CHEMISTRY COMMUNITY

nehashetty_2G

For A, you have to ensure that only the concentrations for A are changing. Therefore, the concentrations for B must be the same (The concentration of C doesn't matter in this question specifically, so only worry about B). Based on this logic, you can use a combination of experiments such as Rate 2 a...

nehashetty_2G

I think it really depends on the step mechanism itself. However, for our exam purposes, I believe they will most likely have the slow step in the middle somewhere (my speculation).

nehashetty_2G

Thank you for everything Lyndon!!!

nehashetty_2G

They will tell you if a step is low usually. You can use that information to decide that the slow step is what determines the rate of the reaction.

nehashetty_2G

I think open book means you have access to lecture notes, but I don't think it includes chemistry community. I am unsure about your other questions; Lavelle may speak on those in later emails!

nehashetty_2G

No, I believe you have to calculate it based off of the rate law equation, or through the method of initial rates.

nehashetty_2G

The answer key does not include rate laws for the reverse reaction; however, in the book it says every forward reaction has a reverse reaction associated with it. So, I am wondering when do we use/write down a reverse reaction rate and when do we not?

nehashetty_2G

You can use the method of initial rates that we learned in class to determine n and k!

nehashetty_2G

You just use stoichiometric relations to find the concentration of A. If 0.034M B was created, that means that two times that amount of A was used in creating B. Therefore, you subtract 0.068 from 0.153 to find [A]t (0.085M).

nehashetty_2G

Also, when the states are the same, you put the higher charge on the left and lower charge on the right

nehashetty_2G

I used the following half reaction:
2H2O + 4e- + O2 --> 4OH-
I used this half reaction because the cell diagram shows that there is an O2 and an OH-. This is why I didn't use the other half rxns that include species like H+.

nehashetty_2G

I found this helpful: viewtopic.php?f=142&t=60522

nehashetty_2G

Both the half reactions for In3+/In2+ and U3+/U4+ use only one electron and it balances out correctly. So, why does the solution manual uses 2e- for n?

nehashetty_2G

When does the Pt go on both sides of the cell diagram and when does it not? For example, in Problem 6M.5 the Pt is only on the right side of the diagram.

Also, in general, how do you determine when to use Pt? Are there specific rules for it?

nehashetty_2G

I used different half rxns and got the same E. Does it matter which ones you use? If so, how would you tell for this problem?
The overall rxn is: H+ + OH- --> H20
The answer key uses O2 in the half rxns
I used the following half rxns:
2H+ + 2e- --> H2
H2 + 2OH- --> 2e- + 2H20

nehashetty_2G

In the answer key, the Ag(s) and AgBr(s) are separated by "|" however, I thought we only use that line when two species are in different phases. Can someone clarify why we use the "|" between Ag(s) and AgBr(s) in this question?

nehashetty_2G

It depends on if the reaction is occurring in an acidic or basic solution. If it is an acidic solution you use H+ and if it is a basic solution you use OH-

nehashetty_2G

A lot of times we separately divide the final answer by moles in order to get those units. It's similar to how in the thermo unit, when we calculated delta H of formation we divided by mol to get J/mol even though you normally get just joules by doing products-reactants.

nehashetty_2G · Re: Notes from 02/14/2020 Sending it right now!

Sending it right now!

nehashetty_2G

I would use the equation: G=-RTlnK. You calculate G using appendix 2A, and solve for K.

nehashetty_2G

A salt bridge comes is significant to a galvanic cell because it eases the charge buildup. It allows the ions to transfer when which allows the solutions to stay neutral overtime.

nehashetty_2G

In terms of entropy, when a system is reversible it means that the delta S total is = 0.

nehashetty_2G

Delta S total = 0 to in a reversible reaction not delta S. Remember Delta S implies Delta S of the system.

nehashetty_2G

Yes! Lyndon told us this during a review session (capital letters indicate a state function): "if you are Depressed or Pressured, all you want is TV and HUGS." State functions are: density, pressure, temperature, volume, enthalpy, internal energy, gibbs free energy, and entropy.

nehashetty_2G

I don't think it matters for entropy. I believe we always use the RTln(V2/V1).

nehashetty_2G

I believe it is just for molarity, that way all the values in your ICE table are consistent and comparable.

nehashetty_2G

No, I believe that W is the number of ways a molecule can be arranged. That's why BF3 has W=1 not three. Whereas COF2 has W=1 because the oxygen can be in 1 of three positions.

nehashetty_2G

I believe residual entropy is the energy that remains even when a sample is brought to 0 Kelvin caused by disorder in the sample.

nehashetty_2G

How do you calculate the delta n? And what is the delta n?

nehashetty_2G

The answer key doesn't have an answer for the change in enthalpy. Can someone with the solutions manual confirm or deny if the answer is +765J for delta H? If it isn't, how do we find the correct answer?

nehashetty_2G

Yes! When external pressure is constant, it is an irreversible process.

nehashetty_2G

When the system does work on something outside of the system, it is negative work because they system is losing energy (ex. the piston moves out/expansion). When something else is doing work on the system, it is positive work because the system is gaining energy (ex. piston moves in/compression). I ...

nehashetty_2G

For an irreversible expansion, the external pressure is constant and for reversible expansion, external pressure can change.

nehashetty_2G

When we talk about oxygen or nitrogen gas, we always assume it is in its diatomic form. This is because it the diatomic form the standard state. Therefore, I think of it as being like the standard or 0 kJ/mol because the element is just naturally just found in that state.

nehashetty_2G

The question asks for the delta U, but they just give us the delta H and the equation. I don't understand how you calculate this? Can we just assume delta U = delta H? And if so, why?

nehashetty_2G

No, definitely not for Test 1. We just need to know in terms of enthalpy (delta H).

nehashetty_2G

This is another link you can refer to: https://lavelle.chem.ucla.edu/wp-conten ... ussion.pdf

nehashetty_2G

Do sig figs count on the test 1? If so, how do sig figs work with pH and decimals points. For example, if pH = 5.1 and the question asks for the [H+], how many sig figs would the [H+] have?

nehashetty_2G

The question asks to use Table 5E.2. Is it supposed to be 5G.2? Because I can't find a Table 5E.2.

nehashetty_2G

Is the K in the second column just K calculated with partial pressures instead of concentrations?

nehashetty_2G

Does Le Chatelier's Principle encompass a change in product/reactant concentration, pressure, and temperature? Or is it just a change in product/reactant concentration?

nehashetty_2G

Practically, I would not worry about a cutoff especially for the purpose of this class, because liquids like water are not included in the equilibrium constant expression.

nehashetty_2G

No, it has to be concentration. If you just use moles it will mess up your calculations when the stoichiometric coefficient is something other than 1 due to the ^n part of the equilibrium equation.

nehashetty_2G

No, the B versus the A subscript are just indicative of whether the reaction is an acid or base.

nehashetty_2G

When the problem gives the partial pressure of one of the compounds in barr units do we divide by the volume to get the initial concentration value? I know in this question it isn't much of an issue because it is 1.0L, but I'm asking for future chem problems.

nehashetty_2G

Nope! Only the mole fractions of all the components of a mixture must add to 1, but we are not dealing with that (right now at least).

nehashetty_2G

I think the K would just be 1. On a ChemLibre website it just says that solids and liquids are essentially just 1, so for a reaction with all solids and liquids I assume 1 makes the most sense.

nehashetty_2G

For a reaction that involves just gases you would use Kp.

nehashetty_2G

I think it will most likely by R=8.314 J/(Kmol) because that is what Lavelle gave on the formula/constant sheets in 14A. In general just look at the units to figure out what to use, or to see if you need to manipulate the constant.

nehashetty_2G

Yes, it should be! Because, the only reason we use Kp versus Kc is based on the phases of the reactants/products (and whether the concentration or partial pressure is give to us for gases).

nehashetty_2G

Yes! Kp and Kc are really both just different input values to get to K so they must have similar implications in practice.

nehashetty_2G

Yeah I personally think it is a little redundant, but I believe we are still required to write both.

nehashetty_2G

I put that it was a strong base because the hydroxides of Group 1 and 2 elements are strong bases.

nehashetty_2G

Why does the diaqua go before the bisaxalato?

nehashetty_2G

I have to find the pH with the information that [H30+] is 1.50 mol/L. The correct answer is a pH of 0.176, but I keep getting -0.176. What am I doing wrong?

nehashetty_2G

Yes it can act as a base because it has the potential to receive an H+! So, the conjugate acid would be H2CO3.

nehashetty_2G

Matt Sanruk 4F wrote:And O has two hydrogen bonding sites right?

Yes, that's usually correct, but it still depends on how O is arranged on the molecule/compound.

nehashetty_2G

No I don't think so. Being labeled polyprotic vs monoprotic just refers to how much one CAN donate; not how they normally donate. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Monopro...

nehashetty_2G

I think chloro is the correct form (rather than chlorido) when it is a ligand in a coordination compound. Same with fluoro and cyano.

nehashetty_2G

Yes we should know how to calculate Ka especially since we have the equation for it. Just insert the concentrations.

nehashetty_2G

A ligand is just something that attaches onto a transition metal central atom in a coordination compound.

nehashetty_2G

I personally always draw out the formula (unless it's a common compound that I know is non polar or polar like CH4 or H2O respectively). The reason is that a lot of times there are lone pairs that are not reflected in the compound's formula that are discovered only after drawing the structure. For e...

nehashetty_2G

Seesaw: <120 degrees for equatorial atoms and very slightly 180 for the axial atoms.
T-shaped: <90 degrees
Square pyramidal: 90 degrees for atoms on the equatorial plane.
Square planar: 90 degrees.

nehashetty_2G

To form a hydrogen bond, the NOF atoms need to have a free lone pair. In butanol, the OH is on the end so it has a free long pair to form hydrogen bonds. However, in diethyl ether the oxygen is in the middle of the atom and has no lone pairs therefore it cannot form a hydrogen bond.

nehashetty_2G

How do you know the arrangement of the H and Cl for a compound like CH2Cl2?

nehashetty_2G

For problem b, why is it not less than 120 degrees instead of 120 degrees for the atoms in the trigonal plane? I thought that because there is a lone pair, the Cl-Te-Cl bond angle would be less than 120 degrees?

nehashetty_2G

We have to divide by Avagadro's number to get it from J/mol to J/atom, where atom is 1 Au atom.

nehashetty_2G

I'm not exactly sure about radicals and shapes, but I know that for 2E.5 when they ask OClO they aren't asking about a molecule ClO2, they are just taking about the bond formed between those three atoms (O, Cl, and O). Hope that helps!

nehashetty_2G

Yes I believe we will. I think Lavelle will go over the other shapes in class (like with the long pairs).

nehashetty_2G

Yes, we definitely will for the exam/final.

nehashetty_2G

The London Forces! It's a type of intermolecular force and it is interchangeable with dispersion forces, van der waals, and induced dipole-induced dipole forces.

nehashetty_2G

I remember seeing the midterm average on the last slide of last lecture's powerpoint. I think that was last year's midterm average so I'm sure he will release that stat for our quarter too.

nehashetty_2G

I think that when calculating formal charge, the concept is not to look at how many electrons an atom has, rather it is to look at the number of SHARED bonds that an atom is involved in.

nehashetty_2G

Electronegativity is not an interaction between molecules. Electronegativity is a relative measure of an atom's tendency to pull electrons to itself. On the other hand dipole-dipole is an interaction that is characterized by the partial charge of one atom being attracted to the opposite partial char...

nehashetty_2G

Since boiling point is the change from liquid to the solid state it is identified when all the intermolecular bonds are broken. Therefore, the stronger the intermolecular forces (IMF) are the higher the boiling point will be. Hydrogen bonding is one of the stronger intermolecular forces and causes h...

nehashetty_2G

Sometimes the work function is given in joules per mole; however, we need it to be just joules in the problem for the equation. Therefore, ensure to divide the joules/mole number by avagadro's number before you input that value into the problem!

nehashetty_2G

Yes, you use the electronegativity difference to determine if a bond is ionic, covalent, polar covalent, etc. Anything with an EN difference > 2.0 is ionic. In terms of determining C-O vs C-S and which is greater you would need to compare the electronegativity of O and S. Remember that we learned th...

nehashetty_2G

Online it says that the two primary exceptions are that 1) N has an ionization energy greater than O and 2) Be has an ionization energy greater than B. Those are probably the most important to remember since they are located in the first three periods of the periodic table.

nehashetty_2G

I have also seen questions that give the work function in the units eV, and in that case just remember that you can convert from eV to Joules using the conversion factor give on the conversion sheet which is 1ev is equal to 1.602 × 10-19 J.

nehashetty_2G

Yes! Because all of the elements in that group have the same valence electron orbital formation; therefore, all of them will exhibit similar characteristics when it comes to arranging valence electrons.

nehashetty_2G

So formal charge is different from ionic charge. Formal charge takes into consideration the bonding in terms of electrons for each individual atom in the compound. Therefore, Cl in a compound does not always have a formal charge of -1. It could also have a formal charge of 0. We don't have to know h...

nehashetty_2G

Professor Lavelle has not put up any information about the midterm yet. However, until then I would look over everything we have covered in class so far (and compare your notes with peers to ensure you did not miss anything). I believe the midterm will also cover the content from Exam #1, but I woul...

nehashetty_2G

Yes! Those l values will always correspond to their respective s, p, d or f subshells. The only thing that changes when you move up shells (ex. n=2 to n=4) is the possibility of subshells or l-values that work for that shell. For n=2, an electron can be in l=0 or 1 and for n=4, an electron can be in...

nehashetty_2G

That's correct! Because the compound is neutral and abides by the normal electron states of the atoms.

nehashetty_2G

Also remember that if you are calculating the total formal charge of an ionic compound by adding all the individual formal charges of the elements, you want the final value to equal to the charge of the ionic compound. For example in SO4 2- the final value should add to -2 NOT ZERO. Instead, to dete...

nehashetty_2G

Before considering formal charge, you usually use double bonds when you are trying to make every element satisfy the octet rule. So for example, if a compound has only 24 electrons and with your initial Lewis Structure with only single bonds you notice you are at 26 electrons, you would most likely ...

nehashetty_2G

The number of orbitals in a subshell can be determined by ml which is equal to 2l+1. Therefore, because we know that l equal to 0 refers to the s-orbital, 1 is the p-orbital,...3 is the f-orbital, we can use this information in the question. For a) since we are looking at p, which has an l value of ...

nehashetty_2G

Nodal planes are any areas where the is zero probability of electron density. Therefore, there will be no electrons in that area.

nehashetty_2G

The question is basically testing you on if you know the meaning of l in the context of orbitals. Remember that l equals 0 refers to the s subshell, 1 to p, 2 to d, and 3 to f. And within each of the subshells, s has 1 orbital, p has 3, d has 5, and f has 7. Therefore, for D24a the answer should be ...

nehashetty_2G

No, Lavelle said we are not required to know how to draw any of the orbitals. Although, I think it's probably good to be familiar with what they look like (especially s and p)

nehashetty_2G

psi by itself represents a wave function, or the position of a wave at (x,y,z). In simpler terms, one can just think of it as the height of a wave, or its position. psi^2 represents the probability of finding an e-. Also, Professor Lavelle mentioned in class that psi^2 is always positive because, #1...

nehashetty_2G

Basically yeah. It always depends on the threshold frequency; any frequency less that the threshold value won't be enough energy to eject the electron.

nehashetty_2G

Yes - I believe at this point we will consider the speed of light/radiation as a constant because there are no changing mediums in the problems we are dealing with.

nehashetty_2G

How do you do 1B.15(c)? You are given that the frequency of the minimum radiation needed to eject an electron is 2.50 x 10^16 Hz and the velocity of an electron that is emitted from a metallic surface by a photon is 3.6 x 10^3 km/s. I thought this was a simple problem but I keep getting 1.2x10^-8 m ...

nehashetty_2G

Yeah! Essentially, electrons are described as a circular standing wave around the nucleus. Therefore, the connected line indicated a wave in line or in phase, which is stable whereas the disconnected line is a wave out of phase which is not stable. It is basically explaining why electrons have quant...

nehashetty_2G

Does the speed of light always stay the same? If not, under what circumstances can it change?

nehashetty_2G

What exactly is meant by option C: "The extent of the change in the electrical field at a given point decreases"? I don't know what it has to do with electromagnetic radiation.

nehashetty_2G

If you want to get into specifics, Avagadro's number is the number of Carbon-12 atoms in 1 mol of Carbon-12 (6.022x10^23 atoms). I believe that's how the number was determined way back in the day!

nehashetty_2G

I think it depends, but definitely follow rules both depending on the step you are on. Usually, you do sig figs at the end of a sequence (like in dimensional analysis); in that case you usually do the normal sig figs rules at the very end of the multiplication. But if you are adding and subtracting ...

nehashetty_2G

I believe combustion reactions always only produce H20 and CO2. I have never seen anything other than that. The only other product other than H20 and CO2 that I can think of would be heat.

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