CBSE Class 11 Chemistry VBQs Hydrocarbons

CBSE Class 11 Chemistry VBQs Hydrocarbons. Value based questions are very important and always form part of examinations and class tests. Students are requested to download the VBQs and practice them to get better marks in examinations. 

UNIT – 13 (HYDROCARBONS)

1. Saturated aliphatic hydrocarbons are called alkanes. They are mainly used as fuels. Depending upon their physical properties such as boiling point, density, solubility, they have many other uses. Now answer the following questions.

(a) Out of propane and butane why is propane used as fuel for barbecue grill? 
Answer: Barbecue is usually done in open. In cold countries, although both propane and butane can be used but propane is preferred because it works better at low temperatures. The reason being that due to lower boiling point (231K), propane always remains as gas in the cylinder in spite of cooling produced by Joule Thomson effect during its use and thus ensures a constant supply of gas. Incontrast ,butane due to its higher boiling point(272.5K)and cooling produced by Joule-Thomson effect, it becomes a liquid in the cylinder and thus the supply of gas becomes slower and slower as the gas is being used.

(b) Which alkane is used in cigarette lighters?
Answer:  Butane is used in cigarette lighters since it can be easily liquefied at room temperature.

(c) What is mineral oil? Give its uses. 
Answer:  Mineral oil is a mixture of liquid alkanes containing 16-18 carbon atoms and is used as a laxative and as a lubricant.

(d) What are the uses of solid alkanes? 
Answer:  Alkanes with 18 or more carbon atoms are waxy solids. These are used as waxy coatings on fruits and vegetables to help them to retain moisture, inhibit mould growth and enhance appearance.

(e) What is Vaseline? What are its uses?
Answer:  Vaseline is a mixture of solid alkanes having low melting points. It can prevent moisture loss and chapped lips and soften nail cuticles. It is also used in ointments and cosmetics.

2. Benzene is the simplest and ideal aromatic compound. It is also one of the most basic petrochemicals. It is mainly used to prepare a number of important chemicals such as toluene, phenol, aniline, biphenyletc which are used in the manufacture of dyes, detergents, drugs, explosives, pesticides, rubbersandlubricants.But it is carcinogenic. Now answer the following questions.

(a) Mention 3 uses of benzene in petrochemical industry. (b)Why benzene is used as additive in gasoline?
Answer:  Benzene is mainly used as an intermediate to make other chemicals. About 85 of benzene is consumed in the production of 3 chemicals, ethyl benzene, cumene and cyclohexane. Ethyl benzene is a precursor to styrene, which is used to make polymers and plastics. Cumene is converted to phenol and acetone. Cyclohexane is used to manufacture nylon.

(b)Why benzene is used as additive in gasoline?
Answer: Because of its high octane number and anti-knocking properties benzene is used as an additive in gasoline.

(c) Comment on the bad effects and carcinogenicity of benzene.
Answer:  Exposure of benzene vapours to humans is a global problem. It affects liver, kidney, lungs and heart. Benzene increases the risk of cancer and other illness. In the body, it gets oxidized to benzene oxide which is not excreted readily and can interact with DNA to produce harmful mutations. Benzene is also a notorious cause of bone marrow failure.

(d) In view of toxicity and carcinogenicity of benzene, should its production be banned? Give your suggestions.
Answer: Although benzene is a potential carcinogen, its production cannot be banned because it’s wide spread application in the manufacture of dyes, detergents, nylons, explosives and pesticides.Instead,benzene should be used in fuming cupboards and ventilated laboratories.

3. Kamala and Reena are best friends. One evening Kamala went to Reena’s house and found her in the kitchen. She saw that the burner was emitting yellow flame and a black soot was getting deposited on the bottom of the cooking vessel .She advised Kamala to put off the gas and helped her in cleaning the holes of the burner. The gas was then switched on and they found that the burner was now emitting clean blue flame.

(i) Why was the burner emitting yellow flame?
Answer:  The holes of the burner were blocked due to soot or some oily material. This caused incomplete combustion of the fuel.

(ii) What was the purpose of cleaning the holes of the burner?
Answer:  By cleaning the holes of the burner the deposits were removed which led to the complete combustion of the fuel. 

(iii) What value is expressed by Kamala?
Answer: Kamala helped her friend in saving fuel and in preventing wastage of gas.

Please refer to the link below for CBSE Class 11 Chemistry VBQs Hydrocarbons