CARBON AND ITS COMPOUNDS

Transcription

1 CARBN AND ITS CMPUNDS Catenation: The unique property of carbon atom to form covalent bonds with other carbon atoms giving rise to large molecules is called catenation. The compounds formed may have long straight chains of carbon, branched chains of carbon or rings of carbon. In these compounds, carbon atoms may form single, double or triple covalent bonds. Tetravalency: Carbon has a valency of four (i.e., tetravalent). Hence, it can form bonds with four other atoms of carbon or atoms of other monovalent elements. Carbon, being small in size, forms strong bonds with many elements such as hydrogen (H), oxygen (), nitrogen (N), sulphur (S), chlorine (Cl), etc. Isomerism: Compounds with identical molecular formula but different structures are called isomers and the property is known as isomerism. Eg. Butane has molecular formula C 4 H 10. It forms two isomers: n-butane and isobutene. In n-butane, all carbon atoms are linked in a straight chain. The structure of n-butane is: H C C C C H In isobutene, three carbon atoms are linked in a straight chain and one carbon atom forms branched chain. The structure of isobutene is: H H C C C H C H Nomenclature of carbon compounds: Rule for naming carbon compounds are as follows: (a) Select the longest chain of carbon atoms containing the functional group. (b) Name the carbon chain based on the number of carbon atoms in the longest chain. (c) Indicate the functional group (if present) in the name of the compound with either a prefix or a suffix. Functional group Prefix/ Suffix Chloro Prefix => chloro Bromo Prefix => bromo Alchohol Suffix => ol Aldehyde Suffix => al Ketone Suffix => one Carboxylic acid Suffix => oic acid Double bond (Alkene) Suffix => ene Triple bond (Alkene) Suffix => yne (d) If a suffix is to be added, then replace the final e in the name of the carbon chain with the suffix. (e) If the carbon chain is unsaturated, then replace the final ane in the name of carbon chain with ene for alkenes or yne for alkynes. (f) Indicate the position of the functional group or side branch on the carbon chain by assigning it the lowest possible numerical prefix. QUESTINS: 1) What is hydrogenation? What is its industrial application? (a) The addition of hydrogen to an unsaturated hydrocarbon in the presence of a catalyst such as nickel or palladium to form a saturated hydrocarbon is called hydrogenation E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 1

2 (b) Industrial application: hydrogenation is used for preparing vegetable ghee (vanaspati) from vegetable oils, which have long unsaturated carbon chains. R R \ / C = C + H 2 / \ Vegetable oil (liquid state) R C C R Nickel catalyst eat Vegetable Ghee (solid state) 2) Give a test that can be used to differentiate chemically between butter and cooking oil. [CBSE 2011] (a) Butter generally has saturated carbon chains while cooking oil generally has unsaturated carbon chains. (b) Unsaturated hydrocarbons undergo hydrogenation (i.e. addition of H 2 ) in presence of a catalyst such as nickel or palladium. (c) Thus, cooking oil can be hydrogenated to form fats (vanaspati ghee) while butter cannot be hydrogenated. See reaction in Q. No. 2. Note: The presence of unsaturation (in cooking oil) can be detected using bromine water test. 3) What are the two properties of carbon which lead to the huge number of carbon compounds we see around us? The two properties of carbon which lead to the huge number of carbon compounds are: (a) Catenation: The unique property of carbon atom to form covalent bonds with other carbon atoms giving rise to large molecules is called catenation. Due to this property, straight chains, branched chains or rings of carbon atoms are formed. This results in huge number of carbon compounds having carbon atoms linked to each other. (b) Tetravalency: Carbon has a valency of four (i.e. tetravalent). Hence, it can form bonds with four other atoms of carbon or atoms of other monovalent elements. Carbon, being small in size, forms strong bonds with many elements such as hydrogen (H), oxygen (), nitrogen (N), sulphur (S), chlorine (Cl), etc. in addition, carbon atom can form multiple bonds with itself and atoms of other elements. This increases the number of carbon compounds. 4) Why is the conversion of ethanol to ethanoic acid an oxidation reaction? The conversion of ethanol to ethanoic acid involves addition of oxygen. Hence, it is an oxidation reaction. CH 3 CH 2 H + 2 Alkaline KMn 4 +eat r Acidified K 2 Cr heat Etanol xygen (from te oxidizing agent) CH 3 C H + H 2 Ethanoic acid Water 5) A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used? (a) Ethyne is an unsaturated hydrocarbon. n combustion in air, it produces a yellow flame with lots of black smoke. (b) Due to this incomplete combustion, heat produced is low and temperature obtained is also low. (c) For welding purpose, high and oxygen is used, complete combustion is achieved and 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 2

3 rovery high temperature required for welding is obtained. Hence, a mixture of ethyne and oxygen is used for welding instead of a mixture of ethyne and air. 6) What is oxidising agents? (a) The substances which are capable of adding oxygen to other substances are called oxidising agents. (b) The oxidising agent loses oxygen or gains hydrogen during the chemical reaction. (c) Alkaline potassium permanganate (KMn 4 ) or acidified potassium dichromate (K 2 Cr 2 7 ) are oxidising agents that can oxidise alcohols to carboxylic acids. 7) Unsaturated hydrocarbons contain multiple bonds between the two C-atoms and show addition reactions. Give the test to distinguish ethane from ethane. [NCERT Exemplar] Ethane and ethane can be distinguished based on the flame obtained when they undergo combustion (i.e. when they are burnt). Ethane, a saturated hydrocarbon, gives a clean, blue flame. Ethane, an unsaturated hydrocarbon, gives a yellow flame with lots of black smoke. 8) Intake of small quantity of methanol can be lethal. Comment. [NCERT Exemplar] (a) Methanol undergoes oxidation to form methanol in the liver. (b) Methanol reacts rapidly with various components of the cell and causes the protoplasm to get coagulated. (c) Methanol also affects the optic nerve which leads to blindness. Hence, intake of small quantity of methanol can be lethal. 9) What is meant by saponification? Give an example. [CBSE 2011] When an ester is heated with dilute sodium hydroxide (an alkali) solution, it gets converted back into the original alcohol and sodium salt of the original carboxylic acid. Such a reaction is known as saponification. Eg. CH 3 C CH 2 CH 3 + NaH Ethyl ethanoate Sodium hydroxide eat CH 3 CH 2 H + CH 3 C Na Ethanol Sodium ethanoate 10) List any four differences between soaps and detergents. [CBSE 2011] Soaps Detergents 1 Soaps are sodium or Detergents are potassium salts of ammonium, sulphate long chain carboxylic acids (fatty acids). salts of long chain hydrocarbons. 2 Soaps cannot be used Detergents can be in hard water. used in soft as well as hard water. 3 Soaps take time to Detergents dissolve dissolve in water. faster in water. 4 Soaps biodegradable. Detergents are generally nonbiodegradable. 11) Why detergents are better cleansing agents than soaps? Explain. [NCERT Exemplar] Soaps react with calcium and magnesium ions present in hard water to form insoluble substance called scum. As a result, more amount of soap is required. The scum formed also sticks to the clothes to be washed making it difficult to get clean clothes. However, detergents do not form insoluble precipitates with calcium and magnesium ions in hard 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 3

4 water. So, detergents can be used even in hard water. Detergents also lower the surface tension of water to a greater extent than soaps because detergents have better solubility in water. Hence, detergents are better cleansing agents than soaps. 12) A wax candle burns with yellow flame. Explain. When a candle is lighted, the wax (hydrocarbons) melts and rises up the wick, which then gets converted to vapours. However, due to insufficient supply of oxygen, the wax vapours undergo incomplete combustion resulting in the formation of unburnt carbon (soot). These carbon particles raises in the flame, gets heated and glow to impart yellowish colour to the flame. Hence, a wax candle burns with yellow flame. 13) Draw the structure of the micelle that would be formed if you dissolve soap in a hydrocarbon. When soap is dissolved in a hydrocarbon (non-polar solvent), the micelle formed will have the ionic ends in the interior of the cluster and the hydrophobic tails are on the surface of the cluster. (a) A homogeneous solution is formed because acetic acid is completely miscible in water. (b) The colour of the solution remains as it is and there is no evolution of gas or precipitation indicating that acetic acid does not react with water. 15) Write the structural formulae of all the isomers of hexane. [NCERT Exemplar] The structural formulae of the isomers of hexane are as follows: n-hexane 2-Methylpentane 3-Methylpentane Fig: Structure of micelle 14) 5 ml of dilute acetic acid was added to 5 ml of water in a test tube and the mixture was shaken for some time. Write two observations with appropriate reason. [CBSE 2016] 2,3-Dimethylbutane 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 4

5 2,2-Dimethylbutane 16) With the help of an example, explain the process of hydrogenation. Mention the essential conditions for the reaction and state the change in physical property with the formation of the product. [CBSE 2015] (a) Hydrogenation: Addition of hydrogen (H 2 ) to an unsaturated carbon compound in the presence of a catalyst such as nickel (Ni) or palladium (Pd) to give a saturated carbon compound is known as hydrogenation. Eg. Preparation of vegetable ghee (Vansapati) from vegetable oils, which have long unsaturated carbon chains. \ / C = C + H 2 / \ Vegetable oil (liquid state) R C C R Vegetable Ghee (solid state) Nickel catalyst eat (b) Essential conditions required for the addition reaction to occur are as follows: Double or triple bonds must be present between carbon atoms in the chain of hydrocarbon. Addition of hydrogen should be carried out in the presence of catalyst such as nickel or palladium. (c) The change observed in the physical property during hydrogenation of state (reactant) to solid state (product) [i.e. an increase in melting or boiling point]. 17) What are esters? How they are prepared? List two uses of esters. [CBSE 2014] (a) Esters are organic compounds having formula R C R Where R represents hydrocarbon chain. (b) Preparation of esters: When carboxylic acids are heated with alcohols in the presence of an acid catalyst, esters are produced. Eg. When ethanoic acid (acetic acid) reacts with ethanol in the presence of concentrated sulphuric acid as a catalyst, an ester (named as ethyl ethanoate, also called as ethyl acetate) and water are formed. Chemical equation: The class of compounds to which the fruity smelling compound belongs is esters. CH 3 C H + CH 3 CH 2 H Ethanoic acid Ethanol Conc. / Heat CH 3 C CH 2 CH 2 + H 2 Ethyl ethanoate (Eter) Water The name of the product formed is ethyl ethanoate E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 5

6 (c) Uses of esters: They are used in foods as flavours and fragrances. They are used as solvents. 18) List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed. [CBSE 2015] Tests for distinguishing between an alcohol and a carboxylic acid: (a) Litmus test: Take two strips of blue litmus paper. Place a drop of dilute aqueous solution of the alcohol and the carboxylic acid on these strips separately. Carboxylic acid turns blue litmus paper red while alcohol does not change the colour of blue litmus paper. (b) Sodium hydrogen carbonate test: A carboxylic acid and an alcohol can be distinguished using sodium hydrogen carbonate. Take about o.5 g of sodium hydrogen carbonate in two separate test tubes- A and B. Add 2mL of one compound to test tube A and 2mL of the other compound to test tube B. bserve for brisk effervescence or rapid evolution of gas bubbles. The compound, which produces brisk effervescence of carbon dioxide, on reacting with sodium hydrogen carbonate is carboxylic acid. Alcohols do not react with sodium hydrogen carbonate to give effervescence. 19) A hydrocarbon C 2 H 4 decolourises bromine water. (a) Name the hydrocarbon. (b) Mention the family to which it belongs. (c) Write the chemical equation of the reaction of this hydrocarbon with hydrogen. [CBSE 2016] (a) The name of the given hydrocarbon is ethane. (b) The family to which the given hydrocarbon belongs is alkenes. (c) Ethane reacts with hydrogen in presence of a catalyst such as nickel or palladium to form ethane. H C = C H + H H Nickel 473K H C C H Hydrogen Ethene Ethane 20) Explain the following: (a) CH 3 H is a weak acid. (b) Propene undergoes addition reaction. (c) The gas stoves have inlets for air. [CBSE 2015] (a) Ethanoic acid (CH 3 H) is a weak acid as it does not ionise completely in water. (b) Propene (CH 3 CH = CH 2 ) undergoes addition reaction as it has a double bond between two carbon atoms. (c) The gas stoves have inlets for air in order to provide sufficient air (or oxygen) supply for the complete combustion of fuel. 21) Complete the following equations: (a) CH 3 CC 2 H 5 NaH (b) CH 3 CH + NaHC 3 (c) CH 4 + Cl 2 Sunlight [CBSE 2013] NaH (a) CH 3 CC 2 H 5 CH3 CNa + C 2 H 5 H Ethyl ethanoate Sodium ethanoate Ethanol (b) CH 3 CH + NaHC 3 CH 3 CNa + C 2 + H 2 Ethanoic Sodium Acid ethanoate 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 6

7 (c) CH 4 + Cl 2 Sunlig t CH 3 Cl + HCl Methane Chlorine chloromethane Hydrogen Chloride 22) Elements forming ionic compounds attain noble gas electronic configuration by either gaining or losing electrons from their valence shells. Explain giving reasons why carbon cannot attain such a configuration in this manner to form its compounds. Name the type of bonds formed in ionic compounds and in the compounds formed by carbon. Also explain with reason why carbon compounds are generally poor conductors of electricity. [CBSE 2015] (a) The electronic configuration of carbon is (2, 4). It has four electrons in its valence shell. It could form C 4 anion by gaining four electrons and thus, completing its octet. However, it would be difficult for the nucleus of C 4 anion having six protons to hold on to ten electrons. It could form C 4+ cation by losing four electrons and thus, completing its octet. However, it would be difficult to remove four electrons to form C 4+ cation having six protons to hold on to just two electrons. Hence, carbon cannot attain noble gas electronic configuration by either gaining or losing electrons from its valence shell. (b) The types of bonds formed in ionic compounds are ionic or electrovalent bonds while in compounds formed by carbon are covalent bonds. (c) Carbon form covalent compounds by sharing of electrons between the atoms. These compounds do not form ions (or charge particles) when dissolved in a solvent or when melted. Hence, carbon compounds are generally poor conductors of electricity. 23) (a) Give a chemical test to distinguish between saturated and unsaturated hydrocarbons. (b) Name the products formed when ethane burns in the air. Write a balanced chemical equation for the reaction showing the types of energies liberated. (c) Why is the reaction between methane and chlorine in the presence of sunlight considered a substitution reaction? [CBSE 2016] (a) Saturated hydrocarbons give a clean flame on burning while unsaturated hydrocarbons give a yellow flame with lots of black smoke on burning. (b) Products formed when ethane burns in the air: Carbon dioxide C 2 and water vapour H 2 2C 2 H C 2 + 6H 2 Ethane xygen Carbon-dioxide Water vapours + Heat and light (c) The reaction between methane and chlorine in the presence of sunlight is considered a substitution reaction because chlorine replaces the hydrogen atoms one by one. When one hydrogen atom of methane is substituted by chlorine atom, chloromethane is formed. Sunlig t (d) CH 4 + Cl 2 CH 3 Cl + HCl Methane Chlorine chloromethane Hydrogen Chloride 24) Describe the addition reaction of carbon compounds with its application. State the function of catalyst in this reaction. How this reaction is different from a substitution reaction? [CBSE 2015] (a) The reaction in which two or more molecules react to form a single product is called as addition reaction. Unsaturated 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 7

8 carbon compounds having double or triple bonds undergo addition reactions. Application: Hydrogenation (i.e. addition of H 2 ) is used for converting vegetable oils to fats. This reaction is carried out in presence of nickel catalyst. \ / C = C + H 2 / \ Vegetable oil (liquid state) R C C R Nickel catalyst eat Vegetable Ghee (solid state) (b) The function of the catalyst is to increase the rate or the speed of reaction. (c) Substitution reaction involves replacement of an atom or a group of atoms in a molecule by another atom or group of atoms. In substitution reaction, two or more products are formed. In addition reaction, only one product is formed. 25) Both soap and detergent are some type of salts. What is the difference between them? Describe in brief the cleansing action of soap. Why do soaps not form lather in hard water? List two problems that arise due to the use of detergents instead of soaps. [CBSE 2015] (a) Difference between soaps and detergents: (Refer question no. 10). (b) Cleansing action of soap: The two ends of a soap molecule have different properties. The ionic end of the molecule is hydrophilic and it dissolves in water, while the hydrocarbon tail of the molecule is hydrophobic and it dissolves in hydrocarbons (i.e. it is insoluble in water). Inside water the soap molecules form clusters in which the hydrophobic ends are in the interior of the cluster while the hydrophilic ends are on the surface of the cluster. Such a spherical formation is called a micelle. The micelle formation is responsible for the cleansing action of soaps. (c) Soap does not form lather easily with hard water, which contains calcium and magnesium salts. This is because soap reacts with the calcium and magnesium ions to form insoluble precipitates of calcium and magnesium salts of fatty acid. (d) Two problems that arise due to the use of detergents instead of soaps are: Detergents are generally nonbiodegradable. Detergents cause soil and water pollution. 26) State the reason why carbon can neither form C 4+ cations nor C 4 anions but forms covalent compound. Also state the reason to explain why covalent compounds: (a) Are bad conductors of electricity? (b) Have how melting and boiling points. [CBSE 2014] The electronic configuration of carbon is (2, 4). It has four electrons in its valence shell. It could form C 4+ anion by gaining four electrons and thus, completing its octet. However, it 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 8

9 would be difficult for the nucleus of C 4 anion having six protons to hold on to ten electrons. It could form C 4+ cation by losing four electrons and thus, completing its octet. However, it would be difficult to remove four electrons to form C 4+ cation having six protons to hold on to just two electrons. Hence, carbon can neither form C 4+ cation nor C 4 anion. It forms covalent compounds by sharing its valence electrons with other atoms. (a) Carbon form covalent compounds by sharing of electrons between the atoms. These compounds do not form ions (or charge particles) when dissolved in a solvent or when melted. Hence, carbon compounds are generally poor conductors of electricity. (b) Carbon forms covalent compounds by sharing of electrons between the atoms. The covalently bonded molecules are held together by weak intermolecular forces. Hence, carbon compounds have low melting and boiling points. 27) Make the structure of methane by showing sharing of electrons between carbon and hydrogen atoms. How could you convert methane into chloroform by substitution reaction? Explain with the help of chemical reaction. [CBSE 2014] Electron dot structure of methane (CH 4 ): There are four single covalent bonds formed due to sharing of electrons between carbon and hydrogen atoms. When methane reacts with chlorine in presence of sunlight, a substitution reaction occurs forming chloromethane (CH 3 Cl). CH 4 + Sunlig t Cl 2 CH 3 Cl + HCl Methane Chlorine chloromethane Hydrogen Chloride n further reaction, the other hydrogen atoms will get substituted by chlorine and this will form chloroform (CHCl 3 ) as one of the products. CH 3 Cl + Sunlig t Cl 2 CH 2 Cl 2 + HCl Chloromethane Dichloromethane Sunlig t CH 2 Cl 2 + Cl 2 CHCl3 + HCl Dichloromethane Trichloromethane (Chloroform) 28) Write the balanced chemical equation for the following: (a) Methane is burned in sufficient air (b) Ethanol is treated with sodium (c) Ethanoic acid is reacted with sodium hydroxide. (d) Ethanoic acid is treated with sodium carbonate. (e) Ethanol is mixed with ethanoic acid in the presence of an acid. [CBSE 2012, 2014] (a) CH C 2 + H 2 Methane xygen Carbon-dioxide Water vapours + Heat and light (b) 2C 2 H 5 H + 2Na 2C 2 H 5 Na + H 2 Ethanol Sodium Sodium Hydrogen Ethoxide gas (c) CH 3 C H + NaH CH 3 C Na + H 2 Ethanoic acid Sodium Sodium Water Hydroxide ethanoate (d) CH 3 C H + Na 2 C 3 2CH 3 C Na + C 2 Ethanoic acid Sodium Sodium Carbon carbonate ethanoate dioxide + H 2 Water 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 9

10 (e) CH 3 C H + CH 3 CH 2 H Ethanoic acid Ethanol Cone. H 2 S 4 /Heat CH 3 C CH 2 CH 3 + H 2 Ethyl ethanoate (Ester) Water 29) (i) Complete the following equations: (a) CH 3 CH 2 H Cone.H 2S 4 /Heat (b) CH 3 CH + KH C 3 Sunlight (c) CH 4 + Cl 2 (ii) Write the name of the following: (a) CH 3 CH 2 CH, (b) CH 3 CH 2 CH 2 Br. (iii)draw the electron dot structure of ethane (C 2 H 4 ). [CBSE 2012] (i) (a) CH 3 CH 2 H Cone.H 2S 4 /Heat CH2 = CH 2 + H 2 Ethanol Ethane Water (b) CH 3 CH + KH C 3 CH 3 C K Ethanoic Potassium hydrogen Potassium acid carbonate ethanoate + C 2 + H 2 Carbon dioxide Water (c) CH 4 + Cl 2 Sunligt CHCl3 + HCl Methane Chlorine Chloromethane hydrogen Chloride (ii) (a) Propanoic acid (b) Bromopropane or (1-Bromopropane) (iii) Electron dot structure of ethane (C 2 H 4 ). 30) Explain the given reactions with the examples: (a) Hydrogenation reaction (b) xidation reaction (c) Substitution reaction (d) Saponification reaction (e) Combustion reaction [NCERT Exemplar] (a) The process in which unsaturated hydrocarbons react with hydrogen in the presence of a catalyst such as nickel or palladium to form saturated hydrocarbons is known as hydrogenation. Eg. Ethane on hydrogenation forms ethane. H C = C H + H Nickel /473K H C C H hydrogen Ethane Ethane (b) The process which involves addition of oxygen to any substances is called oxidation. Eg. Ethanol on oxidation form ethanoic acid. CH 3 CH 2 H Ethanol (Ethyl alcohol) Alkaline KMn 4 +Heat CH 3 CH or Acodofied K 2 Cr eat Ethanoic acid (Acetic acid) (c) The reaction in which replacement of an atom or a group of atoms in a molecule by another atom or group of atoms takes place is known as a substitution reaction. Eg. Chlorination of alkanes: When chlorine reacts with methane in presence of sunlight or UV light, from chloromethane ( CH 3 Cl). CH 4 + Cl 2 Sunlig t CH 3 Cl + HCl Methane Chlorine chloromethane (d) Refer Q. no. 9. (e) Carbon and carbon compounds burn in air or oxygen to form carbon dioxide, water vapour along with the release of a large amount of heat and light. Such reactions ae called combustion reactions. Eg. Combustion of ethanol: CH 3 CH 2 H C 2 + 3H 2 + Heat and ligt Ethanol xygen Carbon Water Dioxide vapour 2327 E SUDAMA NAGAING RAD NEAR PHTI KTHI SQUARE , Page 10