Rates and rhubarb

Read our standard health & safety guidance

Lesson organisation

This experiment is probably most suited to younger students or groups of students who do not need to be given the details of the reaction itself. It is difficult to relate the rate back to the equation. This is because the results can be complex, due to the competing reactions taking place. (See the additional teaching notes.) On a simple level the experiment works well, and makes an unusual alternative to hydrochloric acid and marble chips.

All the practical work can easily be completed in a one-hour session, and graphs could be plotted of the results. Alternatively, the practical work and follow-up can be split over two separate sessions. You could also discuss with students in what ways this experiment is not a fair test.

Warn students not to consume anything in the lab – they may be tempted to taste the rhubarb (although it doesn’t taste very good unsweetened).

If you use rhubarb from home or someone’s garden, ensure that the leaves are removed before it is given to students. Rhubarb leaves contain far more oxalic acid than the stalk, and are harmful.

Apparatus and chemicals

Investigating the effect of surface area

Eye protection

Each working group will require:

Beakers (100 cm³), 2 or more
Measuring cylinder (50 cm³)
Timer
White tile or piece of paper

Students will need access to:

Rhubarb stalks (frozen rhubarb also works if the pieces are long enough) (see note 3)
Knives, 4 to 6 per class (ordinary table knives are probably most appropriate.)
Acidified potassium manganate(VII) solution (Irritant) (see notes 1 and 2)

Investigating the effect of concentration: Additional requirements

Students will need access to:

Filter funnels and filter paper or tea strainers

Each working group will require:

Beaker (250 cm³)
Bunsen burner
Heat resistant mat
Tripod
Gauze

Technical notes

Potassium manganate(VII) solution (Irritant) Refer to CLEAPSS Hazcard 81 and Recipe card 56
Sulfuric acid (Corrosive) Refer to CLEAPSS Hazcard 98A
Oxalic acid (Harmful) Refer to CLEAPSS Hazcard 36A

1 Wear eye protection when making and using the solutions. When making up the potassium manganate(VII) solution, the sulfuric acid is 2 mol dm^–3 and corrosive. Once diluted with the potassium manganate (VII) solution, the acid is approximately 1 mol dm^–3 and an irritant.

Each group of students will need approximately 300 cm³ of the solution in total (for both experiments). Adjust the volumes given below accordingly.

2 Put 4 or 5 crystals (or sufficient powder to just cover the tip of a spatula) of potassium manganate(VII) (Oxidising agent, Harmful, Dangerous for the environment) into a beaker with about 500 cm³ of distilled water. Stir until the crystals dissolve. Add about 500 cm³ of 2 mol dm^-3 sulfuric acid (Corrosive). 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.

3 As the oxalic acid content of the rhubarb will vary, it is well worth checking that the solution of potassium manganate(VII) will give appropriate results with the rhubarb you are using.

Procedure

HEALTH & SAFETY: Wear eye protection

Investigating the effect of surface area

a Cut three 5 cm lengths of rhubarb. Leave one piece as it is, cut one piece in half lengthways, and cut the third piece into 4 evenly-sized pieces.

b Measure 30 cm³ of acidified potassium manganate(VII) solution into a beaker. Pour the same quantity of water into another beaker.

c Place the beakers on a white tile. Put the whole 5 cm long piece of rhubarb into the potassium manganate(VII) and start the timer. Stir the solution containing the rhubarb until the purple colour disappears. If you are not sure, briefly remove the rhubarb and compare the colour of the solution to the beaker of water. When they look the same, stop the timer.

d Rinse out and dry the reaction beaker.

e Repeat the experiment using the piece of rhubarb cut into 2 (use both halves). Rinse and dry the beaker.

f Repeat the experiment again, this time using the piece of rhubarb cut into 4.

Investigating the effect of concentration

a Cut the stick of rhubarb (widthways this time) into thin slices (about 0.5 cm) and put them into the 250 cm³ beaker. Cover the rhubarb with distilled water and heat gently.

b Bring the rhubarb to the boil and continue heating gently until the rhubarb falls to pieces. This will take about 5 minutes. Turn off the Bunsen and leave the rhubarb mixture to cool.

c When cool enough to handle, filter or strain the mixture and keep the filtrate (liquid).

d Measure 30 cm³ of acidified potassium manganate(VII) solution into one of the 100 cm³ beakers and the same amount of water into the other.

e Add one drop of the rhubarb filtrate to the potassium manganate(VII) solution and start the timer. Stop the timer when the colour disappears and is the same as the plain water.

f Repeat the experiment for 2, 3, 4, 5, and 6 drops of the rhubarb extract.

Additional teaching notes, hints and tips

Rhubarb contains oxalic acid (ethanedioic acid) which has the formula C₂H₂O₄:

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

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

The potassium manganate(VII) decolourises which provides a convenient and easy-to-measure end-point to the reaction. Aqueous solutions of Mn²⁺ are actually pale pink, but at these concentrations will appear almost colourless.

Students should be able to observe that as the surface area of the rhubarb increases, so does the rate of the reaction. Likewise for increasing concentration of rhubarb juice. The concentration has been varied by putting in more drops of the rhubarb extract, so the total volume has been increased. You may like to discuss the implications of this with the students.

It is worth noting that the reaction is autocatalysed (catalysed by a product of the reaction) by the Mn²⁺ ions. This could lead to some confusing patterns for students if the results are analysed too closely, and an attempt is made to link the results to the equation.

Health and Safety checked, November 2006