Rhubarb Rhubarb Rhubarb is well known for containing

Rhubarb!

Rhubarb! • Rhubarb is well known for containing Oxalic Acid (ethanedioic acid) – the leaves in particular have quite a large concentration and can be toxic if eaten. • The stems also contain some oxalic acid – in lesser amounts. • This experiment looks at factors affecting the rate of the redox reaction between oxalic acid and potassium permanganate – using rhubarb as the source of the oxalic acid.

The Reaction 2 Mn. O 4 -(aq ) + 5 H 2 C 2 O 4(aq ) + 6 H 3 O+(aq ) --> 2 Mn 2+(aq ) + 10 CO 2(aq ) + 14 H 2 O Interestingly, this is an example of autocatalysis – the reaction is catalysed by the Mn 2+ ions so the rate increases as it goes on.

Links to the Curriculum for excellence Materials Chemical Changes Learning in the sciences will enable me to: develop the skills of scientific inquiry and investigation using practical techniques. Level 3 Through experimentation, I can identify indicators of chemical reactions having occurred. I can describe ways of controlling the rate of reactions and can relate my findings to the world around me. SCN 3 -19 a Number, Money & Measurement Level 4 I can apply my knowledge and understanding of measure to everyday problems and tasks and appreciate the practical importance of accuracy when making calculations. MNU 4 -11 a

The Experiments Concentration Surface Area Rhubarb juice is used to decolourise 25 cm 3 of KMn. O 4 Pieces of rhubarb are used to decolourise 25 cm 3 of KMn. O 4 Experiment is carried out with 1, 2, 3, 4, 5 and 6 cm 3 or rhubarb juice. Experiment is carried out with a whole piece of stem and ones cut lengthways into 2, 3 and 4 pieces Each one is repeated 3 times, the average is worked out and a graph is plotted.

Accuracy There are some issues with the accuracy of this experiment – which is (oddly enough) a good reason for doing it! • • The main problem is the indicator of the reaction – the disappearance of the purple colour of the potassium permanganate. It can be tricky to decide exactly when the colour has gone. There is also an issue with the surface area experiment. It seems that the distribution of oxalic acid in rhubarb stems is not even, so the pieces of frozen rhubarb can have quite variable results – however… if you collate the results from the class and plot them, you should get a decent graph.

Why? The advantages of carrying out an experiment that is imperfect – though it does work - are: • It opens up a discussion of what makes a suitable end-point of a reaction and why we choose the indicators we do. • It allows discussion of variability of results – how close together can you expect readings to be and what might be the explanations if they are not close. • The fact that the class average for the surface area experiment ‘works’ whereas that for the individual group may not shows the importance of repeating a test. • It can also reinforce the importance for pupils of not assuming that any result that departs from their expectations is simply due to their own mistakes but rather to look carefully to see if there might be any other reason.