GCHEM clarifications

[sfoksn]

---

Members don't see this ad.

Question 1:

Do enzymes change specific rate constants for reactions? I thought rate constants were only able to be changed by temperatures.

Question 2:

There seems to be many questions about equipment usages in DAT Destroyer, yet their answers seem to conflict. Sometimes they will use buret and sometimes they will use pipet. What are the patterns?

Question 3:

A + B -> C
Which of the following would not affect the rxn rate of this irreversible rxn?
a. Decreasing A
b. Increasing B
c. Adding catalyst
d. removing C
e. increasing Temp.

I know the answer since I have the answer key, but I feel that their explanation is not very good. Can anyone tell me a way to get the right answer (d) logically?

Question 4:

Is there a shortcut to finding the number of constitutional isomers from a given formula? Ex. given C4H10O, how many constitutional isomers can be formed?

Sorry for a long list of questions.

I hope you guys can enlighten me.

Thanks in advance!

 

[UCB05]

Question 1:

Do enzymes change specific rate constants for reactions? I thought rate constants were only able to be changed by temperatures.

Question 2:

There seems to be many questions about equipment usages in DAT Destroyer, yet their answers seem to conflict. Sometimes they will use buret and sometimes they will use pipet. What are the patterns?

Question 3:

A + B -> C
Which of the following would not affect the rxn rate of this irreversible rxn?
a. Decreasing A
b. Increasing B
c. Adding catalyst
d. removing C
e. increasing Temp.

I know the answer since I have the answer key, but I feel that their explanation is not very good. Can anyone tell me a way to get the right answer (d) logically?

Question 4:

Is there a shortcut to finding the number of constitutional isomers from a given formula? Ex. given C4H10O, how many constitutional isomers can be formed?

Sorry for a long list of questions.

I hope you guys can enlighten me.

Thanks in advance!

1.

The assumption that rate constants are only affected by changes in temperature assumes that you're dealing with the same reaction kinetics. As you can see above, k is dependent on two more variables, Ea and A (a frequency-of-collision factor). Enzymes/catalysts will change k because they change the kinetics of the reaction and lower Ea. Since it is no longer the same reaction as the uncatalyzed rxn, k will change as well, even at the same temp.

2. Not too sure about this. My understanding is that pipettes are more accurate. Burettes are used more for larger scale titrations.

3. In a reversible reaction, the concentration of products and reactants compete and eventually establish an equilibrium level. If the reaction is irreversible, the concentration of the product does not matter at all. The formed product just sits there and looks pretty since it can't react in a reverse reaction mechanism. LeChat's principle only deals with equilibrium shifts when you have both a forward and reverse reaction. cut out half of that and all you have left is a forward reaction that only deals with the concentration of reactants, not products.

4. C4H10O (2C + 2 = 10) implies a saturated molecule, so you just have to bounce the O around on a single bond.
1-butanol, 2-butanol (3/4-butanol are the same as the first two), diethyl ether, methyl propyl ether, methyl isopropyl ether, 2-methyl-1-propanol, etc.

I don't think there's a quick way to do it. just find the constitutional isomers of C4H10 first, then put the O around in different spots. No double bonding though. that would be a degree of unsaturation.

 

[UndergradGuy7]

#2 a pipette delivers a specific amount. It only has 1 line marked on it. For example a 25 mL pipette will deliver 25 mL of a solution. You cannot measure any other amount from it. A burette has lots of marks on it. You can measure 20 mL, 20.2 mL etc.

A pipette is more accurate than a buret, I think? A buret is used during a titration when we add a titrant to a solution. We read the final mark of the solution and can tell how much liquid we used.

Question 3
The question says the process is irreversible. So no matter what you do to the product C it will not affect the rate. If you increase the concentration of C or decrease the concentration of C. The problem tells you that the reaction will only go one way.

 

[JBarr29]

you pretty much only use pipettes for titrating. i cant think of any other practical reason. burets deliver specific volumes at fairly high certainty.

 

[sfoksn]

Thanks for great input!