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      04-26-2010, 11:38 AM   #1
NSFW 335i
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Any chemist in here?

Hey this is one of my exam review questions and I cant seem to understand where to even start..

As a fresh naval engineer you want to construct a light ship to save fuel and decide to use magnesium, a very light but strong metal. You plan to test a small prototype in a large pool, but your unfriendly roommate and competitor has secretly dissolved colorless lead dinitrate in the water. You place your prototype in the pool and...the ship “melts” under your very eyes and disappears while a black metallic dust, later identified as lead by the FBI collects at the bottom of the pool.

a) Write the chemical equation that describes your observations
b) Calculate the standard free energy of the reaction and verify that the “melting” of
your ship was expected to occur spontaneously, as anticipated by your
roommate (who obviously knows her chemistry)
c) Based on the above information, demonstrate whether your prototype would
have “melted” if tested in the ocean (consider the ocean salinity: NaCl concentration is 3.5%, 25 C, no wind, no clouds, no roommate present)

anyone have an idea how to do it?
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      04-26-2010, 11:59 AM   #2
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No just BIO's
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      04-26-2010, 12:10 PM   #3
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a) Pb(NO3)2 + Mg = Mg(NO3)2 + Pb

That's all I've got. The last chemistry book/class I had was for strengths of materials 9 years ago.
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      04-26-2010, 01:18 PM   #4
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For the second part you have to know the dHf or dHc... I can't remember which way around it is if dHf = dHf(react)-(products), maybe the other way around... you should have it in your notes... google enthalpy equations... if the result is negative, the system is drawing heat from the surroundings, which could mean a spontaneous reaction, but you better calculate Gibb's free energy (dG = dH-TdS), that, when negative, will mean a spontaneous reaction.

For c, do the same thing, just plug in different numbers based on the equation
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      04-26-2010, 02:03 PM   #5
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Thanks a lot guys... my prof gave me 30 word problems... I did 25 of them now, got 5 more i have to work on. I really want an A on the exam haha.
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      04-27-2010, 11:12 AM   #6
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A mixture of solid KI and KBr is melted at 200 C, two Pt electrodes are immersed into the hot liquid and connected to a battery. As the potential is lowered gradually between 0 and - 5V you observe the appearance of gas A, followed by the appearance of gas B. What is their color?

having trouble with this one... help...
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      04-27-2010, 07:03 PM   #7
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GL op...if you show your work I may help you along.
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      04-27-2010, 08:17 PM   #8
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wow....i feel like an idiot..it has been 20+ years since any chemistry classes for me (4.0 science average)...I would love to help...but can't...that ship has come and long gone
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      04-28-2010, 10:22 AM   #9
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Just let him know you knew of your roommates sneaky ways to phuck up your project and took preventative measures by coating your hull with Plasti-Dip. The ship did not dissolve and you were victorious. Also let him know you made him an iced tea with the bath water....
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      04-28-2010, 03:32 PM   #10
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It's been a while so i'm rusty...

Here's how I would approach the problem:

sysfailur has already answered the first part, so i'll try the second and third.

List out the free energy of these reactions:

Mg+NO3=Mg(NO3)2 delta G: (look it up on wiki or something)
Pb+NO3=Pb(NO3)2 delta G: (your text book might have a table listing these)

now in the pool there's two things happening, the lead dinitrate bond is breaking, and the magnesium dinitrate bond if forming. obviously one will take energy to break and one will give off energy by forming. So take those two values and just add them together, (the energy for the forming equation should be a negative value because it's giving away energy, and the breaking should be positive because it probably requires energy). Your answer should be a negative number, because it is stated that the reaction is spontaneous, it would not be spontaneous if you need energy to be inpoted into the system.

for the last part, the answer is yes. NaCl in water are ions, they're not even covalently bonded. So the Mg will interact with the Cl. Your boat will be reduced. BTW MgCl is use to make energy in some rudimentary batteries, the free energy of this equation is use to generate electricity.
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