[Catalist] Buffers Chemistry Query

Wed May 17 11:52:47 AEST 2017

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

(+61) 8 9526 5000

kjohnson at sjgs.wa.edu.au

Locked Bag 1, Mundijong WA 6123   

www.sjgs.wa.edu.au

Corner Bishop & Soldiers Rd, Mundijong WA 6123

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