CHEMISTRY COMMUNITY

RobertXu_2J

Thank you so much for everything Dr.Lavelle! I had a great time and I’ll miss being in ur class ;-;. I really loved your passion for teaching and I hope you enjoy your break!

RobertXu_2J

Yes, that is correct. Ka for acid HA is equal to [H+][A-]/[HA] and Kb is equal to [OH-][B+]/[BOH]. The a stands for acid, and the b stands for base.

RobertXu_2J

He is using the same grading system, and I think it is safe to assume that he will be using the same scale also, since the midterm scores, the only average he revealed, were actually higher than those for 14A, and 14A didn't have a curve at all (might have been a point or so tho).

RobertXu_2J

Thermodynamics is what I found to be the most tricky and math-intensive, so I think it would be a good idea to study that.

RobertXu_2J

In a zero order reaction, the rate of reaction does not depend on the concentration of the reactants, so the rate of reaction is constant as the concentration decreases. If you want examples and graphs: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Suppleme...

RobertXu_2J

Because it wants to maximize the surface area of the catalyst, so it is able to be involved in more places.

RobertXu_2J

If it didn't give you anything else, you can use the individual formation enthalpy of each molecule (from the back of the book) and use addition and subtraction of the products and reactants to find the net enthalpy.

RobertXu_2J

Honestly, I was pretty fond of Molecular shape and structure back in 14A. Not my best unit, but I had a lot of fun.

RobertXu_2J

n is the number of electrons exchanged in the general reaction. So you look at the oxidation numbers, and then you look at how they have changed.

RobertXu_2J

Concentration cells are basically just galvanic cells, except the anode and cathode are from the same thing, the only difference between the sides is the concentration.

RobertXu_2J

It is because you need a solid METAL. I2 is not a metal, so you need to add a metal, because you need something to conduct the electrons (metals are able to do this because of metallic bonding, if you remember from Chem 14a, the "sea of electrons" )

RobertXu_2J

The electrodes is referring to the solid parts, and changing their masses does not affect the cell potential because solids are not included when calculating Q.

RobertXu_2J

There is no effect because the electrodes are solid.

RobertXu_2J

n refers to the moles of electrons, so you just look at the overall reaction and see how many moles of electrons are transferred, be careful to make sure that you are using the right reaction with the right coefficients for everything!

RobertXu_2J

Yes it does. Take note that ∆G° means that the reaction is spontaneous under STANDARD conditions, hence the "°". However, if it is proceeding under non standard conditions, you would need to use a formula for ∆G in order to find out if it is spontaneous or not.

RobertXu_2J

Gibbs free ENERGY. delta G is literally referring to a change in energy, so it has the same units as those for energy, which is joules, so the answer would be kJ (kilojoules). Additionally, if you use the equation, you will find that it is kJ

RobertXu_2J

Salt bridges balance the charges of both sides, by allowing for the transfer of electrons from one side to the other. If there was no salt bridge, then the build-up of ions (cations at the cathode and anions at the anode) would cease the flow of electrons, but the salt bridge allows for ions to be a...

RobertXu_2J

No, they do not always have to be positive. It is just that if it is negative, the reaction will not occur spontaneously, so usually, we are dealing with positive standard cell potentials.

RobertXu_2J

Yes. From the equation ΔG∘=−nFEcell, you can see that if E.cell is positive, then the gibbs free energy would have to be negative, and if the change in gibbs free energy is negative, the reaction is spontaneous.

RobertXu_2J

If you know the limiting reactant, then you know exactly how much concentration/amount of reactants are involved in the reaction, and you can calculate extrinsic qualities that depend on quantities, for example the amount of heat acquired/released.

RobertXu_2J

pH is always calculated at equilibrium. If it was not taken at equilibrium they would probably explicitly state at what conditions they found that pH at.

RobertXu_2J

You exclude solids and liquids. Even if it is H2O, you include it if it is explicitly stated as aq or gas

RobertXu_2J

Professor Lavelle said the second page of thermo and all of electro (basically everything after the midterm).

RobertXu_2J

Salt solutions dissociate in solutions. A "salt" is basically any ionic molecule with anion and cations.

RobertXu_2J

There is no direct curve on the test, but Lavelle adjusts the grading scheme as needed depending on the performance of students overall on test

RobertXu_2J

Entropy values are given to us.

RobertXu_2J

If you have an equal amount of some molecule on both sides, you can cancel them out, because they don't change through the reaction.

RobertXu_2J

Because internal energy is the sum of the energy of a system, and temperature represents the amount of energy in a system (NOT heat!), so if there is no change in energy (which includes heat and work), then there is no change in temperature (and vice versa).

RobertXu_2J

Because it is sealed, no matter can get out from within, so no mass exchange can occur. However, because it is not insulated, we know that energy exchange can, so it is not an isolated system. Therefore, it is a closed system, that can exchange energy, but not matter.

RobertXu_2J

Cv is for constant volume, Cp is for constant pressure. For MONOATOMIC atoms, Cv is 3/2R and Cp is 5/2R, but for diatomic ones, Cv is 5/2R and Cp is 7/2R

RobertXu_2J

I found this previous post, https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=1396#p220853 , that says that heat capacity is an intensive property, however on the Wikipedia page for heat capacity, https://en.wikipedia.org/wiki/Heat_capacity , it says heat capacity is an extensive property. I'm so ...

RobertXu_2J

They represent the same concept.

RobertXu_2J

Yes, constant external pressure.

RobertXu_2J

If it says it is insulated, we would assume that it is isolated. I don't think the wording of questions will be ambiguous. If it looks like the question is trying to imply that the system does not exchange energy (e.g. heat) with the environment, then it's probably trying to tell you that it is an i...

RobertXu_2J

Closed systems can exchange energy with the environment, but isolated systems cannot. So, like heat for example.

RobertXu_2J

Closed systems cannot exchange mass but they can exchange energy. Think of it as a metal can that's closed. The material inside cannot get out, but if you heat a flame under it, the matter inside will get hotter, thus energy is being exchanged. But if the system is isolated, it cannot exchange energ...

RobertXu_2J

Steam has more energy than water, so the difference between the energy in steam and the energy of your skin is greater, and as such, when it comes into contact with you, it will transfer a correspondingly greater amount, leading to a more severe burn.

RobertXu_2J

words like "insulated" or "no exchange" or "closed to the environment" or any other words that indicate the system does not exchange matter or energy would be strong indications that the system is isolated, whereas words that indicate the system is able to exchange ener...

RobertXu_2J

If you got a negative number, you did something wrong.

RobertXu_2J

Phase changes are literally just changes from different states of matter (solid, liquid, etc.). Temperature does not change, because the energy that is being added to the material is all going toward changing the phase (e.g from solid to liquid, the molecules are excited and they move faster so the ...

RobertXu_2J

Steam does, because the difference in the energy of steam and your skin is greater than the difference between your skin and water, which being a liquid, does not have as much energy at the same temperature as steam does.

RobertXu_2J

It will proceed in the other direction. For example, if products are added, the reaction will proceed toward the reactants, whereas if more reactants are added, the reaction will proceed toward the products.

RobertXu_2J

Hess's law states that the total enthalpy change during the complete course of a chemical reaction is the sum of its parts. Basically, it's stating that enthalpy can be added.

RobertXu_2J

Hess's law states that the total enthalpy change during the complete course of a chemical reaction is the sum of its parts. Basically, it's stating that enthalpy can be added.

RobertXu_2J

Because changes to their concentration are inconsequential for the rate of reaction. Solids just sit there, they don't really have a concentration because they don't mix with the solvent. Liquids are the solvent, so you assume that there will always be enough of it that any decrease will be insignif...

RobertXu_2J

When using approximations for ICE tables, if you find that X is less than 5% of the initial concentration, then the approximation is valid.

RobertXu_2J

A change in temperature changes the direction of the reaction. For example, if a reaction is endothermic, and the temperature is increased, then K is pushed to the right. If the reaction is exothermic and the temperature is increased, then K is pushed to the left.

RobertXu_2J

Find Kp, because the pressure that a gas exerts is used to represent its concentration.

RobertXu_2J

Use it when there is not a complete reaction in one direction (there is a forward and reveres reaction happening at the same time). While most reactions are reversible (depending on the conditions they take place in), ones where we don't use the equilibrium sign usually occur under extreme conditions.

RobertXu_2J

I do not think a perfectly ideal gas exists, but because the difference is negligible at higher temperatures, we just assume that they are so, to make calculations easier.

RobertXu_2J

Yes you would, because the calculations for Q are the same as for K, except that the values used may be different.

RobertXu_2J

No they do not, since multiplication exhibits commutative properties (a*b = b*a).

RobertXu_2J

I believe the importance of homogeneous and heterogeneous equilibria is that for homogeneous equilibria, you do not have to worry about which of the materials are needed for the equilibria equation, whereas for heterogeneous you have to realize that solids and liquids are excluded.

RobertXu_2J

We are manipulating the equation so that we can calculate the value that we want. For example, if we wanted concentration, which is n/V, we would move it to one side and everything else on the other side.

RobertXu_2J

What are the strong bases that we will need to know for the final?

RobertXu_2J

As long as it's a base + acid = salt + H2O, then it is neutralization

RobertXu_2J

Amphiprotic means it can donate or accept a proton, and amphoteric means it can act as a base or acid. All amphiprotic molecules are amphoteric, but not the other way around. An example of a amphoteric molecule that is not amphiprotic would be Al2O3. It does not contain any H's, but it can act as bo...

RobertXu_2J

Yes, one of the ta's yesterday said that we could use the abbreviations when naming them

RobertXu_2J

Yes, but topics covered by the midterm will also be covered.

RobertXu_2J

Lewis acid means that this specific molecule will accept electrons, and a lewis base means that this molecule will donate electrons.

RobertXu_2J

Resonance structure refers to the bonds in that there are different bond combinations that are possible as lewis structures, BUT none of them are correct. Because electrons move around and are not actually held in straight line bonds, structures intermediate to the ones shown as lewis structures are...

RobertXu_2J

Cis means that the molecule is symmetrical, with the same elements are on the same side. Trans means opposite, the elements are on different sides.

RobertXu_2J

Basicallly, know that cis platinum is used as a chemotherapy drug to stop mitosis. It does this by replacing its two chlorines with two hydrogens, and then linking to two guanines. Because it is linked at two areas, it stops the two strands of dna from separating and keeps mitosis from happening, wh...

RobertXu_2J

The "clean"-ness of a coal refers to how high the carbon content is compared to sulfur content. If the coal has a really high carbon content and very low sulfur content, than it is "clean", whereas if it has a high sulfur content, then it is dirty coal

RobertXu_2J

I think the best way is to look at the formula first. If it is definitely polar and does not have symmetry, you can probably assume that there are dipole forces. But of course, drawing a lewis structure would be safer. So draw the structure and see if there is symmetry when you are not sure.

RobertXu_2J

I think there is a high chance that it will show up on the final, so you should definitely review any biological examples that come up in slides or learning material.

RobertXu_2J

Hybridization has to do with molecule shape. You can look at the shape of the molecule to derive the hybridization of the orbitals.

RobertXu_2J

The overlap of a pi bond is less than that of a sigma bond. However the sigma bond only overlaps at one area, whereas the pi bond overlaps at two. So because the pi bond overlaps at two areas, it cannot turn and thus keep the molecule in position.

RobertXu_2J

dipole-dipole forces are when two polar molecules become attracted to each other. An induced dipole is when something is not normally polar, but becomes polar for a small instance because of the attraction of another molecule with a dipole

RobertXu_2J

Hydrogen bonds can only form between hydrogen and one of these three atoms.

RobertXu_2J

all bonds have the same "push", because the space they occupy is between atoms. Lone pairs cause more push because they can move freely on the outskirts of an atom, and thus it has a wider area of effect and a stronger push.

RobertXu_2J

London forces exist between all types of bonds and is universally present between all atoms. It is considered universal because no matter what types of molecules, london forces always exists between them.

RobertXu_2J

Lavelle usually only makes us memorize the ones he teaches in class, so I think knowing only the ones from class will be sufficient for our next test. However, to be safe I would memorize all of them just in case.

RobertXu_2J

You look at the bonds and symmetry. If the bonds are covalent with no dipole moments, then it is non-polar. If they are polar bonds but there is symmetry it is also non-polar. Only if both of these are false then can it be polar.

RobertXu_2J

Why does the shape of a molecule contribute to how strong the bond is between it and other molecules?

RobertXu_2J

Basically, higher melting point means that intermolecular forces are stronger, so molecules aren't as easily separated. Hydrogen bonds is a very strong attraction between different molecules, and because of hydrogen bonds, certain molecules have higher melting points than you would expect otherwise.

RobertXu_2J

London dispersion forces occur in all molecules. It refers to how, when electrons are moving randomly around the nucleus, there will always be an ebb and flow in how negatively/positively charged a particular region of that atom is. These slight imbalances that occur attract each other, so all atoms...

RobertXu_2J

If the difference in electronegativity is greater than two, then it is ionic. If it is not ionic, look at the difference in charge between the atoms to determine if it is polar.

RobertXu_2J

Adding on to what everyone else mentioned, hydrogen atoms are small in size, and because distance is inversely proportional to the strength of a bond, this contributes to the strength of the hydrogen bond.

RobertXu_2J

Electromagnetic radiation travels at the speed of light, therefore we use the speed of light to calculate it. Light is a form of electromagnetic radiation, and the speed of light is really the speed of all these EM waves.

RobertXu_2J

An induced dipole is when an atom that is not usually able to form a dipole becomes a dipole because of interactions with neighboring atoms.

RobertXu_2J

If they form ions, then yes the electron "leaves". However if it's a covalent bond, then no, the electrons don't leave their home atoms, instead it creates kind of a "shared" orbital between the two atoms.

RobertXu_2J

According to a TA, they will most likely not give us just the name and ask us to find the formula from that. They said that even if they did, it would be an easy formula.

RobertXu_2J

They are different ways of writing it, but Lavelle specifically told us in class to write it [Ar] 3d2, 4s2. Whether or not he takes off points if you write it the other way I don't know, but definitely write it this way.

RobertXu_2J

Ionic bonds are much stronger than covalent bonds. This is because ionic bonds involve the complete transfer of electrons so the electrostatic forces are stronger.

RobertXu_2J

The mass of an electron is constant. While the orbit they take can change between elements and orbital levels, electrons are still electrons and their mass remains constant.

RobertXu_2J

Two things to pay attention to are formal charge and octets. Most elements want to have a full shell of valence electrons, so try to make sure that they all have 8 electrons around them. Next is to pay attention to the formal charge. If by forming a double bond, the formal charge is closer to 0, the...

RobertXu_2J

Although they are both correct, you should use the way Dr.Lavelle wants us to do it. Using 3d and 4s as an example, he wants us to do it this way to emphasize that when the 3d orbital is used, 3d actually has a lower energy level than 4s, even though the 4s is filled first.

RobertXu_2J

Hydrogen is never used as the central atom.

RobertXu_2J

Why does Dr.Lavelle want us to put 3d before 4s when we are writing the electron configurations?

RobertXu_2J

Do all the homework questions and make sure you understand the concepts that were mentioned and the relevant equations.

RobertXu_2J

The units for hertz is cycles per second, but the cycles can refer to a number of different cycles so it's -s.

RobertXu_2J

Strictly speaking, 3d is higher in energy than 4s only if there are no electrons in 3d. When the 3d starts filling up with electrons, the relative energies fluctuate and 4s ends up with a higher energy level as 3d becomes more filled.

RobertXu_2J

The way I do it is I convert it to meters first before finishing the calculations. Using your example, lets assume that we have 100 nm. Then 100 nm x (1 m/10^9 nm) x (10^12 pm/1 m) = 100,000 pm. Although this may take longer than converting directly, it makes sure that your answer is correct.

RobertXu_2J

I think as long as your thought process is clear and they can tell what you were trying to do through your work, you should be good.

RobertXu_2J

This is just a fundamental topic that has more to do with labs. Precision does have a connection to significant figures though. Specifically, the more sig figs your data has, the more precise your answer.

RobertXu_2J

Yes and yes. If the energy is not in those specific quanta amounts, it will not absorb it.

RobertXu_2J

theoretical yield is the yield that will be obtained if all of the given reactants are used to completion and everything goes perfectly. However during real experiments, more often than not, things happen; the product sticks to the beaker, escapes into the atmosphere or something, and the yield obta...

RobertXu_2J

Color of light is decided by the wavelength of the light. If the electron loses energy and releases that energy as a photon, the wavelength of the photon decides the color of the light i think.

RobertXu_2J

There is no difference between calculating theoretical yield with grams versus theoretical yield with moles. The only difference would be what units your answer is in. Something to always be careful about is what units the question wants your answer to be in. If you find that your units are differen...

Search found 104 matches