Hydrolysis of starch

Introduction and teacher's notes

This is a relatively simple practical that can be used to illustrate some of the chemistry covered in the Option B: Biochemistry sub-topic B.4 Carbohydrates. It is suitable for both Standard and Higher Level students, and gives some good chemistry to explain involving the structure and hydrolysis of starch and the redox reactions associated with the test for reducing sugars. Many chemists may be more familiar with Fehling’s solution rather than Benedict’s solution but both involve the reduction of blue Cu2+(aq) ions to form a red precipitate of copper(I) oxide, Cu2O(s).

Benedict’s solution is actually used to test for the presence of glucose in urine (a symptom of diabetes).

There are no serious hazards with this experiment and relatively small quantities are used.

Four solutions will be required – the starch and amylase solutions should be prepared freshly beforehand.

Benedict’s solution. This can be prepared by dissolving 17.3 g of sodium citrate, 10 g of sodium carbonate and 1.73 g of pentahydrated copper(II) sulfate, CuSO4.5H2O in water and making up to 100 cm3. Both the amylase and the starch solution should be freshly prepared.

1% Amylase solution. Contains 1.0 g of amylase in 100 cm3 of water.

1% Starch solution. Contains 1 g of starch in 100 cm3 of water. (Mix the starch into a paste then add to 100 cm3 of boiling water).

Hydrochloric acid. I normally use 2.0 mol dm-3 HCl(aq).
 

Answers to the discussion points

1. (C6H10O5)n + nH2O → nC6H12O6

2. Amylose (water soluble) contains α-1,4-linkages between the α-D-glucose units whereas amylopectin contains both α-1,4 and α-1,6 linkages.

3. Enzymes have their maximum efficiency at about 40 oC. Below this temperature the reaction will be slower and above this temperature the structure of the enzyme will be broken down (denatured).

4. Because the IB syllabus only normally requires students to balance half-equations under acidic condition they may well give the following answer:

Cu2+(aq) + e− → Cu+(aq)

RCHO(aq) + H2O(l) → RCOOH(l) + 2H+(aq) + 2e−

Overall: 2Cu2+(aq) + RCHO(aq) + H2O(l) → RCOOH(aq) + 2Cu+(aq) + 2H+(aq)

In fact the reaction is carried out under alkaline conditions so the half-equations should really be:

2Cu2+(aq) + 2OH−(aq) + 2e− → Cu2O(s) + H2O(l)

RCHO(aq) + 3OH−(aq) → RCOO−(aq) + 2H2O(l) + 2e−

To give the overall equation

2Cu2+(aq) + 5OH−(aq) + RCHO(aq) → Cu2O(s) + RCOO−(aq) + 3H2O(l)

5. The sodium hydrogen carbonate neutralises the excess hydrochloric acid

HCO3−(aq) + H+(aq) → CO2(g) + H2O(l)

This is necessary before the copper(II) ions, Cu2+(aq,) are added as the red copper(I) oxide, Cu2O(s), will not form in acidic conditions as it is a basic oxide.

6. The red precipitate formed when both amylase and an acid catalyst are used shows that the blue copper(II) ions have been reduced by the glucose formed during the hydrolysis.

Standard Level Higher Level Student worksheet

THE HYDROLYSIS OF STARCH

INTRODUCTION

In this practical starch, which is a polymer made up of condensed glucose units, is hydrolysed to its constituent sugar molecules. This hydrolysis reaction is generally quite slow but the rate can be increased by using either an enzyme or an acid catalyst. The presence of the sugar molecules produced is tested by using Benedict’s solution. This is a solution of copper(II) ions, which are reduced to Cu(I) ions by the sugar.

ENVIRONMENTAL CARE:

Starch and amylase are natural materials and the hydrochloric acid used mainly reacts to form sodium chloride. However as Benedict’s solution contains copper ions the waste should be placed in the bottle marked ‘ Heavy metal waste’ in the fume cupboard and not disposed of down the sink.

SAFETY:

There are no particular safety hazards except for the usual need for care when handling glassware and 2.0 mol dm-3 strength acids and heating solutions. Be careful too when adding the boiling solutions to the sodium hydrogencarbonate solution in part (b). Copper(II) ions are poisonous.

PROCEDURE:

(a) Hydrolysis with amylase. Add 5 cm3 of starch solution to two test-tubes. To one of them (the control) add 1 cm3 of water and to the second add 1 cm3 of amylase solution. Place both test tubes in a water bath at 40 oC. After five minutes add 2 cm3 of Benedict’s solution and then, keeping the test tubes in the water bath, heat the water until it boils. Record your observations.

(b) Hydrolysis with an acid catalyst. Add 5 cm3 of starch solution to two test-tubes. To one of them (the control) add 1 cm3 of water and to the second add 1 cm3 of dilute hydrochloric acid. Heat them in a water bath containing boiling water for five minutes. Pour the contents of both tubes into separate beakers containing sodium hydrogencarbonate solution. Add 2 cm3 of Benedict’s solution and heat the contents of both beakers. Observe what happens.

DISCUSSION

1. Give the equation for the hydrolysis of starch, (C6H5O)n.

2. Explain the difference between amylose and amylopectin.

3. Why must the solutions not be heated above 40oC in the first part of experiment (a)?

4. Reducing sugars contain an aldehyde group that can be represented as RCHO. Give the two half-equations for the reaction of copper(II) ions with an aldehyde and the overall balanced equation.

5. Why is it necessary to use sodium hydrogencarbonate solution in experiment (b)?

6. Explain your observations in the two experiments.

This worksheet can also be downloaded from:

Standard Level Higher Level Hydrolysis of starch

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