[Catalist] Buffers Chemistry Query

Sat May 20 14:04:38 AEST 2017

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] On Behalf Of Leon Harris
Sent: Friday, 19 May 2017 7:36 PM
To: catalist at lists.stawa.net <mailto: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  <mailto: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 <mailto: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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