Topic 6.1 Investigation of rates of reaction experimentally and evaluation of the results

This blog post follows on from previous ones. Since July 2015, each month I have been working through the list of mandatory labs from the Chemistry IA course.

As discussed previously, the idea behind these labs is great – each student, teacher and school will be exposed to some great labs that will help to enhance the understanding if the IB Chemistry course.

The downside of this is that students can expect to get a question in paper 3 on one of these mandatory labs.

This does create problems as the syllabus statements are so open ended. This means that you may carry out a lab covering the syllabus statement and a teacher in another school carries out a completely different lab but also covers the same statement.

The aim of this series of blog posts is to give you some ideas of alternative labs to carry out (or at least discuss) – I hope you find this month’s post useful!

Start with the four broad types of labs to investigate:

How surface area, temperature, concentration (or pressure) and catalysts affect the rate of a reaction.

Good ideas for surface area are calcium carbonate and hydrochloric acid (why note sulfuric acid? Answers below please!)

Temperature is one I personally find tricky. In my old school we had access to water baths but in my current school we do not. It can be quite a challenge trying to keep the temperature of a water bath constant (can anyone help me out here!?) I tend to react magnesium with hydrochloric acid. I just ask students to heat water to a predetermined temperature, add say, 5cm³ to 5cm³ of hydrochloric acid (at room temperature) and to record the final temperature. Then add the magnesium and time how long it takes to react. It’s not great as the reaction is exothermic as well but does give plenty of discussion and evaluation points.

The classic concentration experiment is ‘the lost cross’. Sodium Thiosulfate and hydrochloric acid. Dilute the sodium thiosulfate to change its concentration. Time how long it takes a cross on a piece of paper to disappear.

We also need to think about how data is collected.

For the surface area experiment data can be collected in a couple of ways – if you have a sensitive enough balance (to 0.001g) you can measure mass lost. The other method is to time how long it takes to collect a fixed amount of gas.

For temperature, I time how long it takes for the magnesium to react (disappear) but you could also time how long it takes for a fixed amount of gas to form.

Both ‘fixed amount of gas’ ideas could involve a gas syringe or upwards displacement of water.

And there we have it – rates of reaction in a nutshell. I’m not saying I have covered everything, as this topic in particular has lots of variations that you could decide to carry out.

So please, if you have a good idea for a lab and it has not been covered, share your ideas with us below! I look forwards to reading your ideas.

See you next month!