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Author Topic: VCE Chemistry Question Thread  (Read 2313352 times)  Share 

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eagles

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Re: VCE Chemistry Question Thread
« Reply #105 on: January 13, 2014, 03:27:02 pm »
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Hahaha thanks a lot, now I'll remember my silly mistake!

Edward21

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Re: VCE Chemistry Question Thread
« Reply #106 on: January 13, 2014, 05:00:37 pm »
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Hahaha thanks a lot, now I'll remember my silly mistake!
Just google some titration curves and get a taste of what gives what :) there's a few in the textbook too but look online for more.
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #107 on: January 13, 2014, 05:37:33 pm »
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Quickly to clarify this: also part of my summary!

Weak Acid + Weak base (need back titration)
Strong Acid + Strong base (no need for back titration)
Strong Acid + weak base (no need for back titration)
Weak Acid + Strong base (no need for back titration).

Essentially, is it accurate to say that we carry out back titrations because the end point is very broad when we react weak acid with a weak base? We actually increase the accuracy of our results by carrying out back titrations.

Thanks for the clarification! :)

SocialRhubarb

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Re: VCE Chemistry Question Thread
« Reply #108 on: January 13, 2014, 05:43:25 pm »
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Yes, that seems accurate, but bear in mind there are also other reasons for conducting back titrations, apart from the strength of your titrant and analyte.
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Spxtcs

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Re: VCE Chemistry Question Thread
« Reply #109 on: January 13, 2014, 05:45:24 pm »
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This has probably already been asked, but what exactly is a sharp end point and a broad end point? I get the idea of end points/equivalence points but titration curves are confusing the hell out of me!
And how do you know what indicators to use just by looking at titration curves/end points?

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Re: VCE Chemistry Question Thread
« Reply #110 on: January 13, 2014, 05:48:49 pm »
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And how do you know what indicators to use just by looking at titration curves/end points?
To know which indicator to use, you check out the pH of the end point.
I think the list of indicators and the pH they best work in are in the data book?

Yeah - page 11 of the data book
http://www.vcaa.vic.edu.au/Documents/exams/chemistry/chemdata_2012-w.pdf
« Last Edit: January 13, 2014, 05:54:57 pm by Snorlax »
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #111 on: January 13, 2014, 05:50:53 pm »
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This has probably already been asked, but what exactly is a sharp end point and a broad end point? I get the idea of end points/equivalence points but titration curves are confusing the hell out of me!
And how do you know what indicators to use just by looking at titration curves/end points?

The end point of a titration is the point at which the indicator changes colour; theoretically speaking, the same as the equivalence point, at which the reactants are in stoichiometrically equal proportions, and neutralisation has taken place. A sharp end point refers to a very precise, quick change in the colour of the indicator, that of which occurs in a reaction between a strong base/acid and another reactant of any strength (i.e. Strong acid + weak base, Strong base + weak acid or Strong acid + strong base). Sometimes, the colour change of the indicator isn't very quick and precise. As a result, the end point is very broad. This is essentially what happens when you react a weak acid with a weak base.

Q2. When using titration curves, look at your end point. This is basically the close-to-vertical gradient of your titration curve. Go to the middle of that curve and look at the corresponding pH. Then, look at the different indicators and pick the one which the corresponding pH of the end point (as read from your curve) is most suitable to. E.g. an end point with pH 7 would mean you'd use an indicator with a range where 7 falls into (e.g. an indicator with a pH range 6-8).

eagles

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Re: VCE Chemistry Question Thread
« Reply #112 on: January 13, 2014, 06:00:56 pm »
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Just google some titration curves and get a taste of what gives what :) there's a few in the textbook too but look online for more.

Thanks for the advice! Also, about the chemical formula of sodium acetate:

Which one is better: NaCH3COO or CH3COONa? Thanks!

Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #113 on: January 13, 2014, 06:01:49 pm »
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Thanks for the advice! Also, about the chemical formula of sodium acetate:

Which one is better: NaCH3COO or CH3COONa? Thanks!

I go with the second one :) I think the conventional way is that of the latter formula for sodium acetate (for some off reason LOL).

eagles

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Re: VCE Chemistry Question Thread
« Reply #114 on: January 13, 2014, 06:04:36 pm »
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That's also what I was thinking. Thanks for verifying!

Spxtcs

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Re: VCE Chemistry Question Thread
« Reply #115 on: January 13, 2014, 06:07:37 pm »
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The end point of a titration is the point at which the indicator changes colour; theoretically speaking, the same as the equivalence point, at which the reactants are in stoichiometrically equal proportions, and neutralisation has taken place. A sharp end point refers to a very precise, quick change in the colour of the indicator, that of which occurs in a reaction between a strong base/acid and another reactant of any strength (i.e. Strong acid + weak base, Strong base + weak acid or Strong acid + strong base). Sometimes, the colour change of the indicator isn't very quick and precise. As a result, the end point is very broad. This is essentially what happens when you react a weak acid with a weak base.

Q2. When using titration curves, look at your end point. This is basically the close-to-vertical gradient of your titration curve. Go to the middle of that curve and look at the corresponding pH. Then, look at the different indicators and pick the one which the corresponding pH of the end point (as read from your curve) is most suitable to. E.g. an end point with pH 7 would mean you'd use an indicator with a range where 7 falls into (e.g. an indicator with a pH range 6-8).

Thank you so much!

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Re: VCE Chemistry Question Thread
« Reply #116 on: January 13, 2014, 07:42:09 pm »
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Slight correction. As the titration curves are so steep near the equivalence point, it is ok if you choose an indicator that has a pH range slightly away from the equivalence point pH. For a sharp enough titration curve, the difference between the endpoint and the equivalence point ends up being tiny and negligible anyway. For this reason, phenolphthalein is a valid indicator for use in sodium hydroxide/nitric acid titrations, for instance.
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eagles

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Re: VCE Chemistry Question Thread
« Reply #117 on: January 14, 2014, 11:57:43 am »
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If writing an ionic equation from a full equation including ethanoic acid, should we dissociate the H+ and CH3COO? And also, please explain? Thanks!

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Re: VCE Chemistry Question Thread
« Reply #118 on: January 14, 2014, 12:02:45 pm »
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No. Ethanoic acid is a weak acid, so it only partially ionizes in water. If you pour ethanoic acid into water, most of the ethanoic acid molecules will retain the proton.
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Re: VCE Chemistry Question Thread
« Reply #119 on: January 14, 2014, 12:05:07 pm »
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Thank you!  :)