[Catalist] Buffers Chemistry Query

Thu May 25 19:17:34 AEST 2017

my apologies for wading into the debate so late:

this is the kind of question that trips kids (and adults :) up pretty
easily as we're conditioned to neutralize the H3O+ with the OH-. Obviously
it does, but as you pointed out, there is bugger all there (~1:4000 at the
very most) so the most likely reaction will be with H2PO4-.

they get it wrong in essential chem - but tell the kids to correct it if
they want to increase it's gumtree value :)

jim clark deals with it (and quantitative buffer stuff) pretty well here
http://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html

i like to teach the kids how to perform the calcs (and think there way
through these type of questions) even though they're not likely to be in
the exam - it can make a buffer investigation feel a little more
challenging.

cheers
andy

On 20 May 2017 at 12:04, Roy Skinner <rsskinner at optusnet.com.au> wrote:

> Yes I agree – the question should not have been set.
>
>
>
> *From:* Catalist [mailto:catalist-bounces at lists.stawa.net] *On Behalf Of *Leon
> Harris
> *Sent:* Saturday, 20 May 2017 10:03 AM
>
> *To:* catalist at lists.stawa.net
> *Subject:* Re: [Catalist] Buffers Chemistry Query
>
>
>
> No, but that is my point Roy. The question is more complex than the
> syllabus allows.
>
> I would teach mine that it reacts with H2PO4-, and the equilibrium shifts
> to convert some of the right hand side H3O+ and HPO42-
> to H2PO4-.  All that we can expect them to know is the effect of
> concentration, pressure and temperature on reaction rate, we haven't done
> rates fully. However reality, and what the ATAR exam requires sometimes
> differ.
> Where the question is problematic is that it speaks of rates not
> concentrations, and forces them to choose between two plausible choices.
> This forces the students into an area that is outside of the scope of the
> curriculum.
>
> My reading of the situation is that our students are required to be able
> to explain the effect on the equilibrium. Asking them to pick the effect on
> the rates pushes them into these grey areas. We have only taught them some
> of the factors that determine rates. The issue arises because this was an
> actual question, I am reliably informed, in sometime around 2009 in one of
> the WATP or EdWest papers.
>
> For myself, I can say that I don't believe that I have the information and
> skills to properly answer that question. I can answer it in a way that will
> earn the WACE marks, because I can figure out what they want. I think that
> the question would work better if it had been written in terms of adding
> acid to the system. I also would have preferred if they gave a pH for the
> buffer (it is the buffer used in phosphate-buffered saline, and operates
> between 5.8-8.0) and that piece of information  would help because it would
> make explicit the low hydronium ion concentration). Not all buffers are
> near neutral: glycine-COO-: glycine-COOH has a pH of around 2.
>
>
> I have enjoyed thinking about this question, it has been interesting, but
> I think it is a bit rough on the kids.
>
> Cheers,
> Leon
>
>
>
> On 20/05/2017 9:01 AM, Roy Skinner wrote:
>
> Is this really a suitable answer for our year 12s?
>
>
>
>
>
> *From:* Catalist [mailto:catalist-bounces at lists.stawa.net
> <catalist-bounces at lists.stawa.net>] *On Behalf Of *Leon Harris
> *Sent:* Friday, 19 May 2017 7:36 PM
> *To:* catalist at lists.stawa.net
> *Subject:* Re: [Catalist] Buffers Chemistry Query
>
>
>
> UPDATE: It seems I am out of date. In 2014, Korean authors Choi et al (in
> the journal The Journal of Physical Chemistry Letters 2014(5) 2568-2572)
> show that H3O+ moves through solution by proton exchange ("proton
> hopping"), while hydroxide ion moves by regular Brownian diffusion.
> Conventional wisdom held that both moved by proton exchange up until that
> time. My training is out of date :)   !!
>
> We teach our students that OH- will react with H2PO4-, as it is the
> highest concentration instead of H3O+. We also teach that as H2PO4- is a
> stronger acid, it will react with OH-. We do not consider that the other
> potential acid, HPO4 2- will react with OH-, as despite being present in
> equimolar amounts as H2PO4-, it is a weaker acid. When you consider what it
> means by weaker acid (ie dissociates less into H3O+), it makes you wonder
> what exactly is reacting with the OH- added. Without taking actual
> measurements, really who knows? Chemical techniques allow us to measure
> concentrations and amounts directly, from which we infer reaction rates. It
> would be hard to measure. Dueterated H2PO4- added to a solution of HPO4 -
> and some sort of NMR measurement as tritiated OH- ? Hard to do, and you
> would need some kind of stopped-flow cell to capture the kinetics? I don't
> know, I am not skilled in this area and have little idea of the limits of
> this sort of technology.
>
> Perhaps in this question we are best to teach what the syllabus requires,
> but in the back of our minds it is probably good to flag this one as not
> yet experimentally determined.
>
> Cheers,
> Leon
>
>
> On 17/05/2017 12:19 PM, leon wrote:
>
> Hi. In and of itself, you cant tell. Due to the lesser mass of OH- versus
> H2PO4- , its velocity at a given temp will be higher an hence all others
> equal you would expect oh to react. Add To this that oh may move through
> solution fasteR than it can diffuse via "proton hopping" where oh exchanges
> protons with water and you end up with oh moving close to the speed of the
> proton diffusion.
>
>
>
> Against this, you have higher conc of h2po4- . I think both should be
> acceptable, as at this level we dont do rate constants, and without them
> you cant really answer.
>
>
>
> And whatever the rate both reactions will occur to at least some extent.
> cheers leon
>
>
>
> Sent from my Samsung Galaxy smartphone.
>
>
>
> -------- Original message --------
>
> From: Karen Johnson <kjohnson at sjgs.wa.edu.au> <kjohnson at sjgs.wa.edu.au>
>
> Date: 17/5/17 9:52 am (GMT+08:00)
>
> To: catalist at lists.stawa.net
>
> Subject: [Catalist] Buffers Chemistry Query
>
>
>
> ATTENTION: Chemistry Teachers
>
>
>
> I was wondering if someone could give a clear cut answer on buffers. I
> have spoken to a couple of experienced Chemistry teachers and they both
> have validated my concerns with this type of buffer question and informed
> me to post my concerns. I want to make sure that I am teaching buffers
> correctly with the concept of adding in hydroxide ions in case is ever
> comes up in the WACE exam.
>
>
>
> Let’s say you have the following buffer below. Can someone explain to me
> what happens when hydroxide ions are added to this buffer? Does it react
> more with H2PO4– (aq)  or   H3O+ (aq) and therefore which equation
> forward or reverse is favoured more? From my understanding, I believe it
> would react with the H2PO4– (aq)  as there is a greater concentration of
> this acid in the buffer compared to H3O+ (aq). Although hydroxide ions
> can react with hydronium it does this to a lesser extent and has less
> effect.
>
>
>
> *H2PO4– (aq)   +   H2O (l)   **⇌**   HPO42– (aq)   +   H3O+ (aq)*
>
>
>
> I have looked through the last 5 years worth of WACE papers and there is
> literally only this question (see attached 2012 WACE question) that
> discusses this type of question. Most buffer questions in the WACE exams
> are the more straight forward ones, in which they ask about how the effect
> of adding a small amount of acid to a buffer equation that has hydronium
> ions already. I know that the Essential Chemistry book is stating that the
> hydroxide ions react with the hydronium ions but take a look at the WACE
> exam and is begs to differ!!
>
>
>
> This question in the 2012 WACE exam does look at it in question (f). I
> presume that the first equation in (f) links to the first equation in (e).
> This shows that hydroxide ions react with the acid and not the hydronium
> ions.
>
>
>
> If anyone could give me a clear cut answer to this question so I can make
> sure I am teaching this correct, it would be greatly appreciated.
>
>
>
> Looking forward to your responses.
>
>
>
> Kind regards,
>
> Karen
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> 
>
>
> 
>
> *Karen Johnson*
>
> Science Teacher
>
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> *kjohnson at sjgs.wa.edu.au* <kjohnson at sjgs.wa.edu.au>
>
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