY COMMUNITY

Serena Song 1A

Thanks for the reminder!! I'm from Arizona and we don't have Daylight savings so I always forget.

Serena Song 1A

Nothing's been updated as far as I know! It seems to include all the recent equations we learned (rate laws, half life, arrhenius), so I think it'll be the same one as midterm 2. Plus in chem 14a, we only had 1 equation sheet throughout.

Serena Song 1A

I feel like burnout has definitely hit me hard these past few weeks. I've been finding it hard to actually take the time and learn the material instead of just taking notes head empty. A method that works for me is to write out a list of tasks on a sticky sticky note and then crossing them off as I ...

Serena Song 1A · Re: chem community points We should have 50 posts by Sunday 11:59pm I believe.

We should have 50 posts by Sunday 11:59pm I believe.

Serena Song 1A

I try do do as many practice problems (both homework and textbook problems) as I can and get a good night of sleep beforehand. For some reason, I find eating a piece being a test helpful? It helps me calm down.

Serena Song 1A

Activation energy and enthalpy aren't interchangeable. I believe later on in the lectures, we'll see how the two are used together though.

Serena Song 1A

They will usually explicitly state if the solution is acidic or basic in the problem luckily! At least this is what I've noticed based on the practice problems.

Serena Song 1A

Yup! A 1st order reaction can have more than 1 reactant as long as only 1 of the reactants affects the rate of reaction (1st order). For example, if rate = k [A]^n [B]^m where n=1 and m=0 and A and B are reactants, then it's considered a first order reaction!

Serena Song 1A

You could also use t_1/2 = [A]₀/2k!

Serena Song 1A

Having more than 3 reactants can definitely happen, but we most likely won't have to worry about for this class!

Serena Song 1A

Thank you so much for the part 2! Reading this is really inspiring and motivating, especially right now during week 8. Wishing you the best of luck!!

Serena Song 1A

Yes, Le Chatlier's principle still applies for redox reactions. Because E = E0 - RT/nF lnQ where Q is the reaction quotient (concentration of products/reactants), changing the concentration will affect voltage potential as well.

Serena Song 1A

Yup, it does! We probably won't be tested on sig figs though which is nice

Serena Song 1A

yes, E's value only changes when the reaction is flipped, in which case you multiply by -1. Otherwise, E remains constant.

Serena Song 1A

Yup! A big part of balancing redox reactions is making sure the number of electrons lost in the oxidation half reaction equals the number of electrons gained in the reduction half reaction.

Serena Song 1A

Only gases apply for Le Chatelier. For example, if pressure increased, only the concentration of gases would be affected since they can be compressed unlike solids, liquids, and gases.

Serena Song 1A

E changes based on the concentration of the products and reactants since E = E0 - (RT/nF)ln(Q) where Q is the reaction quotient.

Serena Song 1A

I think the anode is usually on the left and the cathode is on the right, but it isn't a strict rule. It makes the diagram simpler since it's more consistent. If you reversed everything else as well (ex. flow of electrons), the anode can be on the right and the cathode on the left.

Serena Song 1A

I don't think he's covered it just yet, but he probably will either Monday or Wednesday. The organic chemistry tutor on youtube does a great job explaining it though, if you want to get ahead! https://www.youtube.com/watch?v=fdbrhQAM9Gw

Serena Song 1A

Hi! For MnO4-, the overall oxidation number is -1 and the oxidation number of O is always -2 (except for in peroxide). Knowing that, we can calculate the oxidation number of Mn. Mn + 4(-2) = -1. The oxidation number of Mn in MnO4- is +7. On the other hand, the oxidation number of Mn2+ is 2+ since th...

Serena Song 1A

Both will help solve for the same delta G value! It mainly depends on what they give you to work with. Since they solve for the same thing, you can also set them equal to one another

Serena Song 1A

I'm only taking BL next quarter, but I know people who are taking both BL and C this quarter. It seems pretty manageable based on what I've heard!

Serena Song 1A

Thank you so much! I was having a hard time memorizing all of them, this really helps!

Serena Song 1A

As others have said, it helps to keep track of the units and make sure they all cancel out. .00826 is used when pressure is in terms of atm and volume is in terms of L. 8.314 is used when pressure is in terms of Pa and volume is in terms of m^3. 8.314 J/mol K is the SI unit form of R and is generall...

Serena Song 1A

During phase changes, the temperature stays constant, so we use deltaS = q/T. When temperature is changing we use deltaS = n*C*ln(T2/T1).

Serena Song 1A

Doing the Chem 14A problems for the textbook definitely help! For me at at least, paying attention to formal charge really helps with making sure the structure is correct, but the more you practice and draw, the more intuitive it gets!

Serena Song 1A

Heat capacities can't be negative because thermal energy/heat and temperature have a direct relationship. For temperature to increase, the energy must be added to the system.

Serena Song 1A

If work is done on the system it's positive, for example compressing a container. If work is done by the system it's negative, for example a gas pushing on a container, leading to the container's volume increasing.

Serena Song 1A

Steam has more (thermal) energy compared to boiling water. A lot of energy is required to break/weaken the hydrogen bonds in water so that it can transform into vapor. In addition, once water has transformed to steam/vapor, the vapor can absorb even more energy. Steam releases a lot more energy than...

Serena Song 1A

Water molecules have hydrogen bonds between then. This keeps them relatively close together due to the attraction between molecules. When water is boiled, energy from the heat allows these hydrogen bonds to break/weaken. As a result, the vapor molecules can be much farther from one another.

Serena Song 1A

Kelvin = (Celcius + 273). Say that an object is originally x1 degrees, but is now x2 degrees. If we were to calculate the change in temp, it would equal x2-x1. If we were to convert these temperatures to kelvin, the initial temp = x1 + 273 and the final temp = x2 +273. Delta T = (x1 + 273) - (x2 + 2...

Serena Song 1A

I believe that kJ/mol is the correct unit standard reaction enthalpy since more moles of a molecule would either release more energy (for most) or require more energy.

Serena Song 1A

I would try using the answer button, which equals the value of your previous answer, if your calculator has one. This way you might face less rounding errors from your calculator since it should store the exact value. Hope you're able to resolve this issue soon!

Serena Song 1A

Scratch paper was allowed last quarter, and it's probably the same this quarter. You just have to show your TA that there's nothing written on it before hand.

Serena Song 1A

Yup as others have said, just download it and you should be good to go! My TA had us take a mini quiz with it to make sure it works. Once the midterm is available on CCLE, there should be a launch lockdown browser button in blue which will activate it and show the exam!

Serena Song 1A

I can't think of a scenario where we would use total pressure as is, but as others have said, there could be a problem where we would need to find a partial pressure given the total pressure.

Serena Song 1A

Yup! If there's no note saying that A, B, or C shouldn't be factored into the equation, then it should be safe to assume they're either gases or aqueous solutions!

Serena Song 1A

I prefer putting the concentrations into the ice box just so that I don't forget to it later on.

Serena Song 1A

Hi! The value we use for R generally depends on the units! .08206 (L atm)/(mol K) and 8.314 J/(mol K) are probably the ones we'll use most tho. Just make sure that the units on the right and left are the same, and you'll be good!

Serena Song 1A

The concentration for solids and liquids are fixed, unlike gases or aqueous solutions. Since the initial and final concentrations for solids and liquids are about the same, they would cancel out anyways in the K equation, so we omit them altogether.

Serena Song 1A

Try plugging both x values back into the concentration equations from the ICE table and see if they make sense. Usually one of the values will produce a negative concentration value which is impossible so we go with the other one. For example, if the concentration of a compound once it reaches equil...

Serena Song 1A

Pressure only applies to gases, especially in the context of volume changing. The volume of aqueous solutions can't change, but the volume of gases can.

Serena Song 1A

1 molecule of Ba(OH)2 has one atom on Barium, meaning that the ratio of their stoichiometric coefficients is 1:1. Because of this, the concentration of Ba2+ and Ba(OH)2 should be equal.

Serena Song 1A

A larger Ka value means a stronger acid. This is because Ka is calculated by dividing the concentration of products by the concentration of reactants generally speaking. A acid's strength is determined by how much of it dissociates in water, so a stronger acid should produce more products, increasin...

Serena Song 1A

I agree. For this class, we can assume the dilution solvent to be water (unless stated otherwise).

Serena Song 1A

305538876 wrote:Re: Sapling Homework

Try emailing the ucla store (textonline@asucla.ucla.edu) and ask them to send you the code. Last quarter my sapling code never arrived physically, but I was able to get the code by emailing them!

Serena Song 1A

For heterogeneous reactions it's important to exclude any solids or liquids in the reaction when calculating for K since their concentration remains constant. For homogeneous reactions you use all the reactants and products when calculating for K.

Serena Song 1A

Yup the only homework problems we have to submit are the ones from Sapling, although we also have the 5 discussion posts per week requirement. We get unlimited tries which is really nice. The problems listed in the syllabus are completely optional, but they're quite helpful when trying to master the...

Serena Song 1A

For Ideal Gas Law, we should use Kelvin. Since pressure and volume can't be negative, the value plugged in for temperature must also never be negative so that the two sides of the equation can be equal. Because Kelvin can't go below zero, we use it as opposed to Fahrenheit or Celsius.

Serena Song 1A

I agree that it most likely means much less than. For example going back to VSPER, trigonal pyramid has a bond angle x < 109.5, while bent (2BP,2LP) had a bond angle x << 109.5 since it usually had a smaller bond angle than molecules with the trigonal planar shape.

Serena Song 1A

It's probably best to not round in this case just to make sure the answer is accurate. They'll probably make it clear which way they want us to round (up or down) on the final if they really do want us to follow sigfigs.

Serena Song 1A

I would first look at intermolecular forces. If they're of the same type, then examine the bond length.

Serena Song 1A

Yup, he probably meant tetrahedral since octahedral has 6 bonds

Serena Song 1A

I personally prefer using OH2 since it makes clear that oxygen is the element that's bonded, but either one is valid!

Serena Song 1A

Thank you so much! I really appreciate it!

Serena Song 1A

Yup, I think it's a good idea to memorize the shapes and the corresponding number of bonded pairs and lone pairs they each have. It's probably also helpful to memorize their corresponding bond angles.

Serena Song 1A

Yup! Nonpolar molecules can have polar bonds. As long as the differences in electronegativity cancels out, it's considered nonpolar. This is common in molecules that have the VSPER shape of linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, and square planer when the surrounding...

Serena Song 1A

I think we'll have to know the bond angles for molecules with the VSPER shape of linear, trigonal planar, tetrahedral, trigonal bipyrimidal, square planar, and octahedral. However for molecules with other shapes, we probably aren't expected to know the exact bond angle. For example, for the water mo...

Serena Song 1A

Yup, flashcards are definitely helpful, especially since the naming rules require hard memorization at first. Doing multiple practice problems from the book and online is really useful too.

Serena Song 1A

Thanks so much! I really needed some practice

Serena Song 1A

Yup! Zeroes at the end of a digit (trailing zeros) are only significant if there is a decimal point present.

Serena Song 1A

As others have said quizlet/flashcards are pretty helpful when trying to memorize the prefixes. Unit conversion problems help out a lot too. Good luck!

Serena Song 1A

Just to add on, although both transport oxygen, hemoglobin can bind to more oxygen molecules at a time. I believe hemoglobin can bind to 4 oxygen molecules while myoglobin can only bind to 1 oxygen molecule at a time.

Serena Song 1A

There's definitely a possibility it could show up on the final since there's been so many examples given in class and in sapling. Hopefully they'll given us a clue on the test if they do want us to draw a ring structure.

Serena Song 1A

I like using this one! Hope it helps!
https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/752/2016/09/26194726/vsepr-geometries.png

Serena Song 1A

I agree! He'll most likely go over it in depth during next week's lectures. He's just finishing up the basics for now.

Serena Song 1A

Since H can only form one single bond, it's never the central atom. As a rule of thumb, the element with the lowest ionization energy is often the central atom.

Serena Song 1A

As said above, the two fluorines in COF2 cancel each other out somewhat. Oxygen is very electronegative as well, so overall the polarity in COF2 isn't as strong (relatively speaking). In comparison, the hydrogen in COHF is much less electronegative than the oxygen or fluorine, leading to the dipoles...

Serena Song 1A

I like using the FC = # of VE - # of dots - # of lines equation as well! I find it to the the fastest method once the lewis structure is drawn out.

Serena Song 1A

Yup! Resonance structures have sigma and pi bonds. I believe the pi bonds in resonance structures are specifically called delocalized pi bonds.

Serena Song 1A

For VSPER, the molecular shape is dependent on the number of lone pairs and bonded pairs surrounding the central atom, more so than whether the bonds are single bonds or double bonds. I believe this is because the type of bond affects the bond length and strength but not the amount of repulsion it f...

Serena Song 1A

A trick I like to use is to look at the Lewis structure and count the number of lines and dots surrounding an atom. Since each line represents a bonding pair of electrons, we can count the number of shared electrons/2 by counting the number of lines surrounding the element. Similarly, since each dot...

Serena Song 1A

I don't think we're required to, but it is useful to have the most common ones memorized. Here's a list of the most common polyatomic ions!
https://ch301.cm.utexas.edu/help/ch301/polyatomics.pdf

Serena Song 1A

Dipole - dipole force happens between two polar molecules, making it an intermolecular force. Polar covalent bonds form between 2 atoms with different electronegativities , making it an intramolecular force. A molecule that is considered polar can experience dipole dipole with another polar molecule.

Serena Song 1A

If two structures have the same overall formal charge, then examine the location of the elements' individual formal charge. Theoretically, elements with higher electronegativity should have a negative formal charge since they're drawing in more electrons, so it's most likely the structure that follo...

Serena Song 1A

There aren't any exceptions as far as I know. I believe molecules/salts with larger molar mass and larger surface areas have bigger London Dispersion forces.

Serena Song 1A

Formal Charge = # of valence electrons - (shared electrons/2 + lone pairs). A short cut to this if you have the lewis structure drawn out is FC = VE - (number of lines + number of dots) since the lines represent the bonded electrons and the dots represent the lone electrons.

Serena Song 1A

The formal charge of an element can vary, even within the same molecule. A great example of this is resonance structures, such as nitrite (NO2^-). In the lewis structure of nitrite, one of the oxygen has 3 lone pairs and a single bond with nitrogen, giving it a formal charge of -1. The other oxygen ...

Serena Song 1A

Lewis acids accepts electron pairs. They're more likely to have a positive charge. The central atom in Lewis acids tend to have no lone pairs. Lewis bases donate electron pairs. They tend to have a negative charge. The central atom in Lewis bases tend to have lone pairs that are donated to Lewis aci...

Serena Song 1A

The favored resonance structure should have the lowest formal charge. If two resonance structures have the same formal charge, choose the structure where the negative formal charge is on the more electronegative element.

Serena Song 1A

To add on, for polyatomic ions, the sum of oxidation numbers should equal the charge of the ion. For example, in sulfate (SO4^2-), we know that the sum of the oxidation numbers should equal -2 and that the oxidation number for O is -2. We can get the equation X +4(-2) = -2 where X equals the oxidati...

Serena Song 1A

Within the context of this class, covalent bonds are generally stronger than ionic bonds, but it can depend on the environment. For example, within a solution, ionic bonds are weaker than covalent bonds. Since many of the chemical reactions in biology involve solutions, we consider covalent bonds to...

Serena Song 1A

As you go across a period, atomic radius decreases since effective nuclear charge increases. The increased attraction between the protons and electrons leads to the electrons being drawn in closer to the nucleus, decreasing atomic radius. As you go down a group, atomic radius increases due to the in...

Serena Song 1A

Atomic radius decreases as you go across a period due to increasing nuclear charge. The stronger nuclear charge leads to an increased attraction between the positively charged protons in the nucleus and negatively charged electrons surrounding it. This leads to the electrons, including the valence e...

Serena Song 1A

I believe oxygen has a lower ionization energy compared to nitrogen due to increased electron electron repulsion within the orbital. For nitrogen, there's 1 electron in the 3 orbitals of the 2 p subshell. However, in oxygen there's suddenly 2 electrons in one of the orbitals, which increases electro...

Serena Song 1A

Each subshell has 1 or more or orbitals. Within an orbital, there's a high probability that an electron can be found. Also depending on the type of subshell, the number and shape of orbitals changes. (s subshells have 1 orbital, p subshells have 3 orbitals, d subshells have 5 orbitals, and f subshel...

Serena Song 1A

To be on the safe side, it's probably best to memorize the more common conversions. One trick is to memorize the prefixes! (ex. kilo detonates 1000x, so 1 km = 1000m and 1 kg = 1000g). There's also angstrom, which represents 10^-10.

Serena Song 1A

Not 100% sure, but even if there isn't a question dedicated to it, I imagine that we could get points docked off if our answers don't have the correct number of sig figs.

Serena Song 1A

Yup, since the limiting reactant will run out first, it determines how much product will be created. It's possible that less product will form than expected in an actual experiment though due to factors such as the reactants sticking to the side of the tubes and not properly reacting.

Serena Song 1A

I find the questions from the textbook and the step-up sessions very useful! I would also recommend writing down all the important formulas on a piece of paper and the memorizing and understanding the formulas. I think we will be given a formula sheet (maybe?), but flipping back and forth between pa...

Serena Song 1A

This looks super helpful! Thank you so much!! (Side note but your handwriting is so nice ;-;)

Serena Song 1A

I think based off the equation E=-(hR)/n^2, E and n have an inverse relationship (As n increases, E will decrease. As n decreases, E will increase). So as n gets larger and larger and eventually approaches infinity, E will get smaller and smaller to the point where it is so minuscule, that it's appr...

Serena Song 1A

Yup! The number of photons increases when the brightness increases, but the wavelength/frequency of each photon remains the same. If the wavelength/frequency changed, then that could've changed what type of EM wave it was instead (ex. from visible light to UV light if the frequency increased/wavelen...

Serena Song 1A

Yes, all multiple choice. According to my TA, they're going to try to give partial credit, but I'm not exactly sure how.

Serena Song 1A

I think it could depend on your TA. Mine is allowing us to use our phone or tablet as an external camera.

Serena Song 1A

I placed my order around the end of week 0, and I still haven't received it in the mail. According to the store, my order has been shipped though, so I hope it'll arrive soon. I'll probably email them soon for a code.

Serena Song 1A

I would say it's best to check because better safe than sorry! I've had so many instances where I forgot to check in the beginning and had to redo all the calculations. It ends up saving lots of time in the long run!

Serena Song 1A

Yup, mol is just a shorthand for mole! Unfortunately mol almost looks like an abbreviation for molecule (which I've seen abbreviated as mlcl before).

Serena Song 1A

It probably wouldn't make a significant difference in most cases, but if you want to be on the safe side use 15.999.

Serena Song 1A

Thank you so much for your advice! I'll try to take more notes during lecture from now on!

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