Microscale Calorimetry: Prep Room Friday

I’m a big advocate for microscale chemistry. Smaller quantities of chemicals means less waste and fewer safety concerns; simpler set-ups reduce the cognitive load of practical work, letting students focus on the science itself, rather than whether they’re using the right kind of .

Inspired by this RSC article about measuring enthalpy changes, I expanded on their brief idea for microscale calorimetry for a year 12 IB Chemistry class.

Students often don’t understand calorimetry as measuring energy changes; this class believed that calorimetry was specific to flame calorimetry, or the burning of substances to measure their enthalpy of combustion. However, we can do calorimetry of almost any chemical reaction with a change in temperature.

Endothermic reactions also aren’t commonly experienced by students in school and many options are unsafe, using ammonium nitrate or thermally decomposing solids. Instead, we can use the sherbert reaction: sodium hydrogen carbonate (or baking soda) and citric acid.

The same chemistry as the fizzing of bath bombs or sherbert is safe, simple and can be done on the microscale! To build our own calorimeters, we simply wrapped a sample vial in cotton wool, and placed it into a 50ml beaker.

After some tweaking, I pre-weighed around 1g of citric acid and 1.15-1.3g of sodium hydrogen carbonate. This let the students focus less on the practical technique and freed up their cognitive load for the theory involved – the neutralisation reaction, the enthalpy changes and the pitfalls of this microscale apparatus. Since the reaction only begins when water is added, they can be weighed in advance – but not too far, since they will start to deactivate each other when left to sit mixed for too long.

If the students use a measured amount of water (in this case, 5ml), they should be able to work out a final enthalpy of reaction from their data. That 5ml acts just like the water in a flame calorimeter, used as the mass heated (or cooled) to calculate the energy change using Q=MCdeltaT. I even created a fully-worked method and question sheet, which can be found below.

The whole practical took twenty minutes from explanation to end, and clean-up was simple. When two solids were used, rather than (for example) solid citric acid and a solution of baking soda, the drop in temperature reached around 10°C – large enough to be perceived by touch.

In the end, this practical was a massive success. The microscale meant the class got results very fast, and could conduct this investigation fully independently. Now, what to microscale next?

As a Chemistry specialist technician with experience in teaching, I’d love to work on more accessible practicals with purpose. If you have ideas, let me know here.


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