CHEMISTRY COMMUNITY

Kelly Kiremidjian 1C

8.) In a 2nd order reaction, a reactant has a known half-life of 3.00 minutes and an initial concentration of 5.00 M. How much time (in seconds) will it take for the concentration of this reactant to drop to 2.00 M?

Could someone explain how they did this question?

Kelly Kiremidjian 1C

For this question I used the equation deltaS=nCvln(T2/T1) You need to find the moles first, however using PV=nRT. Everything is given besides moles, so you can solve for n: n=(1.00 atm)(53.48L)/(.08206)(303K) to get n=2.15 moles. You can then use n to solve for change in entropy, using Cv as 3/2R be...

Kelly Kiremidjian 1C

For this question I used deltaU=q+w Since the internal energy of a system decreased by 1763 kJ, deltaU=-1763. It also gives you the value for w, saying that the system did 1357 kJ of work. When a system does work its negative. so w=-1357. From there you can solve for q, and you should get q= -406 kJ...

Kelly Kiremidjian 1C · Re: Reaction Mechanisms on test 3? no the test only covers 15.1-15.6

no the test only covers 15.1-15.6

Kelly Kiremidjian 1C

My TA told us that test 3 would cover #1-39 from chapter 15

Kelly Kiremidjian 1C

my TA said it was #1-39

Kelly Kiremidjian 1C

my TA told us questions #1-39 will be covered on test 3!

Kelly Kiremidjian 1C

You would have to find delta G for both half reactions and then add them to find delta G of the entire reaction. Once you have that you can solve for E (standard potential of cell). This problem is very similar to 14.27 if you have the solutions manual

Kelly Kiremidjian 1C

When finding average rate of consumption of R, I know the equation is (- delta[R]/delta t) because the concentration of R would be decreasing during the reaction. But when stating the rate, is it always positive? Textbook question 15.3 part a has the rate of consumption of NO2 as a positive rate so ...

Kelly Kiremidjian 1C

Both of the reactants are first order because their concentrations are directly proportional to the rate increase factor.

Kelly Kiremidjian 1C

The normal rate of the reaction is the concentration of a reactant or product to be consumed or formed over a given time unit.

Half-life of a reaction is the amount of time needed for the reactant concentration to decrease by half its original concentration.

hope this helps!

Kelly Kiremidjian 1C

I think they put that just to distinguish that they were referring to moles of oxygen. I don't think we would have to put that in every problem

Kelly Kiremidjian 1C

Generally if you have a metal solid in your reaction you don't need to add the Pt(s). If you only have elements in the (aq) or (l) state you must add the metal conductor so electron transfer can occur.

Kelly Kiremidjian 1C

You don't have to switch the sign of Eº if when calculating Eºcell you always use Eºcathode-Eºanode.

If you prefer to use the method where you add the Eºcathode and Eºanode you must flip the Eºanode.

Kelly Kiremidjian 1C

the question states that the reaction is at 1200K

Kelly Kiremidjian 1C

yeah every week

Kelly Kiremidjian 1C

If a reaction goes in both directions, it is at equilibrium, and therefore it is reversible.

Kelly Kiremidjian 1C

Bond enthalpies are the least accurate way to calculate enthalpy of formation because they are averages

Kelly Kiremidjian 1C

He said we won't have to specifically derive any formulas but need to know how the formulas relate to each other and be able to substitute parts of equations when necessary

Kelly Kiremidjian 1C

yes, Dr. Lavelle announced in lecture today that there will be no class on Wednesday.

Kelly Kiremidjian 1C

For part b you calculate the delta S for changing temperature and then add it to your answer from part a.
so you do deltaS=nRln(t2/t1)
n= moles of both the argon gas and the neon gas

Kelly Kiremidjian 1C

I asked about this at a step-up session and the UA said we would not be expected to know that!

Kelly Kiremidjian 1C

For part C, it asks if the entropy of the system is primarily a result of changes in positional disorder or thermal disorder. The system here is the solution, so if you imagine potassium nitrate dissolving in water it makes logical sense that the entropy, or disorder of the system is mostly the pota...

Kelly Kiremidjian 1C

I got this too. I think it's a mistake in the answer key

Kelly Kiremidjian 1C

Thermodynamic stability occurs when a system is in its lowest energy state. Therefore, compounds with positive free energy are said to be unstable while compounds with negative free energy are said to be stable.

Kelly Kiremidjian 1C

I also got -2.74 kJ.mol-1. I think the there's a mistake in the answer key

Kelly Kiremidjian 1C

In #21, where do they get the value of Kb? Also can someone explain this question I'm confused what we are calculating

Kelly Kiremidjian 1C

In addition, when a system is irreversible, we can see it as expanding into a vacuum. This means that the ∆Ssurroundings will be equal to zero because there is no change of entropy in a pressure-less reservoir. However, this is very different from a reversible system, for which the ∆Stotal of the un...

Kelly Kiremidjian 1C

When looking at a heating curve, you have heat supplied on the x axis and temperature on the y axis. The plateau's represent the phase change. The first plateau is solid changing to liquid, and the second plateau is liquid changing to gas. Notice there is no temperature change during the phase chang...

Kelly Kiremidjian 1C

I think iron rusting would be exothermic since it gives off heat

Kelly Kiremidjian 1C

q is postitive is heat is added to the system and q is negative is heat is removed from the system.

Kelly Kiremidjian 1C

For part B, you must use the gas law, PV=nRT. In this problem we are given everything except for moles, so you can rearrange the equation to look like n=PV/RT. Then you can plug the given numbers in. n=(1.00 atm)(5.45 L)/ (0.08206)(273K) n=0.243 mol Now that you have moles of Nitrogen, you can multi...

Kelly Kiremidjian 1C

the standard enthalpies of formation should be in the back of the textbook in appendix 2A

Kelly Kiremidjian 1C

In #49, how do you know that PΔV is negative?

Kelly Kiremidjian 1C

hi, I was confused about this too but a TA explained it to me. Since there is no ammonia (NH3) in the desired reaction, the ammonia from the reactions with known enthalpies must be cancelled out. To do this, the ammonia must be on different sides of the two reactions it appears in. Since H2 appears ...

Kelly Kiremidjian 1C

Hess's law states that the change of enthalpy in a chemical reaction is independent of the pathway between the initial and final states. This is useful for us in calculating change in enthalpy's because we can then use reactions with known enthalpy's and manipulate them to find the enthalpy of the d...

Kelly Kiremidjian 1C

No. In this question, it says that the cooling system that surrounds the cylinder absorbs 974 kJ of heat. If the system that surrounds the cylinder is absorbing heat, this means that the cylinder is releasing this heat and that is why it is negative. Heat absorbed by the cylinder would be positive, ...

Kelly Kiremidjian 1C

For part a, you know that the vaporization of 0.579 moles of Ch4 requires 4.76 kJ of heat, so all you have to do is divide the amount of heat by the moles to find the enthalpy (kJ/mol). 4.76 kJ/0.579 mol=8.22 kJ/mol For part b it is very similar but instead of being given moles, you are given grams....

Kelly Kiremidjian 1C

ad trouble with this question too. Reading page 468-469 in the textbook really helped me. It says at the end of section 12.3: metals form basic oxides, nonmetals form acidic oxides, and the elements on a diagonal line from beryllium to polonium and several d-block metals form amphoteric oxides. Ther...

Kelly Kiremidjian 1C

Dr. Lavelle mentioned in lecture a quick way to do this problem. Since pressure is increasing and volume is decreasing. When that happens, you know that if there are more moles on the left, the reaction shifts right. If there are more moles on the right, the reaction shifts left. In this case there ...

Kelly Kiremidjian 1C

I had trouble with this question too. Reading page 468-469 in the textbook really helped me. It says at the end of section 12.3: metals form basic oxides, nonmetals form acidic oxides, and the elements on a diagonal line from beryllium to polonium and several d-block metals form amphoteric oxides. h...

Kelly Kiremidjian 1C

I believe what you learned in high school was right. A conjugate base should only have the different of one hydrogen

Kelly Kiremidjian 1C

Chapter 12 #39 asks: 12.39 Using data in Tables 12.1 and 12.2, place the following acids in order of increasing strength: HNO2, HClO2, NH3OH, (CH3)2NH2 Considering we won't have these tables to use during the exam, are we expected to be able to place acids in order of strength? If so, could someone ...

Kelly Kiremidjian 1C

Can anyone explain how to write the hybridization of a compound with a lone pair? Does hybridization rely solely on the amount of regions of electron density or just the number of bonds excluding lone pairs?

thanks

Kelly Kiremidjian 1C

oh I see. thank you!

Kelly Kiremidjian 1C

Hey Sabrina, so when you convert everything to molarity values you get 0.167 M of Carbon and .05 M of H20. Those are the initial amount for of C and H20 in the ice table. Since Carbon is a solid it does not have an effect on the equilibrium values. We know the equilibrium concentration of H2 is 0.04...

Kelly Kiremidjian 1C

how did you solve for x without knowing the value of k?

Kelly Kiremidjian 1C

Can anyone explain #29 from the Chemical Equilibrium Part 2 Post Assessment? Here it is 29. A researcher fills a 1.00 L reaction vessel with 1.84 x 10-4 mol of BrCl gas and heats it to 500 K. At equilibrium, only 18.3 % of the BrCl gas remains. Calculate the equilibrium constant, assuming the follow...

Kelly Kiremidjian 1C

yes each lone pair counts as another region of electron density

Kelly Kiremidjian 1C

By replacing the -CH- in Benzene with a Nitrogen atom the dipole moments no longer cancel so it becomes a polar molecule.

Kelly Kiremidjian 1C

Yes, this molecule has an odd number of electrons(19). So the two oxygens would both have octets. But there are not enough electrons for chlorine to have an octet. Chlorine then has one lone pair, and one single electron.

Kelly Kiremidjian 1C

Also remember that atoms with a similar number of valence electrons are more likely to equally share electrons and have a more covalent character. This is in contrast to ionic character, where one atom is monopolizing the electrons.

Kelly Kiremidjian 1C

Can anyone explain 3.49? I had the correct lewis structure but am not getting the formal charge correct. For part A, I had Formal Charge of Oxygen= 6 valence e - (1 lone pair +6/2) which gives a +2 charge, and the book says Oxygen has a +1 charge. For Nitrogen I had Formal Charge of Nitrogen=5 valen...

Kelly Kiremidjian 1C

Can anyone explain 3.9 from the homework problems? I am confused on what the question is asking for. Also for the given electron configurations
a.) [Ar] 3d7
b.[Ar]3d6

are these possible? Doesn't the 4s orbital need to be filled for to have an electron in 3d?

thanks

Kelly Kiremidjian 1C

Can anyone explain #59 from Chapter 2? Aren't the ions isoelectronic?

thanks!

Kelly Kiremidjian 1C

I had the same question....so you can determine wether the electron is in the excited state or not by seeing if it correctly spaced in the boxes?

Kelly Kiremidjian 1C

Hi!
I did electron configuration in high school using the 2p61, 2p62, 2p63 model... can someone please explain the x,y,z we are using in this class more clearly? thanks!

Kelly Kiremidjian 1C

For 32B you know Kinetic energy of the electron and you know the work function so you are able to solve for the energy of the photon. KE=4.200 x10^-19 J Work function=3.607 x 10^-19 J, which you know from the previous question because there was zero kinetic energy. So then you do: Energy(photon)=Kin...

Kelly Kiremidjian 1C

Can anyone help me with question 34 from this post assessment? I said C for #33 which was correct but couldn't figure out #34. Thanks! 33. Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 s-1 before it can emit an electron from its surface. Answer the following two ques...

Kelly Kiremidjian 1C

dividing by 246.48 g.mol^-1 gives the molar mass of the entire thing, and it just asks for oxygen so you multiply by 11 because there are 11 moles of Oxygen in the equation. Then after you convert to atoms

Kelly Kiremidjian 1C

I have the same question.Couldn't quite follow the solutions manual

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