Chemistry 9 Lesson 52: Starch and cellulose

1. Theoretical Summary

1.1. Natural state and physical properties

– Starch:

• There are many seeds, tubers and fruits of food crops such as corn, potatoes, cassava, rice, …
• Solid, white color, soluble only in hot water to form starch solution.

– Cellulose:

• There are many in vegetables, stems, cotton fruit (fiber) such as bamboo, …
• Solid, white, insoluble in water.

1.2. Molecular formula characteristics

a) Starch

– Branch circuit

– The number of links in a starch molecule is n \(\approx\) 1200 – 6000

b) Cellulose

– Unbranched circuit

– The number of links in a cellulose molecule is n \(\approx\) 10 000 – 14 000

1.3. Chemical properties

a) Hydrolysis reaction

• When heated in a dilute acid solution starch and cellulose are hydrolyzed to glucose

(–C₆H_tenO₅ –)n + n₂O \(\xrightarrow[t^{0}]{Acid}\) nC₆H_twelfthO₆

• At room temperature, starch and cellulose are hydrolyzed to glucose by enzymes

b) Effect of starch on Iodine

When a few drops of iodine solution is added to a test tube containing starch, the solution is blue-black. When the mixture is heated, the solution in the test tube loses its color. Cooling the mixture, the blue-black color reappears.

Explanation: Due to the adsorption of Iodine into the empty slots in the starch, it gives a blue-black color. When heated, Iodine molecules are released which lose their blue-black color; When cooled, Iodine is re-adsorbed to give a blue-black color.

⇒ Method used to identify starch paste and vice versa (used to identify Iodine)

1.4. Applications of starch and cellulose

• Plant photosynthesis: 6nCO₂ + 5nH₂O \(\xrightarrow[anh\,\,sang]{chlorofin}\)(-C₆H_tenO₅ – )n + 6nO₂​
• Starch is an important food for humans, in addition to being an important raw material for the production of glucose and ethyl alcohol.

• Cellulose has the following main applications:

1.5. summary

2. Illustrated exercise

2.1. Form 1: Hydrolyzed starch, cellulose

When hydrolyzing 1 kg of rice flour containing 80% starch, how much glucose is obtained? Assume that the reaction is complete.

Solution guide

The mass of starch contained in 1 kg of rice flour is:

\({m_{crystalline\,\,bot}} = 1.\frac{{80}}{{100}} = 0.8(kg)\)

Chemical equation:

(–C₆H_tenO₅ –)n + n₂O \(\xrightarrow[t^{0}]{Acid}\) nC₆H_twelfthO₆

M: 162.n 180.n

m: 0.8 (kg) → \(\frac{{0,8.180n}}{{162n}} = \frac{8}{9} \approx 0.89(kg)\)

So the mass of glucose obtained is 0.89 kg

2.2. Recognition exercise

State a method to identify the following white solids: Starch, glucose, and sucrose.

Solution guide

• Add substances to water, the insoluble substance is starch
• Put the solutions of the remaining 2 substances into action with AgNO . solution₃ in NH₃, the reactant with silver coating is glucose, the reactant is sucrose

OLD₆H_twelfthO₆ (dd) + Ag₂O → 2Ag + C₆H_twelfthO₇

3. Practice

3.1. Essay exercises

Question 1: State the method to distinguish the following substances: Starch, cellulose, sucrose.

Verse 2: Hydrolyzed 1 kg of potatoes containing 20% ​​starch in acidic medium. If the reaction yield is 75%, what is the mass of glucose obtained after the reaction?

Question 3: To produce glucose, one goes from starch based on the chemical equation for the following reaction

(C)₆H_tenO₅)_n + n₂O ^acid → nC₆H_twelfthO₆

Know the process efficiency is 80%.

a) Calculate the amount of glucose obtained from 1 ton of starch

b) How many tons of starch are needed to produce 1 ton of glucose?

Question 4: How much sawdust (containing 50% cellulose) is needed to produce 1 ton of C2H5OH, knowing the process efficiency reaches 70%.

Question 5: Starch is produced in green plants according to the following chemical equation: 6nCO₂ + 5nH₂O a/s→ (C₆H_tenO₅)_n + 6nO₂

Ask to make 8.1 tons of starch, how many tons of CO . do green plants absorb?₂ and how many tons of oxygen are released. From the above data, what do you think about the effect of trees on the environment.

3.2. Multiple choice exercises

Question 1: Which of the following reagents is used to identify starch?

A. Cu(OH)₂

B. AgNO₃/NS₃

C. Br₂

D. I₂

Verse 2: Choose the most correct sentence?

A. Starch and cellulose are easily soluble in water.

B. Starch is soluble in water and cellulose is insoluble in water.

C. Starch and cellulose are insoluble in cold water but soluble in hot water

D. Starch is insoluble in cold water but partially soluble in hot water. Cellulose is insoluble in both cold and hot water.

Question 3: To distinguish between starch and cellulose we use:

A. purple litmus.

B. iodine.

C. NaCl.

D. glucose.

4. Conclusion

After the lesson, you should know the following:

– General formula, structural characteristics of starch and cellulose.

– Understand the chemical properties, physical properties and applications of starch and cellulose.

– Write hydrolysis reactions and reactions to form these substances in green plants.