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Author Topic: VCE Chemistry Question Thread  (Read 2313014 times)  Share 

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Reus

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Re: VCE Chemistry Question Thread
« Reply #885 on: May 19, 2014, 07:57:34 pm »
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I know we are required to know Chloroalkanes, but what about Chloroalkenes?
I presume it won't make a difference in theory except the alkyl group...
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #886 on: May 20, 2014, 10:21:11 pm »
+1
Hey can someone check if my understanding of the Collision Theory and how to increase reaction rates is accurate? Please feel free to edit/add/remove anything that is relevant to the VCAA course. Thanks!


Increasing concentration of reactants/ increasing pressure of gaseous reactant molecules:
By increasing the concentration of reactants, there is an increase in the number of collisions between reactant molecules. As a result of the increased collision rate, there is an increase in the rate of the overall reaction.

Adding a catalyst:
Catalysts lower the minimum quantum of energy required to disrupt bonds within reactant molecules. In the presence of a catalyst, the activation energy is lowered. As a result, a lower amount of energy must be absorbed by the bonds in the reactant molecules in order for the bonds to be broken. Once this has occured, the molecules which have absorbed the quantum of energy needed to activate the reaction collide with one another, increasing the rate at which the reaction occurs as collision rate increases.

Increasing temperature:
By increasing temperature, the amount of kinetic energy available also increases. As a result, molecules collide more frequently with one another. As the collision rate increases, the overall rate of the reaction also increases.

Increasing surface area of solids:
As surface area increases, the amount of the solid molecules exposed to other molecules' surfaces increases. As a result, there is a greater number of collisions between molecules at their surfaces. The increased collision rate leads to an increase in the overall reaction rate.

Help would be much appreciated. Thank you :)

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #887 on: May 20, 2014, 10:50:09 pm »
+2
Hey can someone check if my understanding of the Collision Theory and how to increase reaction rates is accurate? Please feel free to edit/add/remove anything that is relevant to the VCAA course. Thanks!


Increasing concentration of reactants/ increasing pressure of gaseous reactant molecules:
By increasing the concentration of reactants, there is an increase in the number of collisions between reactant molecules. As a result of the increased collision rate, there is an increase in the rate of the overall reaction.

Adding a catalyst:
Catalysts lower the minimum quantum of energy required to disrupt bonds within reactant molecules. In the presence of a catalyst, the activation energy is lowered. As a result, a lower amount of energy must be absorbed by the bonds in the reactant molecules in order for the bonds to be broken. Once this has occured, the molecules which have absorbed the quantum of energy needed to activate the reaction collide with one another, increasing the rate at which the reaction occurs as collision rate increases.

Increasing temperature:
By increasing temperature, the amount of kinetic energy available also increases. As a result, molecules collide more frequently with one another. As the collision rate increases, the overall rate of the reaction also increases.

Increasing surface area of solids:
As surface area increases, the amount of the solid molecules exposed to other molecules' surfaces increases. As a result, there is a greater number of collisions between molecules at their surfaces. The increased collision rate leads to an increase in the overall reaction rate.

Help would be much appreciated. Thank you :)

Increase temperature => increase average kinetic energy of molecules => increase proportion of molecules that have more energy than activation energy to react => increase reaction rate. This is the second effect of temperature and is MUCH more important. In fact, the dependence on particle speed with temperature is v^2 proportional to T (for an ideal gas), but the relationship between the proportion of particles with more energy than the activation energy Ea is e^(-Ea/RT). See how important T is?

Fun fact: you can actually have a reaction that slows down if temperature is increased. Doesn't occur for many reactions but it happens :P

Also, solids aren't always molecules. Often, they're ions like in sodium chloride or even ammonium nitrate. Of course, some are, like iodine, ice and sulfur. I'm being picky here :P

Bear in mind that catalysts sometimes disrupt bonds themselves. Don't say quantum of energy as the use of quantum implies here that you can only absorb discrete multiples of a certain amount of energy. (ironic how the public uses "quantum" to mean large. Uninformed people)
You should then say that there are more particles with energy greater than the activation energy now that it has been lowered, instead of the bolded sentence.

I know we are required to know Chloroalkanes, but what about Chloroalkenes?
I presume it won't make a difference in theory except the alkyl group...

No difference really, except for iffy cases when you have isomerism about the double bond. There is a reason why alkenes aren't mentioned much in VCE :P
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Saikyo

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Re: VCE Chemistry Question Thread
« Reply #888 on: May 20, 2014, 11:00:02 pm »
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Can anyone help me with this question:

A student titrated an aliquot of standard sodium carbonate solution with hydrochloric acid in a burette. State whether the concentration determined for the hydrochloric acid would be likely to be higher than, lower than or unchanged from the actual value if the student had previously washed with water, but not dried, the following apparatus:

a) the pipette used to deliver the aliquot of sodium carbonate solution
b) the burette.

I'm confused why the answer says for a) would have a higher concentration than actual value?? To my understanding, wouldn't the sodium carbonate solution from pipette be diluted from the water and therefore having a less concentration??

Also, for b) if I dilute the acid in burette, wouldnt the concentration be also less therefore bigger titre is needed? But yet the answer says lower?

I'm not sure what I'm saying is correct, so could any of you clarify or point out any thing wrong I said. I would appreciate that :)

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #889 on: May 20, 2014, 11:10:41 pm »
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Can anyone help me with this question:

A student titrated an aliquot of standard sodium carbonate solution with hydrochloric acid in a burette. State whether the concentration determined for the hydrochloric acid would be likely to be higher than, lower than or unchanged from the actual value if the student had previously washed with water, but not dried, the following apparatus:

a) the pipette used to deliver the aliquot of sodium carbonate solution
b) the burette.

I'm confused why the answer says for a) would have a higher concentration than actual value?? To my understanding, wouldn't the sodium carbonate solution from pipette be diluted from the water and therefore having a less concentration??

Also, for b) if I dilute the acid in burette, wouldnt the concentration be also less therefore bigger titre is needed? But yet the answer says lower?

I'm not sure what I'm saying is correct, so could any of you clarify or point out any thing wrong I said. I would appreciate that :)

So, in a), the pipette actually had less sodium carbonate than what the student thought. As a result, the student would have used less hydrochloric acid than they should have for the titration. As they would not have factored any issues in experimental technique into their calculations, they would assume the sodium carbonate concentration hadn't changed, so the number of moles of sodium carbonate, and thus hydrochloric acid, are both the correct values. However, the volume of hydrochloric acid has decreased, resulting in a higher calculated concentration than what it should have.

For b), a more dilute acid in the burette will require a higher titre volume than normal, yes. However, remember you're calculating the concentration of this HCl solution. You have a constant number of moles of sodium carbonate and thus HCl, yet your volume of HCl solution has increased due to this error. Therefore, the effect of this error is to decrease the calculated concentration of HCl.
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #890 on: May 20, 2014, 11:51:18 pm »
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Increase temperature => increase average kinetic energy of molecules => increase proportion of molecules that have more energy than activation energy to react => increase reaction rate. This is the second effect of temperature and is MUCH more important. In fact, the dependence on particle speed with temperature is v^2 proportional to T (for an ideal gas), but the relationship between the proportion of particles with more energy than the activation energy Ea is e^(-Ea/RT). See how important T is?

Fun fact: you can actually have a reaction that slows down if temperature is increased. Doesn't occur for many reactions but it happens :P

Also, solids aren't always molecules. Often, they're ions like in sodium chloride or even ammonium nitrate. Of course, some are, like iodine, ice and sulfur. I'm being picky here :P

Bear in mind that catalysts sometimes disrupt bonds themselves. Don't say quantum of energy as the use of quantum implies here that you can only absorb discrete multiples of a certain amount of energy. (ironic how the public uses "quantum" to mean large. Uninformed people)
You should then say that there are more particles with energy greater than the activation energy now that it has been lowered, instead of the bolded sentence.

Thanks so much for that :)

Rod

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Re: VCE Chemistry Question Thread
« Reply #891 on: May 21, 2014, 08:53:39 pm »
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Hey can someone check if my understanding of the Collision Theory and how to increase reaction rates is accurate? Please feel free to edit/add/remove anything that is relevant to the VCAA course. Thanks!


Increasing concentration of reactants/ increasing pressure of gaseous reactant molecules:
By increasing the concentration of reactants, there is an increase in the number of collisions between reactant molecules. As a result of the increased collision rate, there is an increase in the rate of the overall reaction.

Adding a catalyst:
Catalysts lower the minimum quantum of energy required to disrupt bonds within reactant molecules. In the presence of a catalyst, the activation energy is lowered. As a result, a lower amount of energy must be absorbed by the bonds in the reactant molecules in order for the bonds to be broken. Once this has occured, the molecules which have absorbed the quantum of energy needed to activate the reaction collide with one another, increasing the rate at which the reaction occurs as collision rate increases.

Increasing temperature:
By increasing temperature, the amount of kinetic energy available also increases. As a result, molecules collide more frequently with one another. As the collision rate increases, the overall rate of the reaction also increases.

Increasing surface area of solids:
As surface area increases, the amount of the solid molecules exposed to other molecules' surfaces increases. As a result, there is a greater number of collisions between molecules at their surfaces. The increased collision rate leads to an increase in the overall reaction rate.

Help would be much appreciated. Thank you :)
Your schools up to unit 4, aos 2 already ? :O
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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #892 on: May 21, 2014, 08:54:43 pm »
+1
Your schools up to unit 4, aos 2 already ? :O
doubtful. knowing Yacoubb, pretty sure he is personally up to it  :P
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Re: VCE Chemistry Question Thread
« Reply #893 on: May 21, 2014, 09:03:38 pm »
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Thanks so much for that :)

doubtful. knowing Yacoubb, pretty sure he is personally up to it  :P

How do you find the time Yacoubb! I struggle with the regulated AoS homework we're doing right now!  ::)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #894 on: May 21, 2014, 09:04:17 pm »
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Your schools up to unit 4, aos 2 already ? :O

That's aos 2? It's certainly listed first in my chemistry textbook before equilibrium. At least, I've run through/am running through these with my chem students before the equilibrium bit.
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thushan

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Re: VCE Chemistry Question Thread
« Reply #895 on: May 21, 2014, 09:15:32 pm »
+1
Your schools up to unit 4, aos 2 already ? :O

Nah, that's U4AOS1. U4AOS1 is reaction kinetics, equilibria and industrial chemistry.
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Re: VCE Chemistry Question Thread
« Reply #896 on: May 21, 2014, 09:55:26 pm »
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Probably a silly question, but just wanted to clarify something that only occured to me then...

are haloalkanes (eg chloroalkane) part of the alkane homologous series or are haloalkanes a completely separate homologous series? Going by the chemistry definition of homologous series:having the same functional group but differing in composition by a fixed group of atoms. This would suggest that chloroalkanes are a homologous series itself, and then say, bromoalkanes are another one...
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Re: VCE Chemistry Question Thread
« Reply #897 on: May 21, 2014, 10:07:06 pm »
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Nah, that's U4AOS1. U4AOS1 is reaction kinetics, equilibria and industrial chemistry.
Our school is only up to mass spec lol. But we did organics (aos 2) first.

Thushan how would you rate unit 4? Smaller in length? Larger? Harder? Easier? Than unit 3.

I have all my unit 4 sacs in the same term.
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Re: VCE Chemistry Question Thread
« Reply #898 on: May 21, 2014, 10:13:59 pm »
+1
Smaller, more conceptual and less memory, and easier.

Check out the vTextbook videos for Unit 4 - I gave them :D
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #899 on: May 21, 2014, 10:19:04 pm »
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How do you find the time Yacoubb! I struggle with the regulated AoS homework we're doing right now!  ::)

Haha my school just finished unit 3. I'm up to area of study 2 now! V-textbook is AWESOME!

I really just find spare time (i.e. when I don't have SACs) to just go ahead. I'm aiming to finish the VCAA Chem course by the start of term 3, and dedicate all of term 3 to prac exams/revision of units 3 and 4 of Chem.