Rates and rhubarb

Rhubarb contains oxalic acid, which has the formula C₂H₂O₄:

Oxalic acid reacts with acidified potassium manganate(VII) and is oxidised to carbon dioxide and water:

2 MnO₄ ^– + 5 C₂H₂O₄ + 6 H₃O⁺ → 2 Mn²⁺ + 10 CO₂ + 14 H₂O

The potassium manganate(VII) decolourises, which provides a convenient and easy to measure end point for the reaction.

This experiment is probably most suited to less able students who do not need to be given the details of the reaction or to try to relate the rate back to the equation. The reaction is autocatalysed (catalysed by a product of the reaction) by the Mn²⁺ ions.

This could lead to some confusion if students analyse the results too closely. More able students could be expected to plan their own experiment. It is also possible to use this reaction to look at how a change in temperature affects the rate of reaction.

Equipment required

• Rhubarb – fresh if possible (frozen also works if the pieces are long enough and should be fine whatever the size for the concentration experiment)
• Beakers (at least 2 per pair of students), 100 cm³
• Measuring cylinders, 50 cm³ 
• Dilute acidified potassium manganate(VII) solution (Irritant) – see note below
• Timer (1 per pair)
• White tile (1 per pair)
• Knives (4–6 per class should be fine)
• Beaker, 250 cm³
• Bunsen burners, heatproof mats, tripods and gauzes
• Filter funnels and filter paper or tea strainers
• Eye protection.

Potassium manganate(VII) solution

Put 2 or 3 crystals of potassium manganate(VII) into a beaker with about 500 cm³ distilled water and stir until the crystals dissolve.

Add about 500 cm³ 2 mol dm^–3 sulfuric acid (Corrosive) and stir to mix.

The solution should be a light purple colour. If necessary, dilute further with a little more water. The exact concentration is not critical.

Health, safety and technical notes

• Read our standard health and safety guidance.
• The sulfuric acid used for making the potassium manganate(VII) solution is 2 mol dm^–3 and is corrosive. See CLEAPSS Hazcard HC098a
• Once it is diluted with potassium manganate(VII) it is approximately 1 mol dm^–3 and the resulting solution is an irritant.
• Wear eye protection when making and using the solution.
• Warn students not to consume anything in the laboratory – they may be tempted to taste the rhubarb.
• If you use home-grown rhubarb, ensure that the leaves are removed before it is given to students as they contain far more oxalic acid than the stalk and are toxic.

Rhubarb contains a number of acids which give it a sour taste (this is why sugar is almost always added to it when it is cooked).

One of these acids is called oxalic acid. It is toxic in large quantities, but rhubarb stalks are safe to eat because they only contain a small amount. It is enough, however, to react with potassium manganate(VII) and decolourise it.

You are going to investigate how the rate of this reaction changes when two factors are changed:

• surface area
• concentration

Surface area

You will need

• 1 stick of rhubarb
• Beakers (or 2 if you wash and re-use them), 4 x 100 cm³ 
• Measuring cylinder, 50 cm³ 
• Dilute acidified potassium manganate(VII) solution
• Timer
• White tile
• Knife
• Eye protection.

What to do

1. Cut 3 x 5 cm lengths of rhubarb. Leave one piece as it is, cut one in half lengthways and the other into four even sized pieces (again, cut lengthways).
2. Measure 30 cm³ acidified potassium manganate(VII) into a beaker. Pour the same quantity of water into another beaker.
3. Place the beakers on a white tile, put the 5 cm long piece of rhubarb into the potassium manganate and start the timer. Stir the solution with the rhubarb until it goes colourless. If you are not sure whether all the colour has disappeared, briefly remove the rhubarb and compare the solution to the beaker of water. When they look the same, stop the stop clock.
4. Repeat using the piece of rhubarb cut into two (use both halves) and then the piece cut into four (use all four pieces).
5. Record all your results in a results table.

Concentration

You will need

• Approx 15 cm stick of rhubarb
• Beaker, 250 cm³
• Distilled water
• Bunsen burner, heatproof mat, tripod and gauze
• Filter funnel and filter paper or a tea strainer
• Beakers, 2 x 100 cm³
• Measuring cylinder, 50 cm³
• Dilute acidified potassium manganate(VII) solution
• Timer
• White tile
• Knife
• Eye protection.

Health, safety and technical notes

• Wear eye protection when using a Bunsen burner or potassium manganate(VII) solution.
• Do not taste anything in the laboratory.

What to do

1. Cut the stick of rhubarb into thin (about 0.5 cm) slices and put them into the 250 cm³ beaker. Cover the rhubarb with distilled water and heat gently using a Bunsen burner.
2. Bring the rhubarb to the boil and continue to boil it gently until it falls to pieces, which will take about 5 minutes. Turn off the Bunsen burner and leave the mixture to cool.
3. When the beaker is cool enough to pick up easily, filter or strain the mixture. Keep the filtrate (liquid).
4. Measure 30 cm³ acidified potassium manganate(VII) into one of the 100 cm³ beakers and the same amount of water into another. Put both beakers on a white tile.
5. Add one drop of the rhubarb filtrate to the potassium manganate(VII) and start the timer. Stop when the colour disappears and the solution looks the same as the plain water.
6. Repeat with 2, 3, 4, 5, and 6 drops of rhubarb filtrate. Record your results in a table.
7. Plot a graph of your results.