Mc Ques Eng Detergents

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

Detergents Public Exam Questions :

Directions : Question 1 and question 2 refer to the making of soap as represented by the following reaction : H

H

H

C

OOC(CH2)16CH3

H

C

OOC(CH2)16CH3

H

C

OOC(CH2)16CH3

+

3NaOH

H

H

C

OH

H

C

OH

H

C

OH

+

3CH3(CH2) 16COONa

H

1. What is / are the function(s) of the sodium hydroxide in this reaction? (1) to act as a catalyst (2) to neutralize the acid formed during the reaction (3) to lower the solubility of soap in water so as to salt out the soap A. (1) only B. (1) and (2) only C. (1) and (3) only D. (2) and (3) only 2. Soap has a hydrophilic head and a hydrophobic tail. Which of the following combinations is correct? hydrophilic head A. B. C. D.

Na+ COONa+ CH3(CH2)16COO-

hydrophobic tail CH3(CH2)16 CH3(CH2)16 CH3(CH2)16COONa+ (HKCEE 1992)

3.

When a little detergent is added to a drop of water on a piece of woollen cloth as shown above, the drop spreads. Which of the following statements correctly explains this observations? A. The detergent dissolves readily in water. B. The detergent has a polar end which is hydrophilic. C. The detergent can form an emulsion with water. -1-

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

D. The detergent can reduce the surface tension of water. (HKCEE 1996)

4. A detergent has the following structure :

H H3C

CH3

H

C C

H

C C

H CH3

CH3

C C

H CH3

CH3

H CH3

+

SO 3-Na

Which of the following statements concerning the detergent is correct? A. Its hydrocarbon chain is hydrophilic. B. It can be manufactured from vegetable oil. C. It is readily degraded by micro-organisms. D. It acts as an emulsifier in the cleaning process. (HKCEE 1998)

5. Which of the following problems are associated with the excessive use of soapless detergents? (1) They can cause skin allergies. (2) They form foam when discharged into rivers and lakes. (3) They form scum when discharged into the sea. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 1998)

6. Which of the following statements concerning a soapless detergent are correct? (1) It can be prepared by heating a cooking oil with sodium hydroxide solution. (2) It acts as a wetting agent by reducing the surface tension of water. (3) It acts as an emulsifying agent in the cleaning process. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 1999)

7. 1st statement : Local tap water produces a scum with soap. 2nd statement : Water containing calcium ions can form an insoluble compound with soap. -2-

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224 (HKCEE 1999)

8. Which of the following statements concerning soaps are correct? (1) They are esters. (2) They can reduce the surface tension of water. (3) Their aqueous solutions are alkaline. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 2000)

9. Which of the following statements is correct for a soapy detergent but incorrect for a soapless detergent? A. Its structure consists of a hydrophilic part and a hydrophobic part. B. It forms lather when shaken with distilled water. C. It can be made by reacting a vegetable oil with an alkali. D. It acts as an emulsifier in the cleaning process. (HKCEE 2001)

10. Soap was prepared by heating fat with sodium hydroxide solution for some time. Concentrated sodium chloride solution was then added to the resulting mixture. The purpose of adding concentrated sodium chloride solution is A. to help the precipitation of the soap. B. to enhance the cleansing power of the soap. C. to reduce the alkalinity of the soap. D. to act as a preservative for the soap. (HKCEE 2003)

11. A detergent has the structure shown below :

Which of the following concerning this detergent is correct? A. It is non-biodegradable. B. It functions well in hard water. C. It can be manufactured from vegetable oils. D. The portion, CH3(CH2)10CH2, is hydrophilic. (HKCEE 2003) -3-

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

12. 1st statement : Sodium carbonate can be used to soften hard water which contains calcium ions. 2nd statement : Sodium carbonate reacts with calcium ions in hard water to form a precipitate. (HKCEE 2003)

13. 1st statement : Sodium chloride is used in manufacture of soap. 2nd statement : Sodium chloride helps the precipitation of soap from soap solution. (HKCEE 2004)

14. Which of the following substances is NOT used for the preparation of soaps? A. vegetable oil B. sodium hydroxide solution C. concentrated sodium chloride solution D. concentrated sulphuric acid (HKCEE 2005)

15. Which of the following statements concerning soaps are correct? (1) Soaps are biodegradable. (2) Soaps have good cleaning power in hard water. (3) The structure of a soap particle consists of a hydrophilic part and a hydrophobic part. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3) (HKCEE 2005)

16.

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PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

Suggested Answers :

1.

B

2.

B

3.

D

4.

D

5.

A

6.

11.

B

12 .

A

13 .

A

14 .

D

15 .

B

16 .

17 .

18 .

26 .

27 .

28 .

21 .

22 .

23 .

24 .

25 .

-5-

C

7.

C

8.

C

9. 19 .

C

10 . 20 .

A

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

Explanation to M.C. Questions :

1. statement (1) : correct. The tri-ester can be broken down to alkanoic acid and alkanol by water (This process is known as hydrolysis) but the rate is very slow. Adding NaOH can increase the rate of this reaction. NaOH is, in fact, a catalyst. statement (2) : correct. After hydrolysis of tri-ester by water, an alkanoic acid is first formed. It is then neutralized by NaOH. CH3(CH2)16COOH(aq) + NaOH(aq) → CH3(CH2)16COO-Na+(aq) statement (3) : wrong. It is not used to decrease the solubility of soap in water but it is the role of concentrated sodium chloride. Ans. : B 2. Na+ is not the hydrophilic head of the soap molecule, it is just the counter-ion of the soap ion. Ans. : B 3. In this case, detergent acts as a wetting agent which can be explained by its presence of hydrophobic tail and hydrophilic head. The hydrophobic tail can dissolve in oil, while the hydrophilic head can dissolve in water. A : wrong. Detergent can dissolve readily in both oil (hydrophobic tail) and water (hydrophilic head). B : wrong. This statement is correct but it is not the explanation of this case. C : wrong. This statement is correct but it is not the explanation of this case. It can explain why detergent can remove oil and dirt from an object. D : correct. Before adding detergent, water can’t spread easily over the surface of an object because of its high surface tension. After adding detergent, detergent can help to interrupt the interaction between water molecules and hence reduce the surface tension of water. In other words, water can spread over the surface of the object. Ans. : D 4. A : wrong. The hydrocarbon chain is hydrophobic. B : wrong. It is a soapless detergent, which is manufactured from petroleum products. C : wrong. It is hard to be degraded by micro-organisms because of its presence of side-chains. D : correct. It is detergent, it can acts as a wetting agent and an emulsifying agent during the cleaning process. Ans. : D -6-

PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

5. statement (1) : correct. No matter it is soapy detergent or soapless detergent, when people use it for a long time, they will suffer from skin allergies. statement (2) : correct. statement (3) : wrong. Soapless detergents usually do not form scum with Ca2+ and Mg2+ ions which are present in sea water. Ans. : A 6. statement (1) : wrong. This method is used to prepare soapy detergent. But for soapless detergents, they are prepared using petroleum fractions. For example :

statement (2) : correct. No matter it is soapy detergent or soapless detergent, it can reduce the surface tension of water by interrupting the interaction between water molecules. statement (3) : correct. No matter it is soapy detergent or soapless detergent, it can serve as an emulsifying agent to remove grease or oil from the surface of an object during cleaning process. Ans. : C 7. 1st statement : false. Local tap water is soft water. 2nd statement : true. If water contains Ca2+ ions, soap will form insoluble scum with it. 2RCOO-(aq) + Ca2+(aq) → (RCOO)2Ca(s) Ans. : C

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PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

8. statement (1) : wrong. Soaps are produced from ester but they are not ester, they are metal carbonate compounds. For example : H

H

H

C

OOC(CH2)16CH3

H

C

OOC(CH2)16CH3

H

C

OOC(CH2)16CH3

+

3NaOH

H

H

C

OH

H

C

OH

H

C

OH

+

3CH3(CH2)16COONa

H

tri-ester

glycerol

soap

statement (2) : correct. Soap can reduce the surface tension of water by interrupting the interaction between water molecules. statement (3) : correct. Carbonate compounds are slightly alkaline. Ans. : C 9. A : wrong. Both soapy and soapless detergents consist of hydrophilic part and hydrophobic part. B : wrong. In distilled water, no or very low [Mg2+] or [Ca2+]. Both soapy and soapless detergents do not form scum in distilled water. They form lather when shaken with distilled water. C : correct. It is the way to manufacture soapy detergent but not soapless detergent. Soapless detergent can be manufactured from petroleumproducts. D : wrong. Both soapy and soapless detergents serve as emulsifying agent in cleaning process. Ans. : C 10. A : correct. Concentrated sodium chloride can decrease the solubility of soapy, thus soap can be precipitated out. B : wrong. The cleansing power of a soap depends on the structure of the soap. C : wrong. Alkalinity depends on the hydrophilic head of the soap. D : wrong. Sodium chloride is not a preservative. Ans. : A

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PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

11. A : wrong. It is biodegradable as the hydrocarbon chain is straight chain. B : correct. It does not form insoluble scum with Ca2+ or Mg2+ ion found in hard water. In other words, it can still form lather, i.e. its cleaning properties haven’t lost. C : wrong. It is manufactured from petroleum products. D : wrong. This portion, CH3(CH2)10CH2−, is hydrophobic. Ans. : B 12. 1st statement : true. There is an ion exchange between Na+ ion and Ca2+ ion when sodium carbonate is added to hard water. nd 2 statement : true. It is the correct explanation for the 1st statement. Ca2+ ions can be precipitated and Na+ ions then go to the water. This can help soften the hard water. Na2CO3(aq) + Ca2+(aq) → CaCO3(s) + 2Na+(aq) Ans. : A 13. 1st statement : true. 2nd statement : true. It is the correct explanation for the 1st statement. NaCl can decrease the solubility of soap in aqueous solution, i.e. soap can be precipitated out. Ans. : A 14. A : correct. B : correct. NaOH acts as a catalyst to speed up the rate of hydrolysis of tri-ester (found in vegetable oils / animal fats) to give glycerol and soap. C : correct. It can help precipitation of soap from soap solution. D : wrong. It is used in for preparation of soapless detergents. Ans. : D

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PLASTICS AND DETERGENTS

Textbook (Book 3) : p.199 – 224

15. statement (1) : correct. Since soaps are manufactured from vegetable oils or animal fats which can be decomposed easily by the bacterial action. Soaps are biodegradable. statement (2) : wrong. Soaps do not work well in hard water as they can form insoluble scum with Mg2+ or Ca2+ ion found in hard water. 2RCOONa(aq) + Mg2+(aq) → (RCOO)2Mg(s) + 2Na+(aq) 2RCOONa(aq) + Ca2+(aq) → (RCOO)2Ca(s) + 2Na+(aq) statement (3) : correct. The hydrophilic part is an ionic head and the hydrophobic part is a hydrocarbon chain. Ans. : B 16.

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