|By Kewlkiwi102 (Kewlkiwi102) on Monday, April 26, 2004 - 06:04 pm: Edit|
Here is a question from the Kaplan AP Chem review book (Practice test 2, #19).
19. Which of the following statements is/are true about the half-reactions shown below?
I. As written, the standard potential for the overall reaction is -1.56 V.
II. As written, the overall reaction is spontantous.
III. The sign of the potential of the overall reaction indicated whether or not the reaction is spontaneous.
Ag --> Ag+ + e- Eo=-0.80 V
Zn+2 + 2e- --> Zn Eo=-0.76 V
A. I only
B. II only
C. III only
D. I and III only
E. II and III only
Their answer: C.
Their explanation: Roman numberal I is incorrect. It does not properly account for the fact that to balance the equation you need to multiply to top by two....II is incorrect because a negative Eo means it is nonspontaneous....III is a correct answer.
SINCE WHEN would the multiplication of the top half reaction change the Eo value? Isnt Eo an intensive porperty so it is unaffected by the multiplication?
Also in the same test, they said that the statement "The absolute temperature of a substance is equal to the average kinetic energy of its particles" is true! Again I ask, SINCE WHEN?! Arent they proportional?
Someone please correct me if I am wrong about these, but i think that these are major errors for a review book to make. STAY AWAY FROM KAPLAN AP CHEM!
|By Artic (Artic) on Monday, April 26, 2004 - 06:32 pm: Edit|
All the AP books have errors. My AP Barrons Chem book was overrun with little dumb errors, but all the books are worth the money to help. The real thing is to make sure there are no errors in the info, not the multi choice.
|By Blee731 (Blee731) on Tuesday, April 27, 2004 - 10:59 pm: Edit|
Anyways, Answer I. was wrong because the Eo potential is +.04V. It's cathode-anode or reduction-oxidation. -.76-(-.80)=.04V The explanation wasn't correct, but the book was correct in saying it was wrong.
Yes, I stress also that ALL review books have errors. However, I must say "STAY AWAY from Chem: The Easy Way or any books from The Easy Way series. The chem book had numerous errors, and I mean numerous. The explanations were horrible; a complete waste of money. I got it a couple years ago and it's now sitting in a corner. My 2002 AP Kaplan for Chem has errors; my friend's 2004 AP Princeton for Chem has errors; it's inevitable since they need to produce a book every year for the exam to make money.
|By Wrathofgod64 (Wrathofgod64) on Wednesday, April 28, 2004 - 04:19 am: Edit|
blee731, dont u add up reduction potentials, which would make the overall voltage -1.56.
|By Zzii (Zzii) on Wednesday, April 28, 2004 - 02:48 pm: Edit|
EDIT: Sorry, misread post.
|By Blee731 (Blee731) on Thursday, April 29, 2004 - 11:59 am: Edit|
No, you DO NOT add up the reduction potentials to get the overall voltage because one half-reaction is oxidation and the other is reduction! You must use the formulas above or switch around the reduction potential for the oxidation half-reaction. Read any AP Chem book.
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