More DAT Chem Questions (LOTS of questions)

[dane4695]

A few questions... I really hope this isn't because I'm really tired and burnt out.
The data shown below were obtained frmo the following reaction:
A+2B->2C+D
Trial-----[A]-----------Rate
1--------1--------1-------.65
2--------2--------1-------1.30
3--------2--------.5------.65
4--------.5-------2-------.65
What is the rate law?

A) k[C]^2[D]/[A]^2 is the "correct" answer, but shouldn't it really be

k[A]

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Catalysts have which of the following effects on a chemical reaction?
A) They lower Activation Eneergy
B) They increase free energy
C) They cause the reaction to proceed Spontaneously
D) they increase the rate at which product is formed
E) They shift eqil. so that the reverse reaction proceeds more easily.

So the answer I put was A, which i'm almost positive is correct. However, the book says B. Also, couldn't D technically be correct, as catalysts increase the rate of reactions?

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In which of the following are the atoms arrange by increasing atomic size
A) Be<Na<Mg
B) Be<Mg<Na
C) Mg<Na<Be
D) Mg<Be<Na
E) Na<Be<Mg

It says A, but I think it's B. Ataoms increase size from Right to left and top to bottom.

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Someone aready mentioned the 4.00 x 10^-8 g C_3_H_8_


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An Alloy contains atoms of:

I won't even write the whole answer. The one they say is correcct is "one elemeent and has the properties of a nonmental"

Shouldn't it be that it's of 2 atoms with metalic properties? Or nonmentalic? I'm 99% sure it's not just 1 metal though.


I have a few more questions but i'll just wait to see if I'm just tripping on these ones.

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[mg777]

yea i noticed those too.
now i know they are wrong. i cant beleive barron's made so many mistakes in this section. i dont know if its even worth doing the othe practice test/

 

[mg777]

for that one on atomic size. is atomic size the same thing as atomic radius? i noticed barron;s has them listed by order of the atomic mass. however if we rank them by atomic radius the answer is different than what barron's has in the back.

 

[blumnday99]

Are those questions from the Barron's book you can buy at any bookstore? I was going to do those tests for more practice after my Kaplan ones.

Mark

 

[ems5184]

1) For this one I would agree that the rate = k[A]. However, if the answer has what you listed, are you sure they werent asking for the equilibrium equation (Keq)?

2) This one is a bit trickier. A is not correct because while they appear to lower the activation energy, this is deceiving. Catalysts provide an alternate mechanism which requires less activation. Note this is different than using the same mechanism with a lower Ea. C is not true because if there is some amount of Ea it is really not a spontaneous reaction. D is not true because a property of a catalyst is that it speeds up the rates for both the forward and reverse rxns so you can't say that more product is formed. E is not true because the equilibrium is not changed, just the rate. So, by process of elimination it has to be B that is correct.

3) I think you were just getting tired at this point, this is a simple one. Just glance at the periodic table. Be is atomic number 4, Na is 11, Mg is 12. By this you know each is larger than the previous one so the order has to be Be, Na, Mg which is choice A.

4) Answered in a different post.
http://forums.studentdoctor.net/showthread.php?t=136209

5) I would agree with you here, as far as I know, an alloy is a combination of 2 or more metals.

Any comments/criticisms are appreciated in regards to my solutions to these. Let me know if this helps dane.

 

[dane4695]

It says "increasing atomic size" i listed that verbatum. IS atomic size thus the atomic mass? I would think size would indicate radius.

Anyone else want to confirm our questions?

 

[ems5184]

well by atomic size, Be has a 2s orbital filled. Na and Mg both have 3s orbitals so these are obviously bigger. Since Mg has an extra proton and electron, there will be a little more shielding and the atomic radius will be slightly bigger, so I still think A is right for this one.

 

[blumnday99]

well by atomic size, Be has a 2s orbital filled. Na and Mg both have 3s orbitals so these are obviously bigger. Since Mg has an extra proton and electron, there will be a little more shielding and the atomic radius will be slightly bigger, so I still think A is right for this one.

From what I understand, electrons within the same shell cannot shield each other. Since Mg has one more proton, the effective nuclear charge felt by the electrons will be more than the effective nuclear charge felt by the electrons in Na. Therefore Mg will be smaller than Na.

Is this correct?

 

[mg777]

atomic radii inceases to the left in the periodic table, and down a group

 

[mg777]

catalysts lower activation energy

 

[sinned]

atomic radii inceases to the left in the periodic table, and down a group

correct, but kaplan was teaching me that an atom with a bigger electron cloud was the correct answer...even though those electrons are more tightly pulled towards the nucleus...

so now I wanna know what the real DAT goes by...the electron cloud or the trend for atomic radii?

 

[sinned]

catalysts lower activation energy for both forward and reverse reactions, and lets the reaction proceed faster...because if you draw the energy diagram, if you lower the Ea for forward rxn, then the reverse Ea gets lowered as well...

so I would say A,D,E

 

[blumnday99]

correct, but kaplan was teaching me that an atom with a bigger electron cloud was the correct answer...even though those electrons are more tightly pulled towards the nucleus...

so now I wanna know what the real DAT goes by...the electron cloud or the trend for atomic radii?

Isn't the electron cloud what determine the atomic radii?

 

[sinned]

thats why he is asking that question...according to that then sodium should have the biggest atomic radii

 

[blumnday99]

thats why he is asking that question...according to that then sodium should have the biggest atomic radii

BUMP

 

[JayMiranti]

I dont think its whether they "go by" one method or another, both the electron cloud (sheilding) and the number of protons in the nucleus are factors in determining atomic radius.

And yes within the same orbital shielding cannot occur, so that is why as you go from left to right int he table, the atomic radius decreases, more protons are added and that just pulls the electrons in further, no shielding involved when you are talking about elements in the same row.

WHen you go down a column, the radius increases, since shielding occurs. Yes, protons are also being added as you go down, lots of protons, but this does not outwiegh the effects of shielding, and leads to a bigger atomic radius, even though there are more protons. SO it seems like shielding is the more signicant factor overall, but both are important.

 

[rocknightmare]

atomic radii is atomic size i thought or almost the same..

 

[drat]

To the OP: Size is usually the atomic radius. Across a period, Mg is smaller than Na. Down a column, Be is smaller than Mg. So I would say Be<Mg<Na is a good guess.

Also, I think you're right about catalysts lowering the activation energy. The free energy is the difference between the products and reactants, and a catalyst DOES NOT change the energy levels of these states. Therefore, the free energy stays the same.

I noticed the error about the rate law for that other problem, too. You have it under control.

drat!