Beagle

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Does anyone have answers for these ?s

1. Which of the following statementst about straight-chain alkanes is true?
A. As the molecular weight increases, the symetry of the molecules decrease
B. As the molecular weight increaes, the surface area per molecule decreases.
C. As mol. weight increases, dispersion forces between molecules become weaker
D. As the mol. weight increases, the boiling pt decreases
E. As the MW increases, dispersion forces between the molecules become stronger

2. What property do CH3CH2COOH, HCL, HCOOH, and HNO3 have in common?

A. they are all strong acids
B. they accept protons
C. They can all act as bronsted acids
D. They can all act as bronsted bases
E. They are nonpolar molecules

3. When a nonvolatile hydrocarbon is dissolved in benzene, the partial pressure of benzene over the resulting solution

A. is greater than the vapor pressure of pure benzene
B. Decreases as the mole fraction of benzene decreases
C. Is lower than the vapor pressure of pure hydrocarbon
D. Increases as the mole fraction of benzene decreases
E. Does not change


4. An excited electron drops from a high energy level, Ea, to ground level state, Eb. The wavelength of the photon emitted is:
A. h/(Ea-Eb)
B. (Ea-Eb)/h
C. (Eb-Ea)/h
D. hc/(Ea-Eb)
E. c(Ea-Eb)/h

5. Catalysts have which of the following effects on chemical reactions.

A. They cause the reaction to proceed spontaneously
B. they increase the available free energy
C. They increase the rate at which products are formed
D. They lower the energy of formation of the products

I am stuck--- I read in barrons that B was correct. However, C seems to be a good answer?? Or is rate not acted on with catalysts?. And D sounds good as well?


6. CaF2 is added to 0.1 M Ca(NO3)2 solution. at waht concentration of F- will CaF begin to precipitate. (Ksp of CaF2=
4 x 10^-11

A. 4 x 5 ^-5
B. 2 x 5 ^-5
C. 4 x 10 ^-10
D. 2 x 10 ^ -10
E. 4 x 10 ^-11

I have no idea how to approach this problem

7. The eq for this reaction
N204 (g) -----> 2 NO2 (g) delta H= 13.1 kcal/mol can be shifted to the right by
A. decreasing the volume
B. decreasing the pressure
C. decreasing the temp
D. two of the above?


Thank you
 

DesiDentist

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Originally posted by Beagle
Does anyone have answers for these ?s

1. Which of the following statementst about straight-chain alkanes is true?
A. As the molecular weight increases, the symetry of the molecules decrease
B. As the molecular weight increaes, the surface area per molecule decreases.
C. As mol. weight increases, dispersion forces between molecules become weaker
D. As the mol. weight increases, the boiling pt decreases
E. As the MW increases, dispersion forces between the molecules become stronger

E sounds like the best answer. This is for straight chain alkanes. The physical properties of alkanes is that the longer the chain gets there is an increase in surface area and boiling point.

2. What property do CH3CH2COOH, HCL, HCOOH, and HNO3 have in common?

A. they are all strong acids
B. they accept protons
C. They can all act as bronsted acids
D. They can all act as bronsted bases
E. They are nonpolar molecules

The definition for a bronsted acid is that it can donate a proton. All of the above look like they can donate a proton so I would say the answer is C.

3. When a nonvolatile hydrocarbon is dissolved in benzene, the partial pressure of benzene over the resulting solution

A. is greater than the vapor pressure of pure benzene
B. Decreases as the mole fraction of benzene decreases
C. Is lower than the vapor pressure of pure hydrocarbon
D. Increases as the mole fraction of benzene decreases
E. Does not change

Benzene is one of the most stable aromatic/aryl structures, hence its predominance in nature. If you are using a non-volitile hydrocarbon (one that doesn't react explosively) then ...i'm confused with this???


4. An excited electron drops from a high energy level, Ea, to ground level state, Eb. The wavelength of the photon emitted is:
A. h/(Ea-Eb)
B. (Ea-Eb)/h
C. (Eb-Ea)/h
D. hc/(Ea-Eb)
E. c(Ea-Eb)/h

What the heck is this? I know I saw this before but I was saying to myself like the DAT would never have this crap.

5. Catalysts have which of the following effects on chemical reactions.

A. They cause the reaction to proceed spontaneously
B. they increase the available free energy
C. They increase the rate at which products are formed
D. They lower the energy of formation of the products

Remember, catalysts, or biological catalysts (enzymes) only do one thing. Lower the activation energy thus increase the rate. Catalysts aren't used up and they don't help any reaction to occur that would not occur naturally.

I am stuck--- I read in barrons that B was correct. However, C seems to be a good answer?? Or is rate not acted on with catalysts?. And D sounds good as well?


6. CaF2 is added to 0.1 M Ca(NO3)2 solution. at waht concentration of F- will CaF begin to precipitate. (Ksp of CaF2=
4 x 10^-11

A. 4 x 5 ^-5
B. 2 x 5 ^-5
C. 4 x 10 ^-10
D. 2 x 10 ^ -10
E. 4 x 10 ^-11

I have no idea how to approach this problem

I'm with you on this one.

7. The eq for this reaction
N204 (g) -----> 2 NO2 (g) delta H= 13.1 kcal/mol can be shifted to the right by
A. decreasing the volume
B. decreasing the pressure
C. decreasing the temp
D. two of the above?

I think it has to do with Le Chatliers priniciple.

Thank you
 

vixen

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the last one is lechatliers principle....when you decrease pressure (or increase temp), you go to the side that has MORE moles.
 

vixen

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I *think* 3 is B...not sure though....I think its talking about partial pressures....if you have benzene + X....then the partial pressure for benzene is benzene/benzene + X....so then it would now benzene would have a lower # (its being divided by a larger #)....


This is just what I would think, I could be totally off :)
 

portlander

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1. Dispersion forces become weaker (c). I'm not sure why, I got this wrong on the diagnostic exam because I put that they would become stronger, but the key said (c).

2. Bronsted acids is correct.

3. The answer is (b) as the mole fraction increases, the partial pressure increases also. Volitile means a liquid that will evaporate quickly.

4. Oooh, I know this!! Well, E=hv (E is energy, v is wavelength, and h is planks constant) So wavelength equals energy over planks constant, so you can discount A and D. E is the change in energy, and since you are going from high to low, it should be Ea-Eb, so B is the correct answer.

I have no idea how to approach this problem.

7. The eq for this reaction
N204 (g) -----> 2 NO2 (g) delta H= 13.1 kcal/mol can be shifted to the right by
A. decreasing the volume
B. decreasing the pressure
C. decreasing the temp
D. two of the above?

Well, I think that decreasing the presure would push it to the right. I know that increasing pressure would push it the other way, (because there is 1 mole as compared to 2 on the right) so I assume that doing decreasing pressure would do the opposite. Anyone else??
 

wasabi007

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4) i think the answer to #4 is C instead of B...

portlander was right in that you should use planck's theory: E=hf or E=hV, where E is the energy of the electron, h is planck's constant, and f or V is FREQUENCY. Just remember that f=c/wavelength. so plug in this new value for f, and solve for wavelength. i believe you end up with
C: hc/(Ea-Eb) = wavelength.

7) answer is B....le chatelier's principle

when you decrease P of a system, your rxn will move in the direction where there are more moles of gas (right). decreasing V is the same as increasing P, so that would move the rxn to the left...decreasing T would also shift the rxn to the left because the rxn is endothermic (delta H > 0).
 

Viraj

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For #4
E= h*c/lambda
where E= Energy
h= Planck's const
lamba= wavelength
so according to the question :
4. An excited electron drops from a high energy level, Ea, to ground level state, Eb. The wavelength of the photon emitted is:
A. h/(Ea-Eb)
B. (Ea-Eb)/h
C. (Eb-Ea)/h
D. hc/(Ea-Eb)
E. c(Ea-Eb)/h


Ea-Eb= hc/lambda
this implies that Ea-Eb/hc=1/lambda
this implies that lambda= hc/Ea-Eb
Therefore answer to this question is gonna be choice D
I hope this solves the problem for all of you guys
 

Beagle

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Thanks everyone for your help!
I am still stuck on the catalyst question though.
Does anyone have an answer?

Thank you!!!!
 

DesiDentist

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i believe the answer to the "catalyst" question is C. Catalysts only increase the rate of the RXN, they don't affect anything else, hence my answer in red.

DesiDentist
 

Beagle

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Desi
Do you think increasing the available energy is wrong?
I swear I saw that in barrons and wondered if it was a mistake??
 

UBTom

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Well, all catalysts actually do is lower the activation energy required to get a reaction going... It does not increase available energy.

If one looks at the Gibbs free energy change of a reaction, written as negative-delta-G (don't know how to type the delta symbol) of a catalyzed vs. an uncatalyzed reaction involving the same reactants, it would be absolutely the same.

Desi is right-- a catalyst increases the rate of a reaction. The caveat is that it is still subject to equilibrium laws same as uncatalyzed reactions! :p

HTH!
 

wasabi007

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whoops...sorry...i put the wrong letter answer for #4...but my answer and process in coming to it was correct...i just correlated it with the wrong letter...