Background

This year we’ve ended up teaching the the mole just after the acids and bases work (we’ve tagged together the C2 and C3 stuff from the AQA GCSE chemistry course).

I started off covering the basics and before we got too far into producing salts and doing titrations I thought it was worthwhile skipping ahead to do the quantitative stuff so that my students could put some numbers to those practicals when they do them and get some practice in.

I had only just done the mole with them the previous lesson (I use Vanessa Kind’s approach as reccommended in the RSC Chemical Misconceptions text (section 9.6 – pg 52). If you are unfamiliar with the text, stop now and read it from cover to cover. It’s probably one of the most important pieces of work a secondary chemistry teacher could read (along with the resources that accompany it).

So what I was after was an acid / base reaction to keep the context in their mind and one where they could easily see the importance and value of getting the stoichiometry right and as luck should have it I was reminded of the perfect activity by an article in JCE: Fizzy Drinks – Stoichiometry you can taste (J. Chem. Educ., 2000, 77 (12), p 1608A DOI: 10.1021/ed077p1608A)

 

Running the lesson

I adapted this and the idea was that students would get a chance to practice working out their formula masses for a starter and make up a fizzy drink mixture without thinking about the stoichiometry (hopefully not tasting that great) before calculating the correct mass proportion to taste the deifference.

We worked in the food tech room. I brought over balances in trays to ensure they didn’t contaminate the room. The students measured out into cupcake cases and used disposable cups (but washed up between trials). I also reduced the volumes from the original paper because as it turns out, if you live in Bristol it’s pretty hard to get your hands on citric acid. I gave up after 5 pharmacies were out and three of them suspiciously observed that “people generally want it for cutting heroin”). I was on the verge of popping down to the local Harm Reduction unit when I found our amazing technician had got me some from her village pharmacy. Maybe heroin supplies are less in demand in rural Gloucestershire?

The students slightly over-egged the tablespoon measurement meaning that the ratios could be a little more off from each other. In future I’d want to show them how you measure with a spoon!

I thought the topic was particularly handy as I was also able to use the opportunity to go over the solubility of carbonates and water hardness as my students had done poorly in their mocks on a question on the topic

Resources

The activity worked well so I knocked up a PowerPoint which is included here.

 

 

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Tasty Stoichiometry by Declan Fleming is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
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Alka Seltzer image: http://www.blogcdn.com/www.diylife.com/media/2010/06/alka-seltzer-water-glass-fizz-590kb062110.jpg

Fizzies images: http://www.fizzies.com/