Login

Welcome, Guest. Please login or register.

March 29, 2024, 02:58:23 am

Author Topic: VCE Chemistry Question Thread  (Read 2313245 times)  Share 

0 Members and 2 Guests are viewing this topic.

lzxnl

  • Victorian
  • ATAR Notes Legend
  • *******
  • Posts: 3432
  • Respect: +215
Re: VCE Chemistry Question Thread
« Reply #60 on: January 07, 2014, 02:05:07 pm »
+1
I would probably work it out something like that as well, because for me, the first step, the conversion to chromium hydroxide, is relatively simple to see.

That's not in the textbook though; I do whatever method suits the question.
2012
Mathematical Methods (50) Chinese SL (45~52)

2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

2014-2016: University of Melbourne, Bachelor of Science, Diploma in Mathematical Sciences (Applied Maths)

2017-2018: Master of Science (Applied Mathematics)

2019-2024: PhD, MIT (Applied Mathematics)

Accepting students for VCE tutoring in Maths Methods, Specialist Maths and Physics! (and university maths/physics too) PM for more details

SunnyB

  • Guest
Re: VCE Chemistry Question Thread
« Reply #61 on: January 07, 2014, 03:39:13 pm »
0
what are you guys doing for holiday hw?

PsychoT

  • Victorian
  • Trendsetter
  • **
  • Posts: 177
  • Respect: +12
  • School Grad Year: 2014
Re: VCE Chemistry Question Thread
« Reply #62 on: January 07, 2014, 04:46:09 pm »
0
I would probably work it out something like that as well, because for me, the first step, the conversion to chromium hydroxide, is relatively simple to see.

That's not in the textbook though; I do whatever method suits the question.

Cheers mate. Got that downpat now. Appreciate it alot.
2014 - Chemistry [?] English [?] Further Maths [?] Geography [?] Business Management [?]

Strawberrry

  • Victorian
  • Adventurer
  • *
  • Posts: 11
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #63 on: January 09, 2014, 11:06:40 am »
0
How come this acid + base equation doesnt have water as a product?

HCl (aq) + NH3 (aq) -----> NH4Cl (aq)


Thank you in advance :)

Yacoubb

  • Guest
Re: VCE Chemistry Question Thread
« Reply #64 on: January 09, 2014, 11:13:25 am »
0
How come this acid + base equation doesnt have water as a product?

HCl (aq) + NH3 (aq) -----> NH4Cl (aq)


Thank you in advance :)

HCl dissociates to H+ and Cl-. Your product is NH4Cl; so, added to NH3 are the H+ and the Cl-, meaning there isn't any by-product formed. Plus oxygen isn't involved in the reaction, so even if excess H+ existed, water cannot be formed as a by-product.

Hope this helped.

lzxnl

  • Victorian
  • ATAR Notes Legend
  • *******
  • Posts: 3432
  • Respect: +215
Re: VCE Chemistry Question Thread
« Reply #65 on: January 09, 2014, 11:23:53 am »
+2
How come this acid + base equation doesnt have water as a product?

HCl (aq) + NH3 (aq) -----> NH4Cl (aq)


Thank you in advance :)

Not all bases directly contain OH-. Only those bases which contain a hydroxide, oxide or carbonate ion (for VCE purposes) form water when neutralised by an acid. An acid is merely a species that donates protons and a base is merely a species that accepts protons. An acid base reaction is thus just a proton transfer reaction; water can only be formed in specific circumstances.
2012
Mathematical Methods (50) Chinese SL (45~52)

2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

2014-2016: University of Melbourne, Bachelor of Science, Diploma in Mathematical Sciences (Applied Maths)

2017-2018: Master of Science (Applied Mathematics)

2019-2024: PhD, MIT (Applied Mathematics)

Accepting students for VCE tutoring in Maths Methods, Specialist Maths and Physics! (and university maths/physics too) PM for more details

Yacoubb

  • Guest
Re: VCE Chemistry Question Thread
« Reply #66 on: January 09, 2014, 11:29:27 am »
0
Not all bases directly contain OH-. Only those bases which contain a hydroxide, oxide or carbonate ion (for VCE purposes) form water when neutralised by an acid. An acid is merely a species that donates protons and a base is merely a species that accepts protons. An acid base reaction is thus just a proton transfer reaction; water can only be formed in specific circumstances.

Is that simply because there is actually oxygen present in the base or acid? So for instance, sodium hydroxide's hydroxyl group reacts with dissociated H+ ions from the acid to form water as the by-product of the reaction. That's the logic I'm following so just clarifying to check whether that's right or wrong.

lzxnl

  • Victorian
  • ATAR Notes Legend
  • *******
  • Posts: 3432
  • Respect: +215
Re: VCE Chemistry Question Thread
« Reply #67 on: January 09, 2014, 11:50:54 am »
0
Is that simply because there is actually oxygen present in the base or acid? So for instance, sodium hydroxide's hydroxyl group reacts with dissociated H+ ions from the acid to form water as the by-product of the reaction. That's the logic I'm following so just clarifying to check whether that's right or wrong.

The presence of oxygen is one thing, but in something like the phosphate ion, which also has oxygen, that doesn't form water when reacted with an acid. The oxide ion forms water because it is the conjugate base of the hydroxide ion (I know right?) and accepts two hydrogens to form water. Carbonate ions form water because carbonic acid is unstable.

You don't call hydroxide ions hydroxyl groups; that term is reserved for OH groups that aren't ions.
2012
Mathematical Methods (50) Chinese SL (45~52)

2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

2014-2016: University of Melbourne, Bachelor of Science, Diploma in Mathematical Sciences (Applied Maths)

2017-2018: Master of Science (Applied Mathematics)

2019-2024: PhD, MIT (Applied Mathematics)

Accepting students for VCE tutoring in Maths Methods, Specialist Maths and Physics! (and university maths/physics too) PM for more details

Yacoubb

  • Guest
Re: VCE Chemistry Question Thread
« Reply #68 on: January 09, 2014, 12:38:53 pm »
0
The presence of oxygen is one thing, but in something like the phosphate ion, which also has oxygen, that doesn't form water when reacted with an acid. The oxide ion forms water because it is the conjugate base of the hydroxide ion (I know right?) and accepts two hydrogens to form water. Carbonate ions form water because carbonic acid is unstable.

You don't call hydroxide ions hydroxyl groups; that term is reserved for OH groups that aren't ions.

Ah ok thanks for the heads up :)

Strawberrry

  • Victorian
  • Adventurer
  • *
  • Posts: 11
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #69 on: January 09, 2014, 04:21:24 pm »
0
The first dot point of the study design refers to 'titration curves'... What does it actually mean?
Thanks  :D

clıppy

  • Victorian
  • Forum Leader
  • ****
  • Posts: 708
  • Would you like help?
  • Respect: +68
  • School Grad Year: 2014
Re: VCE Chemistry Question Thread
« Reply #70 on: January 09, 2014, 04:23:36 pm »
0
Graphs that look something like this

You mostly use them in questions asking what type of indicator to use by looking at the equivalence point and comparing to your data book.
2013 : VCE
2014 : VCE
2015 : UoM


Putting this here so I don't forget about it: http://www.codecogs.com/latex/eqneditor.php

Tutoring in Chemistry. PM if interested.

Tomas Sadauskas

  • Victorian
  • Fresh Poster
  • *
  • Posts: 3
  • Respect: 0
  • School: Melbourne High School
  • School Grad Year: 2014
Re: VCE Chemistry Question Thread
« Reply #71 on: January 09, 2014, 04:39:31 pm »
+1
Just wondering, with the titration curve above, why is the graph so steep/vertical around the equivalence point (ph 8-11ish)?
2013: Psychology (48)
2014: English Methods Chemistry Biology French

Yacoubb

  • Guest
Re: VCE Chemistry Question Thread
« Reply #72 on: January 09, 2014, 04:49:04 pm »
0
Just wondering, with the titration curve above, why is the graph so steep/vertical around the equivalence point (ph 8-11ish)?

Because the end point (the point at which the inducator changes colour, and theoretically, this occurs at the same time as the equivalence point, where reactants are in stoichiometrically equal proportions) is sharp. That is, there is a very precise and quick colour change, represented by the steepness.

When looking at back titrations, you notice that they're essentially done because the end point is broad; that is, there is no real quick & precise colour change of the indicator. In the circumstance of a broad end point, there is a nore diagonal slope which indicates the end point, and so to obtain accurate results (this is obtained with a sharp end point), you carry out a back titration. Hope this helped :)

lzxnl

  • Victorian
  • ATAR Notes Legend
  • *******
  • Posts: 3432
  • Respect: +215
Re: VCE Chemistry Question Thread
« Reply #73 on: January 09, 2014, 04:56:21 pm »
+1
Just wondering, with the titration curve above, why is the graph so steep/vertical around the equivalence point (ph 8-11ish)?

Because the end point (the point at which the inducator changes colour, and theoretically, this occurs at the same time as the equivalence point, where reactants are in stoichiometrically equal proportions) is sharp. That is, there is a very precise and quick colour change, represented by the steepness.

When looking at back titrations, you notice that they're essentially done because the end point is broad; that is, there is no real quick & precise colour change of the indicator. In the circumstance of a broad end point, there is a nore diagonal slope which indicates the end point, and so to obtain accurate results (this is obtained with a sharp end point), you carry out a back titration. Hope this helped :)

It's not really that. Let's say we're adding an acid to a base. Near the end point, the amount of base is almost zero. Adding the same amount of acid then leads to a larger drop in pH because less of it is neutralised by the base. This is why the curve would drop more. And vice-versa if we were adding base to acid.

The reason why weak acid and weak base titrations don't have clear endpoints is because when adding the weak acid to the weak base, the concentrations of H+ and OH- are quite low. Therefore, the reaction doesn't take place as readily and adding bits of the weak acid doesn't lead to as great a change in the concentration of H+.
2012
Mathematical Methods (50) Chinese SL (45~52)

2013
English Language (50) Chemistry (50) Specialist Mathematics (49~54.9) Physics (49) UMEP Physics (96%) ATAR 99.95

2014-2016: University of Melbourne, Bachelor of Science, Diploma in Mathematical Sciences (Applied Maths)

2017-2018: Master of Science (Applied Mathematics)

2019-2024: PhD, MIT (Applied Mathematics)

Accepting students for VCE tutoring in Maths Methods, Specialist Maths and Physics! (and university maths/physics too) PM for more details

Yacoubb

  • Guest
Re: VCE Chemistry Question Thread
« Reply #74 on: January 09, 2014, 05:00:52 pm »
0
What is the most VCAA could ask us to so with a titration curve? I know how to read off whether the aliquot is an acid/base and determine its strength, I know how to identify equivalence point and which indicator would be best to observe the end point. Is this sufficient? Is there any more? I mean, concentrations could be read off and then apply values to stoichiometry, but thats about all I found.

It's not really that. Let's say we're adding an acid to a base. Near the end point, the amount of base is almost zero. Adding the same amount of acid then leads to a larger drop in pH because less of it is neutralised by the base. This is why the curve would drop more. And vice-versa if we were adding base to acid.

The reason why weak acid and weak base titrations don't have clear endpoints is because when adding the weak acid to the weak base, the concentrations of H+ and OH- are quite low. Therefore, the reaction doesn't take place as readily and adding bits of the weak acid doesn't lead to as great a change in the concentration of H+.


Thanks :)