CHEMISTRY COMMUNITY

JiangJC Dis2K

I would assume you use the energy of products minus energy of reactants because delta H is a state property not a kinetic property, so therefore the activation energy is not concerned in that aspect.

JiangJC Dis2K

In a review session, the UA said that Lavelle will most likely have 1-2 Kinetics problems, 2 Thermochem/Dynamics, 1 Electrochem. with 8 questions total. Basically everything is fair game for the final with probably one or two questions from each outline.

JiangJC Dis2K

Oxidation and reduction are opposites. So the species that has a higher tendency to oxidize has a higher reducing power. For example if the reduction potential of species 1 was .34v and species 2 was .78v, species 1 is more likely to be oxidized and therefore has a higher reducing power.

JiangJC Dis2K

Yes. For zero order, the rate constant units are mol/Lsec., First order=1/s, 2nd order=L/molsec.

JiangJC Dis2K

The graph of ln[A] vs. time is linear for first order reactions, but the graph of [A] vs time is exponential decay, because e is applied to undo the ln. That is the difference between the two graphs but both of them are for first order reactions, the only difference is the two variables.

JiangJC Dis2K

The rate constant is basically the derivative of the concentration, or d[R]/dt=k. Note that the rate constant is found experimentally and is temperature and reaction specific.

JiangJC Dis2K

In some book examples Pt is included in the cell diagram. How do you know when to include it and when not to, and also which side it appears on? What specific wording in a question would suggest that?

JiangJC Dis2K

In Karen's worksheet session on Sunday she clarified this a little. Solid metals are always written towards the outer and oxidation forms are written closest to the middle. Therefore, the product of the oxidation half reaction would be to the right of the initial form.

JiangJC Dis2K

n is the number of electrons involved in the equation. For example, if given a balanced redox reaction, you would have to reseperate the half reactions to find the number of electrons that are being cancelled out.

JiangJC Dis2K

The e- transfer from one reaction to another will stop due to charge build up and the reaction will stop. The salt bridge serves to maintain neutrality. Adding a salt bridge or porous disk allows the ion transfer to stay neutral.

JiangJC Dis2K

Lavelle did not completely explain this but I'm guessing it has to do with the relative stability of different oxidation states. For example, in the class example the equation required Zn==>Zn+2 + 2e-, but the value was only given positive in the reverse. This shows that the state Zn+2 is probably m...

JiangJC Dis2K

Basically you're going to want to identify which element is reduced and which is oxidized and then write out separate equations. Therefore, you would write one equation for the reduction and another equation for the oxidation. By balancing both with the electrons cancelling out, you therefore have a...

JiangJC Dis2K

For a system at equillibrium, the total delta s equals zero. For n irreversible expansion, given that there is no effect on the surroundings, delta s system equals delta s total.

JiangJC Dis2K

Delta U equals delta H + w when pressure is constant. At constant pressure q=delta H. Another helpful equation in this scenario is delta u=delta h-delta n RT. This can be used when you have gas values other thank P and V.

JiangJC Dis2K

You can assume the work of the system is zero when there is no change in volume, given that w=-pdeltav. Therefore w would equal zero since deltav equals zero.

JiangJC Dis2K

Could someone please explain number 6 on worksheet week 4? Thanks

JiangJC Dis2K

in our lecture he did not do this example, but he gave the example ch4 and ch3cl (similar). for ch4 the degeneracy is only 2^1 if there are two possible partitions NOT 2^5. i think this is because ch4 is acting as a single unit. so in the example ch3cl, the degeneracy is 2^2 for two partitions. if a...

JiangJC Dis2K

i feel like as a general, the only one important to know is torr since that appears most frequently and some constants on the equations sheet are given also in torr. 760 torr=1atm.

JiangJC Dis2K

pressure and volume are inversely related. so at a constant volume, pressure is also constant. think about if you increase the pressure of a bike tire, the tire is also expanding and therefore there is a change in v, delta v. if delta v=0 then w=-pdeltav equals zero because the change in volume is z...

JiangJC Dis2K

if q for the system is positive, then it absorbs heat. if q is negative, it releases heat and heat is therefore lost to the surroundings. if w is positive for the system, work is done on the system by the surroundings. if w is negative, work is done by the system on the surroundings. hope this helps!

JiangJC Dis2K

I am assuming Lyndon will make an announcement sometime this week cause typically the practice midterms are lined up with a certain review session, so must be sometime this week :)

JiangJC Dis2K

The slanted lines show and include all temperatures where only one state exists (ex. solid, liquid, gas). While the straight lines show melting/vaporization. Therefore, at that temperature, both phases that it is in between are present. Think about when ice is melting. It does not immediately go fro...

JiangJC Dis2K

Lavelle gave a great example in class. If you are pumping a tire, you are putting work into the system (given that the tire/pump is the system). Therefore the work for the surroundings is negative because you are losing energy. But the work for the system is positive because it is having the work do...

JiangJC Dis2K

Work=-pressure*change in volume. The units for that are kgm-1s-1m3. The meters cancel out through exponent rules to create the unit kgm2s-2. This equals a joule. Therefore, your answer should be in joules, unless all other units in the equation are multiplied by 1000. But you could convert the final...

JiangJC Dis2K

I know that adding an inert gas to a chemical reaction at equilibrium does not cause a shift, but which gases are inert? Is there a row/period on the periodic table?

JiangJC Dis2K

I was in Lavelle's class last quarter for 14A. Yes, the tests are during discussion section. The format is open ended questions with a possible mix of a few multiple choice questions thrown in there. It's usually around 8-10 questions that have multiple parts. For 14A at least, the tests were worth ...

JiangJC Dis2K

I'm unsure about comparing reactions as a whole. But in terms of reactants and products, if the equilibrium constant k<0 then that suggests that the reactants are more stable than the products. If the equilibrium constant k>0, then the products are more stable than the reactants.

JiangJC Dis2K

You are correct that the speed of the reaction does not effect the amount of product produced. Making more product includes LeChatlier's principle which essentially says that the reaction will adjust to a change in order to reach equilibrium again. This includes pressure/volume, temperature, and con...

JiangJC Dis2K

If you are given the percentage protonation, is that the change value or equilibrium value when making an ICE table? (Given that you multiply the initial concentration by the percentage).

JiangJC Dis2K

If a reaction is endothermic, then it requires heat to complete the reaction, because the products have higher energy than the reactants. Therefore, if temperature is raised (more heat inputted) then equilibrium will favor the products. On the opposite, if a reaction is exothermic, meaning that the ...

JiangJC Dis2K

If you are calculating ka from given concentrations, so the ka is unknown, you employ the 5% rule. You would first calculate the percent ionization by dividing the equilibrium concentration by the initial concentration of one of the products times 100 in order to determine the change in x. If this a...

JiangJC Dis2K

The conjugate seesaw is based upon the statements "the stronger the acid the weaker its conjugate base" and "the stronger the base the weaker its conjugate acid" and inversely. Quantitatively, this can be explained by the equation ka x kb= kw, and kw is a constant (10^-14). There...

JiangJC Dis2K

No. You earn three points per week for chemistry community posts (1 point per posts for up to 3 points a week) and then 7 points a week for homework problems (1 point per problem).

JiangJC Dis2K

Lying or sitting to the right basically means that at equilibrium, there are more products than reactants based upon the equilibrium constant k being greater than 10^3. Shifted to the right implies a change to the reaction or conditions, shifting the reaction away from equilibrium and producing more...

JiangJC Dis2K

I think you can always assume the initial concentration of a product is zero because the reaction has not occurred yet at the beginning and reactants are what are being formed. As for products, it is unlikely that the initial concentration is zero because then there is no reaction occurring because ...

JiangJC Dis2K

R is a constant in the ideal gas law (PV=nRT) and has multiple unit derrivations that can all be found on the equations and constants sheet! So no need to memorize the value or units!

JiangJC Dis2K

Also in regards to your second question, I had Lavelle for 14A and we did have to memorize the chemical formulas of some complex biological molecules. But overall, I would say it's necessary to memorize common polyatomic molecules as well as any specific example he goes over in class. For example, c...

JiangJC Dis2K

No-PV=nRT is listen on the Equations and Constants sheet on Lavelle's website as well as the values for constant R with multiple unit derrivations!

JiangJC Dis2K

Water is amphoteric so it can act as either an acid or base, therefore it depends on the other compound involved in the reaction with water to determine water's behavior.

JiangJC Dis2K

Strong acids produce more protons of H+'s because they are almost completely ionized in solution. HBr, HCl, and HI are some examples of strong acids. Also, in today's lecture, Lavelle talked about how longer bonds associated with larger molecules become increasingly strong acids because the bonds ar...

JiangJC Dis2K

Based on Lavelle's class notes "Between metal oxides (Bases) and Nonmetal Oxides (Acids) there is a diagonal bond of amphoteric oxides closely matching the diagonal bond of metalloids." Examples include BeO, Al2O3, Ga2O3, SnO2, Sb2O5, and PbO2.

JiangJC Dis2K

Its based off of a point system overall, but there is a note on the syllabus that says a 50% is a C- (which would typically be a 70%), so I think the class is curved overall to the letter grade at the end. I'm assuming it can only help and not hurt you because its not a competetive scaling or anythi...

JiangJC Dis2K

No. The octahedral shape is always nonpolar, because the shape is symmetrical. When a molecule has a symmetrical shape, the dipoles cancel out and the molecule in nonpolar.

JiangJC Dis2K

Yes. It would probably be most helpful conceptually too to write out the electron configuration and figure out which orbital and n number the valence electrons exist in. From there you can draw the Aufbau diagram like we did in class for the examples. I am also confused though if its necessary to in...

JiangJC Dis2K

Lavelle went over this in class and said that he personally will accept either way. As long as the total number of orbitals matches the number of electron densities in the right order you should be good!

JiangJC Dis2K

I think we basically just need to memorize all of them, though some of the angles could probably be determined by visualizing. There are a lot of shapes, but what helped me was to make an organized table of the formulas using the VSEPR notation of A, X, and E and then the shape, bond angle(s), and h...

JiangJC Dis2K

Sigma and Pi bonds do not affect the shape or structure because in VSEPR, all bonds are treated as equivalent. Single, double, and triple bonds are all considered one region of electron density. Though Sigma and Pi bonds impact the movement and rotation of molecules. Molecules with only sigma bonds ...

JiangJC Dis2K

Yes, you would need to draw the Lewis Structure and count from there. A single bond has one sigma bond. A double bond has one sigma and one pi. And a triple bond has one sigma and two pi!

JiangJC Dis2K

Just to clarify, all three atom molecules are not bent. For example, BeCl2 has a linear shape with a bond angle of 180 degrees. The difference is in the lone pair of electrons. BeCl2 has no lone pairs around the central atom, where as something like H20 is not linear because of the exertion of force...

JiangJC Dis2K

A hydrogen bond occurs only between a hydrogen atom and nitrogen, oxygen, or fluorine. A hydrogen bond in this sense is a covalent bond between a hydrogen atom and a highly electronegative atom with an available lone pair of electrons.

JiangJC Dis2K

Basically, the more electrons there are, the more likely or easily they are distorted. If you think about it in the diagram of circles as different n levels around the nucleus, the electrons are getting further and further away and filling more n orbitals. Therefore, the further electrons experience...

JiangJC Dis2K

Thank you Karen! This is super helpful for preparing for the midterm today!

JiangJC Dis2K

Commenting on the other replies, remember that 4s is only higher in energy when the 3d orbital has electrons in it. If the 3d orbital is empty, then 4s has lower energy level. Also remember the exceptions Cr [Ar] 3d5 4s1 and Cu [Ar] 3d10 4s1!

JiangJC Dis2K

I don't think we would need to know how to calculate this equation. In the lecture notes, Lavelle basically went over that Schrodinger's equation is used to predict where an electron is, or the probability of finding an e- at some point in space.

JiangJC Dis2K

The four exceptions are hydrogen, lithium, beryllium, and boron. These are the first 5 elements on the periodic table, so you they have only 5,4,3,2, or 1 electrons to start with. Therefore they cannot form more bonds than valence electrons.

JiangJC Dis2K

Just try to draw as many as possible. Usually it's symettric or the question tells you how many resonance structures to consider. We won't be calculating the actual mixed bond lengths or anything because that's exclusively found experimentally.

JiangJC Dis2K

A resonance structure is basically when bonds and electrons can be organized in a variety of combinations. Remember that the actual occurring structure is a mix or blend of the multiple resonance structures, rather than one. But using formal charge, the lowest energy resonance structure factors more...

JiangJC Dis2K

Also make sure to remember the exceptions: Cr=[Ar]3d54s1 and Cu=[Ar]3d104s1 !!

JiangJC Dis2K

I think for ionic bonds or polyatomic ions (basically something with a charge), you use the box method to notate that each individual compound has a charge. Such as KNH4 then [K+][the lewis diagram for NH4+], bad example but hopefully that helps.

JiangJC Dis2K

No, this class is not graded on a curve. On the syllabus it lists and overall point system. Basically, I think the homework points and chemistry community points act as a kind of curve like or buffer to your grade. Honestly, the curve would probably hurt more than help.

JiangJC Dis2K

If you go to the words "Quick links" in the upper left corner, the drop down will provide you with the option "Your posts" and they are all organized there.

JiangJC Dis2K

The UA talked about this a little in one of the review sessions. Basically he said you should know basic naming in addition to polyatomic ions or common compounds. In many practice questions in the textbook or midterm reviews though they often give you the name and then the formula after so don't st...

JiangJC Dis2K

You cannot round down. If you're going from that approach of multiplying the mass ratio of say carbon in co2 and dividing moles, then you'll need to multiply (basically guess and check) what integer will produce a next to close number. 0.15 is a little too high to round down since we are producing w...

JiangJC Dis2K

Basically. If the energy of the photon/light has greater energy than the threshold energy, then energy will be conserved and appear in the form of ejected electron's kinetic energy. If the energy of photon is equal to the threshold energy, it is a break even, and the kinetic energy of ejected electr...

JiangJC Dis2K

No. 1/2mv^2 can only be used for particles that have mass and velocity (essentially particles in motion). This cannot be applied to photons because they have no rest mass. Therefore, the equation hc/E is used instead of h/p.

JiangJC Dis2K

Intensity relates to the quantity of photons. Basically, if the intensity decreases then there are less photons in the stream of light. Since photons are essentially packets of energy, this means that there is less energy going in-leading to either insufficient amount to eject electrons or less elec...

JiangJC Dis2K

The equation you would use is Energy (photon) - Work Function=Kinetic Energy of Electron=1/2mv^2. And then use the known value of mass of electron to solve for velocity to plug into heisenberg's.

JiangJC Dis2K

I believe that bond lengths can only be found experimentally, so it's unlikely that we will be needing them in this class. I felt that Lavelle was using the experimental data of bond lengths to depict to us the general pattern that in terms of length: triple bonds<double bonds<single bonds. Also to ...

JiangJC Dis2K

I was really confused on the Ms number number when I learned it in high school but I feel like Lavelle explained it really well. If you draw out the orbitals it makes a lot more sense. For example if n=2 and l=1 then you know this is the 2p orbital. p has three ml levels or three subshells (room for...

JiangJC Dis2K

I found the way that Professor Lavelle explained this concept in lecture today very helpful. Basically, if you passed something with large mass such as a baseball or human being through a stream of light, the distance and time could be accurately computed by multiple light beams because the track of...

JiangJC Dis2K

Yes, there are other equations for elements that contain more than one electron. But, this equation is not necessarily only for hydrogen, but for single electron atoms.

For example: Li2+, He+, Be3+, etc. also have one electron using the periodic table and their "z" value.

JiangJC Dis2K

Generally, for multiplication and division problems, you adhere to the least number of significant figures stated in the given numbers for the problem. For addition and subtraction, you go to the least number of decimal places given in the problem. But for rounding I am also a bit confused. I would ...

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