CHEMISTRY COMMUNITY

rachelle1K

Dr. Lavelle has been great with updating us on all the adjustments he has been working hard to make, so check your email and you'll find all the info from him. Everything has been moved to online and we will be taking the final exam through CCLE.

rachelle1K

It tells you that it is a first order reaction, and therefore you know to use the equation ln[A] = -kt + ln[A]0
Make sure that you molarity is in relation to the reactant, and then you just plug in what you know to solve for k, which is the constant.

rachelle1K

whenever there is no other conducting solid, Pt is usually used.

rachelle1K

The main reason OH would be in a chemical equation would be for basic solutions. In general always use H2O to add oxygen. If it is a basic solution, what I usually do is wait until everything is balanced according to an acidic solution, then I add an OH for every H+ on both sides, and cancel out the...

rachelle1K

No printing needed! Just log on to CCLE and everything will be accessible there, starting at 11:15 am.

rachelle1K

Depends on the TA, some might leave them in mailboxes for pickup or schedule a time when you can get it from them.

rachelle1K

Probably Electrochemistry, initially I thought cell diagrams were impossible to understand but after learning them I finally began to enjoy it lol

rachelle1K

Dr. Lavelle also states during lecture that catalysts are not formed, but used up. Therefore, a reactant would be a good example of this since it is not the product of the reaction.

rachelle1K

Are reaction rates only determined by the reactants of the slow step?

rachelle1K

Which method did Dr. Lavelle say we should be using? Pre-equilibrium or Steady-state?

rachelle1K

If lectures are supposed to be online, where will we find it? Will they be Bruincasted?

rachelle1K

7B.9 For the first-order reaction A -> 3B + C, when [A]0 = 0.015 mol.L-1, the concentration of B increases to 0.018 mol.L-1 in 3.0 min. (a) What is the rate constant for the reaction expressed as the rate of loss of A? For part a, I did .018/3 = .006 to get the amount that C would increase as well, ...

rachelle1K

Hi, could you please include the entire problem in your posts, both this and future posts? My copy of the textbook does not appear to have a 5A.17 Sorry! I meant 7A. 17 The following data were obtained for the reaction A + B + C -> products, with A in first order and B and C in second order. It giv...

rachelle1K

For 5A.17, I used the formula to calculate for k. However, my answer is 2.85 while the book says it is $2.85\times 10^{12}$
Is there some conversion that I am missing in this problem?

rachelle1K

Nevermind! I just watched this video
https://www.youtube.com/watch?v=1g-vDSWYins
which really helped me understand, we are supposed to use $K= M^{1-n}.t^{-1}$

rachelle1K

7A.7 says Express the units for rate constants when the concentrations are in moles per liter and time is in seconds for (a) zeroth-order reactions; (b) first-order reactions; (c) second-order reactions.

Is this just memorization? Or do we use $Rate = constant \times[conentration]^{a}$ ?

rachelle1K

6N.13 Calculate the reaction quotient, Q, for the following cell reactions, given the measured values of the cell potential. Balance the chemical equations by using the smallest whole-number coefficients. (a) Pt(s) Sn4+(aq),Sn2+(aq) Pb4+(aq),Pb2+(aq) C(gr), Ecell = +1.33 V. I'm having a little troub...

rachelle1K

6N5.a asks for the PH in the equation (a) Pt(s) H2(g, 1.0 bar) H+(pH ?) (salt bridge) Cl2(aq, 1.0 mol.L-1) Hg2Cl2(s) Hg(l), Ecell = +0.33 V. Using the Nerst equation, I came up with 0.33 = .27 - (.05916/2)(log(1/ x^{2} )) After solving I found the pH to be 10, yet the answer is 1. What am I doing wr...

rachelle1K

Justin Sarquiz 2F wrote:I think the book made a mistake because I got K=107. The book said that 2 electrons were transferred, but it should be that one electron was transferred.

I also got K=107

rachelle1K

I was looking at this question in other chemistry posts, and someone said we have to use the equation ln(K2/K1) = (deltaH/R)[(1/T1) - (1/T2)] Though I am not really sure how to apply it.

rachelle1K

part d of 6M.13 asks us to calculate the cell potential for 2 NO3-(aq) + 4H+(aq) + Zn(s) --> Zn2+(aq) + 2 NO2(g) + 2H2O(l) To calculate this, I found the reduction potential of NO3- (0.96) and added it to the oxidation potention of Zn (0.76). However, that gives me 1.72V which is different from the ...

rachelle1K

To determine which one is the cathode or anode, look at the cell potentials of each half-reaction from appendix 2b. The one with the higher cell potential is the cathode. In this case, NO3 -/NO is the cathode because .96 > .79.

rachelle1K

Will content from 5G be on Test 2?

rachelle1K

Are there different techniques we should use for balancing a redox reaction in bronsted neutralization and solubility?

rachelle1K

6L.7 Part C asks to balance the redox reaction in a nickel-cadmium cell as follows:
Cd(s) + 2 Ni(OH)3(s) -> Cd(OH)2(s) + 2 Ni(OH)2(s)
How do we know to assume that it is occurring in a basic solution?

rachelle1K

For G = -nFE, how should we calculate n? Is it just the change in moles (n final - n initial)? Thanks in advance!

rachelle1K

Part d asks to balance the redox reaction of Cl2(g) -> HClO(aq) + Cl2(g) . In the answer key, the product is Cl- . When would we know that it is okay to change the products?

rachelle1K

What is the difference between H and q? When would they be equal?

rachelle1K

Will the midterm include questions and problems from topic 4J?

rachelle1K

4I.3 states The standard entropy of vaporization of benzene is approximately 85 J.K-1.mol-1 at its boiling point. (b) What is the standard entropy change of the surroundings when 10. g of benzene, C6H6, vaporizes at its boiling point?

Why is the answer to part b negative?

rachelle1K

4C.13 says, "An ice cube of mass 50.0 g at 0.0 8C is added to a glass containing 400.0 g of water at 45.0 8C. What is the final temperature of the system?" I was wondering why, when calculating the heat absorbed, do we add (50g)(4.184J.°C-1.g-1)(Tfinal) to (n)(deltaH) ? I was solving the p...

rachelle1K

I just looked it up and there is a chart in the textbook which explains that the pressure should be in Pa, and volume in m^3, so no conversions need to be done for this this equation.

rachelle1K

For $w=-P\Delta V$ , are we expected to memorize that there are 101.325 J in 1L.atm ?

rachelle1K

When would the change in entropy of the surroundings be 0?

rachelle1K

In the homework problem 4B.7, the answer for work is -1626. However, I got 1626. When should I know to make work negative?

rachelle1K

The purpose of using 101.325 is to convert atm to Pa. In SI units, Pa is J.m^-3 . The equation used is w=PAD, and work is in joules, so the units would essential cancel out to joules because w=(J.m^3)(m^2)(m) .

rachelle1K

No need to memorize, the number is on the constants and equations sheet.

rachelle1K

qv implies constant volume, whereas qp is constant pressure.

rachelle1K

Should we always convert to J (using the 101325 Pa value) on exams? Correct me if I'm wrong, but I don't recall hearing that in lecture? I didn't hear him talk about it either in lecture, but it makes sense because work is equal to J and you have to make sure the units work out. Pa is equal to J.m^...

rachelle1K

I did this problem yet I keep getting my final answer as 28.6, which would round up to 29. The answer says it' supposed to be 28. Has anyone else been getting the same thing?

rachelle1K

In homework problem 4A part c, the answer key states that a bomb calorimeter in which benzene is burned is an isolated system. Can someone explain this to me?

rachelle1K

5.39 states, In an experiment, 0.020 mol NO2 was introduced into a flask of volume 1.00 L and the reaction 2 NO2(g) ∆ N2O4(g) was allowed to come to equilibrium at 298 K. (a) Using information in Table 5E.2, calculate the equilibrium concentrations of the two gases. (b) The volume of the flask is re...

rachelle1K

Only the stoichiometric coefficients and temperature can change K. Concentrations do not effect K, because a change in concentration only mean that the reaction has to adjust to move back to equilibrium.

rachelle1K

Kc can also be used with gases, I think in the problem they will specifically ask for Kc/Kp, otherwise, Lyndon told us in a workshop that Kc is usually used.

rachelle1K

005391550 wrote:Do you mean (1.0x10^-14)/3.1? Because I'm not sure where you got the 3.2x10^-14

Thanks! Yeah I meant that, I just fixed the post.

rachelle1K

I was trying to solve part c of 6A19, however, I got $(1.0\times 10^{-14})\div (3.1)=3.2\times 10^{-15}$ The book says the answer is $3.2\times 10^{-12}$ . Is anyone also getting the same thing? Or are my calculations wrong? Thanks in advance!

rachelle1K

Yes, it basically shows that a chemical reaction can adjust in order to minimize the effect and go back to equilibrium.

rachelle1K

Q in itself is the reaction quotient at any time, so initially, a large amount of product was added the reaction would continue in the reverse direction because Q > K.

rachelle1K

PV=nRT is a good formula that shows the relationship between pressure, volume, temperature, concentration, and moles. Increasing moles and temperature will increase the pressure.

rachelle1K

On the syllabus, it says we should review ideal gases. Does any have any tips on how we should do that? Thanks in advance!

rachelle1K

At equilibrium, are the concentrations of reactants and products the same?

rachelle1K

So on homework problem 6C 19 part c it asks for thee stronger acid between HBrO2 and HClO2. Can someone explain to me why the answer is HClO2? I thought that because bromium's electronegativity is lower is would be more willing to give up H and therefore be a stronger acid.

rachelle1K

The question asks for change in pH when HCL concentration is reduced by 12%. I saw a previous chem community post where they plugged in -log(.12) to find the answer, but why don't we plug in H+ (pH= -log([.88])) since it is asking for the value of the change?

rachelle1K

The pH equation is -log([H+]). The equation for change is -log(change in concentration / original concentration). In this case: -log(0.12*[HCl] / [HCl]). We don't need the actual value of [HCl], we can just assume it's 1 to represent the original value. Thus, we have -log(0.12) = 0.92 Why don't we ...

rachelle1K

Is there a difference between nitric acid and nitrous acid?

rachelle1K

Should we know how to name acids/ bases for the final? For example when to use -ic versus -ous in an acid?

rachelle1K

In question 2F 13, it gives the molecule CH2CHCN and asks us to find the hybridization of each carbon atom. To what extent do we take into account double bonds when determining hybridization? Are the shared electrons with double bonds spin paired?

rachelle1K

What are common acids and bases that we should memorize in order to come up with reactions?

rachelle1K

How can I determine the coordination number without drawing the Lewis Structure?

rachelle1K

How can one determine the number of sites to which a ligand can combine to create a polydentate?

rachelle1K

What is the difference between cyano and cyanido?

rachelle1K

If we are given SO4, do we automatically assume the need to create octets, even if it means adding a charge?

rachelle1K

How do I find the oxidation state of a molecule? Does it require drawing out the Lewis structure? Thanks!

rachelle1K

Can someone walk me through how to name $[Fe(CN)_{6}]^{4-}$ ?

I know that you start with naming the ligand before the TM, so I got to hexacyano, but how did iron become ferrate?

rachelle1K

So if nitrate is NO3-, then why is Zinc Nitrate Zn(NO3)2

rachelle1K

How do we know what nitrate is by just looking at the name? Is it just memorization?

rachelle1K

Is the reaction for HBr in water HBr(aq) + H2O(aq) ---> Br-(aq) + H3O+(aq)

In general, how do we know what to write for the reaction?

rachelle1K

Why is NH3 a bronsted base? Doesn't it provide protons?

rachelle1K

Two chemists walk into a bar. One tells the bartender, "I'll have an H2O." The other says, "I'll have an H2O too!" The second chemist dies.

Found this one online and it made me laugh a little bit harder than I should’ve.

rachelle1K

The first bond is always a sigma bond, all bonds following that would be a pi bond.

rachelle1K

After looking a bit more into it I find that it also can be the difference between electron geometry and molecular geometry. They are essentially the same thing, but this naming is easier for me.

rachelle1K

SBr4 is a nonpolar molecule because the central atom (S) is surrounded by 4 identical (Br) molecules that are equivalent in their electronegativity, so the dipoles cancel each other out, and as a result, there is no dipole-dipole interaction. The individual bonds can still be polar but the whole mo...

rachelle1K

Why does SBR4 not have a dipole-dipole interaction?

rachelle1K

If a molecule has resonance structures that would change the number of sigma and pi bonds, which one should we choose? An example where I found this was in SO2.

rachelle1K

What’s the difference between molecular shape and geometry? We talked a bit about it during discussion but I’m just confused on what extent we need to know for the exam. Thank you!

rachelle1K

Will we be going over the VSEPR formulas in class?

rachelle1K

Why are all the bond lengths identical in SOCL2? Doesn't Sulfur have a lone pair so that would repel the other atoms and create a difference in bond length?

rachelle1K

Do we have to memorize the nodal planes for each orbital?

rachelle1K

If a question asks for us to find a dipole moment for a lewis structure, should we draw the unpaired electrons?

For example, in the Dino Nuggets mini supplemental questions for 2a, the answer key does not draw the unpaired electrons.

rachelle1K

Why does the photoelectric effect tell us that light can act as wave when in the experiment frequency/ intensity of light did not play a role in removing electrons from the metal?

rachelle1K

How do you know when do differentiate if you should use the speed of light equation or De Broglie?

rachelle1K

Why do we sometimes multiply \Delta v by an integer? For example, in the Dino nuggets problem, it states that "Lyndon gets so angry at Matt's mistake that he yeets a rock at him that weighs 2.8g with a whopping speed of 373.23 \frac{+}{-} .34 m/s. What is the indeterminancy of position of the r...

rachelle1K

Why is the quantum number sequence {5,0,-1,+1/2} not possible? Also, how do I calculate $m_{\imath }$ to the electron's actual position?

rachelle1K

What are the different elements that do not require an octet?

rachelle1K

Hi! So in this problem, you know that you should use the Heisenberg equation in order to find uncertainty of speed of the electron. Since Delta p * delta x = h/4pi, and delta p can be substituted by m * delta v (velocity), you can rearrange and solve for the minimum uncertainty in a particle's velo...

rachelle1K

What does it mean that an electron drops between levels in the context of the Bohr Frequency condition?

rachelle1K

For this problem, why does the answer key give it in J when the question asks for watts?

rachelle1K

Yes. For example, Chromium has the electron configuration [Ar]3d 5 4s 1 instead of the expected configuration [Ar]3d 4 4s 2 . This fact came mainly from experimental observation so it's not entirely known why it is more stable, but since the 3d orbital is at a lower energy level than the 4s orbital...

rachelle1K

Why are half-filled shells more stable? Can $3d^{^{5}}$ an example of this because an electron will move from the 4s subshell to make the th 3d subshell half-filled?

rachelle1K

I was wondering what equation I should use for part b of 1.31? I tried to use K_{max}= hv-\phi but the value I get for (hv) is so small that my answer just ends up being the work function. This is the problem: ( and I used the 405 nm wavelength ) The agents want to use a handheld laser to illuminate...

rachelle1K

Will our midterm include chemistry bonds?

rachelle1K

I went to the fundamentals review session today, and most of what we did is go over Test 1. So if you are not able to attend another fundamentals review session looking over the concepts covered on Test 1 would be a sufficient substitute.

rachelle1K

I'm not sure how to write the electron configuration when it comes to 4s and 3d orbitals. For example, why does $Ga^{3+}$ end with $3d^{10}$ rather than $3d^{8}4s^{2}$ ?

rachelle1K

How do you tell the number of valence electrons by just looking at the periodic table?

rachelle1K

What is the significance of the number Z?

rachelle1K

Hello, I was wondering why electrons fill 4s before 3d? Is this an important topic to know for the midterm?

rachelle1K

that would be so helpful! my email is rbishay140@gmail.com

rachelle1K

I know professor Lavelle wrote out easier numbers for us to remember wavelength as it relates to a certain type of wave. If someone could include it below that would be very much appreciated! Also, how do we know to use Balmer or Lymer series based on wavelength?

rachelle1K

On homework problem 1A #3, why does speed not also decrease when frequency decreases? I combined equations to obtain \frac{c}{f}=\frac{h}{mv} , with f being frequency and v as velocity. Is it incorrect to combine the de broglie equation with speed of light equation? If so, what is the relationship b...

rachelle1K

According to the photoelectric effect experiment, light can act as a wave.

Wavelength and frequency have an inverse relationship. Amplitude is the height of a wave. Velocity in terms of light is c, the speed of light.
c= λ(wavelength) multiplied by v(frequency).

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