MCAT: Chem

[DrLady]

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Hi, I thought it would be a good idea to make a thread to ask questions in chemistry section for MCAT as I have couple questions.

I have some dumb questions to ask. These probably are easy, but I am not getting them, for some reason. I was hoping if someone can help me with these following problems:

I was doing the end of chapter exams for chap 1 and 2 in Ek and I have few questions. In page 155 of EK chem, I don't understand question # 40, how did they derive the answer as D? I am a bit confused. I thought I understood the concept of rate laws in the chapter, but I had hard time with this prob.

Another question is regarding prob. 40, in chap 3. I don't understand the answer. I didn't get 'B' as my answer. For some reason, I thought that I had to multiply ONLY the enthalpy of Diamond by 2 and not both: Diamond's and Graphite's enthalpies.

Thanks! 🙂

btw, I am doing well in those discretes in the middle of chapter in the EK books, but as I do the end of chap. exams, I am suddenly doing horrible. Also, most of the time, I am down to two answer choices and I am always doubting my first instincts about a certain answer choice and then I don't choose it, hence I get the question wrong. It gets real frustrating!

 

[Bevo]

i haven't looked at those questions since I have my EK books in the car.

But I can understand the frustration. In verbal Im getting 1 problem wrong everytime I do a passage and for the last one I get it down to 2 answers and I always pick the wrong one. I come up with reasons why the right one is wrong in my mind but i cannot for the one I picked.

 

[DrLady]

I know what you mean, Imbebo. I also find reasons why I shouldn't trust my first instinct!


Oh and one more thing about EK chem. I feel that some of the discretes that are asked in the middle of chapter, esp with certain concepts aren't even explained. For example, there was a question on the London Dispersion forces and dipole-dipole interactions in chap. 1, I believe, but those weren't even explained till chap. 4 ( which is where I am right now) and also there are other concepts that aren't explained at all in the chapter. Also EK seems really brief in its chapter and am wondering if that is purposely done as if it is expected of us to know the basics. Oh well, was just wondering, coz I will just go ahead and reread those sections from TPR books. But does anyone else thinks the same way that EK is really brief?

 

[14931]

if u post the questions, i'll try and answer them

 

[DrLady]

Originally posted by immediatespring
if u post the questions, i'll try and answer them

Immy, it takes too much work to type them! am lazy this late at night! But thanks! 🙂

 

[efex101]

"I was doing the end of chapter exams for chap 1 and 2 in Ek and I have few questions. In page 155 of EK chem, I don't understand question # 40, how did they derive the answer as D? I am a bit confused. I thought I understood the concept of rate laws in the chapter, but I had hard time with this prob. "


If you go back to page 36 where it explains how to get rate laws from experimental data, you will see that they compare concentrations of reactants (one at a time) from two different experiments. If the concentration of X reactant is halved and the rate is also halved then that reactant gets and exponent of 1, same if it is doubled and the rate doubles then it gets exponent of 1. Looking at question 40 on page 155, I see that for experiment one and two the concentration of I is halved and looking down at the rate, the rate is also halved then I gets an exponent of 1. Looking at the various answer choices D is the only one where I has an exponent of 1. So this is nice because you do not have to find the other reactant's exponent, for the only possibly choice in regards to I is D, this saves some time.

 

[efex101]

"Another question is regarding prob. 40, in chap 3. I don't understand the answer. I didn't get 'B' as my answer. For some reason, I thought that I had to multiply ONLY the enthalpy of Diamond by 2 and not both: Diamond's and Graphite's enthalpies."


Yes, you have to multiply the enthalpy of diamond times two, but because now you have 2 moles of diamond, you are getting two moles of C02, hence when you calculate the enthalpy of C02 to graphite you have to multiply that enthalpy times two and change the sign from negative to positive because you are going from C02 to graphite, and the way the book has it is going from graphite to C02. So it looks like this:


2 x (C diamond + 02 ---- > C02 ) change in enthalpy = - 792
2 x (C02 ----> 02 + C graphite) change in enthalpy = + 788

Total change in enthalpy = -4

 

[DoctorWannaBe]

DrLady,

From the lectures I've done (1 of each book), I also feel that EK seems brief. I missed the same question about the London dispersion forces. I wonder if it explains many of these basic things later or if we are supposed to remember it from gen. chem. I have the TPR science review, but haven't looked at it yet. Does that book give more details about the basics?

 

[Mudd]

On question #40, you may want to try what is essentially the same standard method that everyone uses, but in an easier-to-view form.

Choose any two trials and divide one set of data by the other. I'll choose trials 1 and 2 (and place the4 larger rate over the smaller rate):

• Rate1/Rate2
  = 6 x 10exp-6/3 x 10exp-6 = 2
  
  [I-]1/[I-]2
  = 0.060/0.030 = 2
  
  [S2O82-]1/[S2O82-]2
  = 0.030/0.030 = 1

The 1 implies no change, so it is ignored. The question is reduced to "2 to what power equals 2?" The answer is the first power, so rate depends on [I-] to the first power (meaning it is first-order with respect to iodide.) Only choice D list a first-order relationship with iodide, so there is no need to go further.

This is the same method explained in words on page 36 of EK as well as in pretty much every review book and text book available. It might be easier to see as a simple math relationship than in words. Hoped this helped.

 

[Ramoray]

Originally posted by efex101
"Another question is regarding prob. 40, in chap 3. I don't understand the answer. I didn't get 'B' as my answer. For some reason, I thought that I had to multiply ONLY the enthalpy of Diamond by 2 and not both: Diamond's and Graphite's enthalpies."


Yes, you have to multiply the enthalpy of diamond times two, but because now you have 2 moles of diamond, you are getting two moles of C02, hence when you calculate the enthalpy of C02 to graphite you have to multiply that enthalpy times two and change the sign from negative to positive because you are going from C02 to graphite, and the way the book has it is going from graphite to C02. So it looks like this:


2 x (C diamond + 02 ---- > C02 ) change in enthalpy = - 792
2 x (C02 ----> 02 + C graphite) change in enthalpy = + 788

Total change in enthalpy = -4

hey DL, dont use so much math just think about thigns conceptually. Like Combusting Diamond gives off 792 total, and it takes 788 to make graphite so you gonna have 4 total left so -4. YOu hav eto times by 2 cause to make two moles u gonna have to combust twice as much. Dont get caught up in formulas just think about whats happening and trust that. I hate trusting 4mulas but eh. ok DL your supre smart im sure ur well on ur way to doing great!

 

[DrLady]

Thank You so much, efex ( for both questions ) and Mudd! Mudd, Thanks for that method. It really seemed to make it easier for me! I reread the question and looked at the answer choices again, and I now see where I made a mistake. I am going to go back and review that section, apparantly I don't have the rate laws down as I thought I had. TPR science workbook has more problems on these, so I will just go ahead and practice in that book.

DWB,
Yes, the TPR science review is more detailed and has more examples and it is great for basics. I just assumed that Ek would have more details but oh well. I am going to use both books for max. practice and reviewing.