CHEMISTRY COMMUNITY

RJ Lopez 2l

A certain reaction has an enthalpy of deltaH=−39 kJ and an activation energy of Ea=53 kJ.
What is the activation energy of the reverse reaction?
How do we figure this out?

RJ Lopez 2l

What do we do with the slow step in reaction mechanisms/profiles. If there are multiple what do we do?

Thank you

RJ Lopez 2l

The forward and reverse rates of reaction depend on the equilibrium of the chemical reaction. it is also important to look at the coefficients of the reactants and products before making any assumptions. Make sure you are looking at the K value and not the Q when determining rates of reaction

RJ Lopez 2l

If you search chill or study under spotify there are some study playlists other people have made. I personally enjoy a person i found on tik tok named "ymoug" he does live streams and has good playlists. helps me focus and get motivated

RJ Lopez 2l

Is there a way to check how many posts you have done this quarter, I know you can see your total amount of chemistry community posts on the side, but I also took this class last quarter and I don't remember how many I had last year.

RJ Lopez 2l

What is the "mechanism" of a catalyst reaction

RJ Lopez 2l

How do we determine catalysts v and intermediates in the steps of a reaction?

RJ Lopez 2l

Depending on the units you are given, make sure to check what type of pressure and temperature is being used. But in the end your could always convert to the correct units at the end of the question despite which one you solve with?

RJ Lopez 2l

At colder temperatures, the movement of particles are slower so there are less collisions. And at higher temperatures, there are more collisions. You can think of this in the order of physical phases too such as solids--> liquids--> gas. Thus there is a relationship between k and T

RJ Lopez 2l

A particular reactant decomposes with a half‑life of 145 s when its initial concentration is 0.393 M. The same reactant decomposes with a half‑life of 215 s when its initial concentration is 0.265 M.

Determine order:

What is the value and units of the rate constant for this reaction?

k?

RJ Lopez 2l

To solve for T you can take the natural log of the final and initial rates of reactants divided by R in the form:

t= -ln(A/Ai)/R

Also you may have to covert your answer into the proper form of time before solving, make sure it is in seconds

RJ Lopez 2l

For the final, if our TA no longer hold online discussions, which zoom link are we supposed to go when we take the exam. Does anyone has a link they can share?

RJ Lopez 2l

The rate of a chemical reaction depends on reactant concentration. For a reaction such as aA → products, the rate law generally has the form rate = k[A]ⁿ, where k is a proportionality constant called the rate constant and n is the order of the reaction with respect to A. In your case it would be a s...

RJ Lopez 2l

A concentration cell at 25 °C is constructed from hydrogen half-cells of different hydronium ion concentration and sealed so that their hydrogen gas pressure can be controlled. In one half-cell the hydronium concentration is 2.0 M and the hydrogen gas pressure is 0.50 atm, and in the other half-cell...

RJ Lopez 2l

What is the best way to balance redox reactions in Basic conditions, I've seen some TAs solve it by treating it as an acidic reaction and other start with balancing water first. There are just a lot of methods and it confuses me when it switches each time I go to a different session

RJ Lopez 2l

What's the difference between the pre-equilibrium approach and the steady-state approach in solving for the rates. We still use the same formula correct?

RJ Lopez 2l

Dinitrogen pentoxide, N2O5, decomposes by first‑order kinetics with a rate constant of 3.7×10−5 s−1 at 298 K.
What is the half‑life, in hours, of N2O5 at 298 K?

How does half life change between zero order and first order kinetics?

RJ Lopez 2l

Could someones explain how to approach this problem? We are given a chart with different concentrations of reaction and suppose to figure out the order and then solve for them?

RJ Lopez 2l

Could someone provide explanation of what zero order, first order, second order, and third order mean? and how does it relate to the units?

RJ Lopez 2l

For this problem you should start with reactants first. Depending on what concentration we are given, we should use the formula concentration over time, then from there we can go on to solve for the products. An easy way to think about it is as a slope equation with difference in concentration over ...

RJ Lopez 2l

Calculate the cell potential for the reaction as written at 25.00 °C ,given that [Cr2+]=0.808 M and [Ni2+]=0.0130 M Use the standard reduction potentials in this table.

Cr(s)+Ni2+(aq)↽−−⇀ Cr2+(aq)+Ni(s)

I am very confused on how to solve this, how do I find Q and Ecell

RJ Lopez 2l

For a particular redox reaction, BrO− is oxidized to BrO3-1 (charge -1) and Cu2+ is reduced to Cu+ . Complete and balance the equation for this reaction in basic solution. The phases are optional. I don't understand how you would balance the oxygens in this equation. Also for Cu2+ would you just add...

RJ Lopez 2l

If you first balance BrO- as if it was in an acidic solution, you get BrO−+2H2O⟶BrO−3+4H+ +4e− Then in order to convert it to a basic solution, you add OH- to the side with H+ to form 4 molecules of water on the right side (add the same amount to the left side). Then you get 4OH- + BrO−+2H2O⟶BrO−3+...

RJ Lopez 2l

In a basic solution there is no H+, only H2_ and OH-.

RJ Lopez 2l

For a particular redox reaction, BrO− is oxidized to BrO−3 and Cu2+ is reduced to Cu+ . Complete and balance the equation for this reaction in basic solution. The phases are optional. I am confused on how you would balance the Cu half reaction, would we just add electrons or do we use OH and water? ...

RJ Lopez 2l

Thank you so much for the clarification, I was struggling with this problem as well. The spontaneous should be in the top boxes, and the Q < K, Q=K, and Q>K should be on the bottom

RJ Lopez 2l

Calculate the equilibrium constant, K , at 25.0°C for each of the reactions. Use the thermodynamic information provided in the table. The first step in the synthesis of trichloromethane (chloroform) from natural gas (methane). CH4(g)+Cl2(g)↽−−⇀CH3Cl(g)+HCl(g) I am having a hard time finding the equi...

RJ Lopez 2l

Calculate the equilibrium constant, K , at 25.0°C for each of the reactions. Use the thermodynamic information provided in the table. The first step in the synthesis of trichloromethane (chloroform) from natural gas (methane). CH4(g)+Cl2(g)↽−−⇀CH3Cl(g)+HCl(g) I am having a hard time finding the equi...

RJ Lopez 2l

The ΔHvap of a certain compound is 26.41 kJ·mol−1 and its ΔSvap is 65.37 J·mol−1·K−1.

What is the boiling point of this compound?

Why is deltaG = 0 when solving this?

RJ Lopez 2l

You set delta G to zero and convert units so they are the same. Delta S should be in kJ, then you plug in the values given for the variables. Lastly for part 2, you use the signs of delta H and delta S to determine whether the reaction is spontaneous.

RJ Lopez 2l

The molar heat capacity for carbon monoxide at constant volume is C V, m =20.17 J/(K·mol) . A 17.00 -L fixed-volume flask contains CO(g) at a pressure of 10.00 kPa and a temperature of 25.0 C. Assuming that carbon monoxide acts as an ideal gas and that its heat capacity is constant over the given te...

RJ Lopez 2l

Could someone explain the difference between q(rev) and regular q from the 1st law of thermodynamics, or are they the same thing?
Also how do we convert to units of S when given work and temperature change in C?

RJ Lopez 2l

Everything looks good, just make sure you use the correct sign for delta G given the type of reaction and equation, should be negative

RJ Lopez 2l

Could someone explain the relationship between delta H and delta U. How does it relate to q?

Thank you

RJ Lopez 2l

At constant volume, the heat of combustion of a particular compound, compound A, is −3223.0 kJ/mol. When 1.397 g of compound A (molar mass =111.21 g/mol) is burned in a bomb calorimeter, the temperature of the calorimeter (including its contents) rose by 4.375 ∘C. Do we start with q = mCdeltaT then ...

RJ Lopez 2l

To add on to the previous answer, make sure to convert your heat of fusion which is measured in joules into Celsius.
Then use heat of fusion + q ice = - q water

q = mC(Tf-Ti)
qice = (49.0g)(4.184 J/g*C)(Tf - 0C)
- q water = - (411g)(4.184 J/g*C)(Tf - 45C)

RJ Lopez 2l

For this types of problems, it helps to use an ICE table. With an ICE table you only need to know whether the you are working with a LEWIS acid or base, the rest of the products wont matter. The equation will always have the for x^2/(Ka of reactant)-x if the concentration is less than 10^-4, it beco...

RJ Lopez 2l

Carbon disulfide can be prepared from coke (an impure form of carbon) and elemental sulfur. 4C(s)+S8(s)⟶4CS2(l)ΔH∘=+358.8 kJ Calculate the amount of CS2 produced when 391 kJ of heat is absorbed. How would we go about solving thus equation, I tried using DeltaH= moles x deltaH formation but i get the...

RJ Lopez 2l

What do we use to calculate standard enthalpy of a reaction if we are given the enthalpy of formation and moles? Do we apply the magnitudes of the elements in the equation?

Thank you

RJ Lopez 2l

The Kb for an amine is 8.835 × 10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.993 ? Assume that all OH− came from the reaction of B with H2O. I was able to determine the concentration of [OH-], but i do not know how we are supposed to percentage pronated s...

RJ Lopez 2l

Could someone explain the relationship between pH and pKa, for example what does a high pKa value mean and what does that mean for the pH.

Thank you

RJ Lopez 2l

To calculate equilibrium its always products over reactants. For the questions that involve finding equilibrium using ICE box, I would always draw out a chart so you can clearly see the values you are given and what you are trying to solve for. Also, make sure that on equilibriums involving weak aci...

RJ Lopez 2l

When you get two positive values for x, if you plug both of the x values into the original equilibrium equations, you will see that one of the value would result in a negative equilibrium. Choose the x value that makes a positive final concentration. negative equilibriums are incorrect, so if you ge...

RJ Lopez 2l

Bronsted Acids use H+ ions whereas Lewis acids focus on the transfer of electrons. A Bronsted acid would be a proton donor while a Lewis acid would be an electron acceptor.

RJ Lopez 2l

Hi
First you would calculate Kc using the initial molarities that are given in the question, so (0.600)^2/0.300^2. That would be your Kc value, then you would set the expression you created equal to Kc and solve for x. Once you find x, do (0.6-2x) to find the new concentration of NO.

Hope this helps

RJ Lopez 2l

Silver walks up to gold in a bar and says "AU get out of here"

RJ Lopez 2l

K represent the equilibrium constant while Q is the equilibrium quotient. They are both achieved by dividing the concentration of products by the concentration of reacts. However, Q can be calculated for a reaction at any time whereas K is calculated at the point of equilibrium. K value is constant ...

RJ Lopez 2l

What happens to K when temperature is increased in a endothermic reaction and exothermic reaction

RJ Lopez 2l

Could someone explain how to use hess law to arrange two of the equations to give the desired overall equation? Such as in achieve question 5.

RJ Lopez 2l

Could someone explain what an ICE chart is, and what do we do with the information on the chart?

RJ Lopez 2l

At a certain temperature, the equilibrium constant, Kc, for this reaction is 53.3. H2(g)+I2(g)↽−−⇀2HI(g) Kc=53.3 At this temperature, 0.800 mol H2 and 0.800 mol I2 were placed in a 1.00 L container to react. What concentration of HI is present at equilibrium? I am confused about how you would create...

RJ Lopez 2l

That is the rydberg equation. It is simply finding the energy of the electron at a particular energy level (n), however it only apply to hydrogen atoms

RJ Lopez 2l

Lone pairs would apply when determined electron density which will help you find the molecular shape of the molecule.

RJ Lopez 2l

If you look up the names of the students/TAs doing the step up sessions in the search bar, you should be able to dint their powerpoints. Julia lok, parsa mokhtar and micheal Nguyen have some good material

RJ Lopez 2l

It may be helpful to look at the parent acid and parent base of the salt to determine whether they are strong acids or bases. But add them with water, you can determine the ph of the salt

RJ Lopez 2l

I understand how acid rain is form through pollutants such as SO2 CO2 and NO2 reacting with water, but how do we reduce acid rain. Is there something that we can emit into the atmosphere that can fix the pH or is the answer just to no burn fossil fuels

RJ Lopez 2l

If air were restricted and no co2 is being exhaled, is it the build up of CO2 or HCO3- that is causing the blood to become acidic?

RJ Lopez 2l

Along with the shapes of coordination compounds, do we also need to know the hybridization of the structures as well?

RJ Lopez 2l

Because of F high electronegativity, less energy is required to make F2- from F- than O2- from O- Correct answer false. Could someone explain the reasoning behind this question. Is it could the electron in f2- are held closer together or is it something else. I’m just confused because I thought f2- ...

RJ Lopez 2l

Lewis acids and bases are related to the electrons being accepted or donated while bronsted acids and bases relate to the protons being accepted or donated.

RJ Lopez 2l

A salt such as NH4Cl is composed of a parent acid HCl(strong acid) and parent base NH3 (weak base). When determining if salts are acidic or basic in a solution, you would need to determine the parent acid and bases by adding water. Generally speaking halogens are strong acids and group 1 hydroxides,...

RJ Lopez 2l

You would use bis tris and tetrakis when the ligand already has prefixes like di and Tri in its name. One example in which you would use one would be dimethylamine.

RJ Lopez 2l

PKA is the -logka. The higher the KA, the lower the pKA. The KA is also the acid dissociation constant that is used to distinguish strong acids from weak acids. Strong acids will have low pKA values, usually an acid of 1 or below is a strong acid

RJ Lopez 2l

Determine the initial molar is of Ba(OH)2 in an aqueous solution that contains 0.43 g of Ba(OH)2 in 0.100 L solution Calculate the molarities of Ba2+, OH-, and H3O+ in the same solution I figure our the molarity of BA2+ (0.025] and OH- (0.050M) but how would we determine the H3O+ if there is none pr...

RJ Lopez 2l

Could someone explain why blood would want to form HCO3-. How would it balance blood pH if the resulting reaction produces an H+ that makes blood more acidic. How does the exchange of CO2 balance blood ph.

RJ Lopez 2l

The dipole dipole forces between NaF are greater due to F being the most electronegative element. Dipole Dipole forces are stronger than the London Dispersion Forces in KCl. It important to look at the difference in the electronegativity as well as size of an atom when determine melting point

RJ Lopez 2l

Counting the electrons and Calculating the formal charge of each atom when creating a lewis structure is a good way to determine the amount of valence electrons Xe will hold. For example, in the molecule XeF2 there should be 22 electrons, if you were to draw the Lewis structure with single bond and ...

RJ Lopez 2l

Yes, we have to memorize bond angles, it its also important to understand that bond angles can be less than what is expected. For example, Bent or angular geometry have less than a 109.5 degree angle (<109.5(, however, if the VSEPR equation is AX2E2, it is a even greater difference (<<109.5 degrees)

RJ Lopez 2l

What is the difference between pi bonds and delocalized pi bonds. Also are there localized and delocalized sigma bonds?

RJ Lopez 2l

Dr. Lavelle mentioned "Atom with lone pair - Spacer - Spacer - Atom with lone pair" during lecture applying to rotation. Does that mean that atoms that are mono dentate cannot rotate. What does "Atom with lone pair - Spacer - Spacer - Atom with lone pair" apply to?

RJ Lopez 2l

I am confused on when to use prefixes when naming compounds. How do you know when to us bi or bis, tri or tris, tetra or tetrakis, etc. I know it has something to do with ligand number, but I am a little confused on the conditions. Thank you

RJ Lopez 2l

I am confused as to why C H2 Cl2 is a polar molecule. I thought that if you put the chlorines on opposite sides and hydrogens on opposite sides that the dipoles would cancel, making a non polar molecule.

RJ Lopez 2l

Could someone explain how you determine whether atoms are on the same plane?
There was a question on achieve stating "The hydrogen atoms of H2CCH2 and H2CCCCH2 lie in the same plane." I was very confused as to why this is?
Thank you

RJ Lopez 2l

I saw this as a key point for chemical bonds outline and was wondering if I could get some clarification.
Understand and describe the parameters that contribute to the intermolecular energy E is proportional to -alpha 1 times alpha 2 / r6.
Sorry couldn’t type the Greek letters

RJ Lopez 2l

Do atoms with more bonds have greater dispersion forces or is it strictly dependent on size. There is a question in achieve using different variations of CH2 and ch3. Order the following molecules from greatest to least:
CH3 (CH2 x 5) CH3 and CH3 (Ch2 x 4) Ch3 and Ch3 C(CH3)2 CH3

RJ Lopez 2l

Do atoms with more bonds have greater dispersion forces or is it strictly dependent on size. There is a question in achieve using different variations of CH2 and ch3. Order the following molecules from greatest to least:
CH3 (CH2 x 5) CH3 and CH3 (Ch2 x 4) Ch3 and Ch3 C(CH3)2 CH3

RJ Lopez 2l

When forming resonance bonds with carbon dioxide, CO2, how many combinations are there? Would it include two double bonds, a single and a triple bond, a triple and a single bond for a total of 3? Or are there more possibilities. How would one determine plausible resonance bonds?

RJ Lopez 2l

Could someone explain how to determine oxidation number and its relationship to formal charge. Im confused with achieve number #9 which uses a perchlorate ion. Thank you

RJ Lopez 2l

where in the H-atom spectrum does one expect to see n = 4 to n = 3 transitions

He told us no calculations were required, but I am still confused on how he got his answer. Based on the diagram he gave us, wouldn’t it be visible since the next line is N=2 which is the Balmer Series

RJ Lopez 2l

The e.coli bacterium is about 2.3 μm long. Suppose you want to study it using photons of that wavelength or electrons having that de Broglie wavelength. What is the energy E photon of the photon?The e.coli bacterium is about 2.3 μm long. Suppose you want to study it using photons of that wavelength ...

RJ Lopez 2l

All the information on UA peer learning sessions is posted on my class website at: https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/Chem14UA_PL_ALL.pdf There are no passwords to attend sessions, but maybe you are referring to a zoom link. Students should email the UA of a session i...

RJ Lopez 2l

Hi I wasn’t able to join the step up sessions because there was a password and I couldn’t find it anywhere. I plan on going to Julia Lok step up session today at 2. Does anybody know the password.

Also where do we find the information about step-up sessions. Thank you

RJ Lopez 2l

Calculate the wavelength, in nanometers, of the spectral line produced when an electron in a hydrogen atom undergoes the transition from the energy level n=7 to the level n=1 I use the equation given on “feedback” but I can’t seem to get the right answer. What is the correct equation I am supposed t...

RJ Lopez 2l

How do we determine electron spin? For example what is the set of quantum numbers for a 2px. Also if given 3,2,0,-1\2 what does this correspond to and what does 3,2,0,1\2 correspond to?

Thank you

RJ Lopez 2l

Where do we find the workshop worksheets? I’ve checked both ccle and Lavelle website and cannot find them

RJ Lopez 2l

Are we supposed to have the correct sig figs for each step of the question or just the answer? Also what about constants we are given such as Avogadro number

RJ Lopez 2l

I’m having a hard time trying to manipulate this equation I saw in the text book, could someone explain it pls

hv = work function = wavelength = ch/work function

RJ Lopez 2l

If we are given the wavelength in a spectrum and we are asked to calculate the values of n for the beginning and ending energy levels, how do determine n1? I understand it is based on the Balmer and Lyman series where n1=1 and n1=2 respectively, but I'm not sure how we get n1 just by being provided ...

RJ Lopez 2l

According to my TA, he said to be prepared for some conceptual questions along with the standard problems

RJ Lopez 2l

How does weins law apply to the Schrodinger equation?

RJ Lopez 2l

How do we work backwards with the rydberg equation? Like how do we Determine the values of n for the initial and final energy levels of the electron during the emission of energy that leads to this spectral line give a wave length of 102.6. How would you rearrange the varaiables

RJ Lopez 2l

Is there a way to check your number of posts and whether you got replies?

Thank you!

RJ Lopez 2l

what is a key identifier to use this equation? In lecture I know that we were able to derive this equation by combining E=pc and E=hv. Is it only used when an object is moving with momentum?

RJ Lopez 2l

Identify the neutral element represented by this excited‑state electron configuration, then write the ground‑state electron configuration for that element.

excited state: 1s2 2s2 2p1 3s1

RJ Lopez 2l

How do we determine ionic radius? for example, how would you find cl- or p3-

RJ Lopez 2l

What is the minimum uncertainty in an electron's velocity (delta v min) if the position is known within 11 A

RJ Lopez 2l

When doing the hw, I saw the units for kinetic energy of an electron in a beam is eV. How do we get these units from the equation? I got joules instead.

RJ Lopez 2l

The electron in a hydrogen atom is excited to the n=5 shell and emits electromagnetic radiation when returning to lower energy levels. Determine the number of spectral lines that could appear when this electron returns to the lower energy levels, as well as the wavelength range in nanometers. how do...

RJ Lopez 2l

What was the unexpected outcome of the photoelectric effect experiment?

RJ Lopez 2l

Could someone how to take a schrodinger equation and turn it into a wave function? How do we calculate probability from a wave function?

Search found 104 matches