Carbon and its Compounds

Transcription

1 4 Carbo ad its Compouds I ed CHAPTER o N be C E re R pu T bl is h the last Chapter, we came to kow may compouds of importace to us. I this Chapter we will be studyig about some more iterestig compouds ad their properties. Also we shall be learig about carbo, a elemet which is of immese sigificace to us i both its elemetal form ad i the combied form. Thigs made of metal Activity 4.1 Thigs made of glass/clay Others Make a list of te thigs you have used or cosumed sice the morig. Compile this list with the lists made by your classmates ad the sort the items ito the followig Table. If there are items which are made up of more tha oe material, put them ito both the relevat colums. o tt Look at the items that come i the last colum your teacher will be able to tell you that most of them are made up of compouds of carbo. Ca you thik of a method to test this? What would be the product if a compoud cotaiig carbo is burt? Do you kow of ay test to cofirm this? Food, clothes, medicies, books, or may of the thigs that you listed are all based o this versatile elemet carbo. I additio, all livig structures are carbo based. The amout of carbo preset i the earth s crust ad i the atmosphere is quite meagre. The earth s crust has oly 0.02% carbo i the form of mierals (like carboates, hydrogecarboates, coal ad petroleum) ad the atmosphere has 0.03% of carbo dioxide. I spite of this small amout of carbo available i ature, the importace of carbo seems to be immese. I this Chapter, we will be lookig at the properties of carbo which lead to this aomaly BONDING IN CARBON THE COVALENT BOND I the previous Chapter, we have studied the properties of ioic compouds. We saw that ioic compouds have high meltig ad boilig poits ad coduct electricity i solutio or i the molte state. We also Sciece

2 saw how the ature of bodig i ioic compouds explais these properties. Let us ow study the properties of some carbo compouds. Meltig ad boilig poits of some carbo compouds are give i Table 4.1. Most carbo compouds are poor coductors of electricity as we have see i Chapter 2. From the data o the boilig ad meltig poits of the above compouds, we ca coclude that the forces of attractio betwee these molecules are ot very strog. Sice these compouds are largely ocoductors of electricity, we ca coclude that the bodig i these compouds does ot give rise to ay ios. I Class IX, we leart about the combiig capacity of various elemets ad how it depeds o the umber of valece electros. Let us ow look at the electroic cofiguratio of carbo. The atomic umber of carbo is 6. What would be the distributio of electros i various shells for carbo? How may valece electros will carbo have? We kow that the reactivity of elemets is explaied as their tedecy to attai a completely filled outer shell, that is, attai oble gas cofiguratio. Elemets formig ioic compouds achieve this by either gaiig or losig electros from the outermost shell. I the case of carbo, it has four electros i its outermost shell ad eeds to gai or lose four electros to attai oble gas cofiguratio. If it were to gai or lose electros Table 4.1 Meltig poits ad boilig poits of some compouds of carbo Compoud Meltig Boilig poit (K) poit (K) Acetic acid (CH 3 COOH) Chloroform (CHCl 3 ) Ethaol (CH 3 CH 2 OH) Methae (CH 4 ) (i) It could gai four electros formig C 4 aio. But it would be difficult for the ucleus with six protos to hold o to te electros, that is, four extra electros. (ii) It could lose four electros formig C 4+ catio. But it would require a large amout of eergy to remove four electros leavig behid a carbo catio with six protos i its ucleus holdig o to just two electros. Carbo overcomes this problem by sharig its valece electros with other atoms of carbo or with atoms of other elemets. Not just carbo, but may other elemets form molecules by sharig electros i this maer. The shared electros belog to the outer shells of both the atoms ad lead to both atoms attaiig the oble gas cofiguratio. Before goig o to compouds of carbo, let us look at some simple molecules formed by the sharig of valece electros. The simplest molecule formed i this maer is that of hydroge. As you have leart earlier, the atomic umber of hydroge is 1. Hece hydroge has oe electro i its K shell ad it requires oe more electro to fill the K shell. So two hydroge atoms share their electros to form a molecule of hydroge, H 2. This allows each hydroge atom to attai the Carbo ad its Compouds 59

3 Figure 4.1 A molecule of hydroge Figure 4.2 Sigle bod betwee two hydroge atoms Figure 4.3 Double bod betwee two oxyge atoms Figure 4.4 Triple bod betwee two itroge atoms electroic cofiguratio of the earest oble gas, helium, which has two electros i its K shell. We ca depict this usig dots or crosses to represet valece electros (Fig. 4.1). The shared pair of electros is said to costitute a sigle bod betwee the two hydroge atoms. A sigle bod is also represeted by a lie betwee the two atoms, as show i Fig The atomic umber of chlorie is 17. What would be its electroic cofiguratio ad its valecy? Chlorie forms a diatomic molecule, Cl 2. Ca you draw the electro dot structure for this molecule? Note that oly the valece shell electros eed to be depicted. I the case of oxyge, we see the formatio of a double bod betwee two oxyge atoms. This is because a atom of oxyge has six electros i its L shell (the atomic umber of oxyge is eight) ad it requires two more electros to complete its octet. So each atom of oxyge shares two electros with aother atom of oxyge to give us the structure show i Fig The two electros cotributed by each oxyge atom give rise to two shared pairs of electros. This is said to costitute a double bod betwee the two atoms. Ca you ow depict a molecule of water showig the ature of bodig betwee oe oxyge atom ad two hydroge atoms? Does the molecule have sigle bods or double bods? What would happe i the case of a diatomic molecule of itroge? Nitroge has the atomic umber 7. What would be its electroic cofiguratio ad its combiig capacity? I order to attai a octet, each itroge atom i a molecule of itroge cotributes three electros givig rise to three shared pairs of electros. This is said to costitute a triple bod betwee the two atoms. The electro dot structure of N 2 ad its triple bod ca be depicted as i Fig A molecule of ammoia has the formula NH 3. Ca you draw the electro dot structure for this molecule showig how all four atoms achieve oble gas cofiguratio? Will the molecule have sigle, double or triple bods? Let us ow take a look at methae, which is a compoud of carbo. Methae is widely used as a fuel ad is a major compoet of bio-gas ad Compressed Natural Gas (CNG). It is also oe of the simplest compouds formed by carbo. Methae has a formula CH 4. Hydroge, as you kow, has a valecy of 1. Carbo is tetravalet because it has four valece electros. I order to achieve oble gas cofiguratio, carbo shares these electros with four atoms of hydroge as show i Fig Such bods which are formed by the sharig of a electro pair betwee two atoms are kow as covalet bods. Covaletly boded molecules are see to have strog bods withi the molecule, but itermolecular forces are small. This gives rise to the low meltig ad boilig 60 Sciece

4 poits of these compouds. Sice the electros are shared betwee atoms ad o charged particles are formed, such covalet compouds are geerally poor coductors of electricity. ed Figure 4.5 Electro dot structure for methae o N be C E re R pu T bl is h More to Kow! Allotropes of carbo The elemet carbo occurs i differet forms i ature with widely varyig physical properties. Both diamod ad graphite are formed by carbo atoms, the differece lies i the maer i which the carbo atoms are boded to oe aother. I diamod, each carbo atom is boded to four other carbo atoms formig a rigid three-dimesioal structure. I graphite, each carbo atom is boded to three other carbo atoms i the same plae givig a hexagoal array. Oe of these bods is a double-bod, ad thus the valecy of carbo is satisfied. Graphite structure is formed by the hexagoal arrays beig placed i layers oe above the other. The structure of diamod The structure of graphite The structure of C-60 Buckmisterfulleree tt These two differet structures result i diamod ad graphite havig very differet physical properties eve though their chemical properties are the same. Diamod is the hardest substace kow while graphite is smooth ad slippery. Graphite is also a very good coductor of electricity ulike other o-metals that you studied i the previous Chapter. Diamods ca be sythesised by subjectig pure carbo to very high pressure ad temperature. These sythetic diamods are small but are otherwise idistiguishable from atural diamods. Fullerees form aother class of carbo allotropes. The first oe to be idetified was C-60 which has carbo atoms arraged i the shape of a football. Sice this looked like the geodesic dome desiged by the US architect Buckmister Fuller, the molecule was amed fulleree. Q 2. E S T I O N S What would be the electro dot structure of carbo dioxide which has the formula CO2? What would be the electro dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hit The eight atoms of sulphur are joied together i the form of a rig.) o 1. U Carbo ad its Compouds? 61

5 4.2 VERSATILE NATURE OF CARBON We have see the formatio of covalet bods by the sharig of electros i various elemets ad compouds. We have also see the structure of a simple carbo compoud, methae. I the begiig of the Chapter, we saw how may thigs we use cotai carbo. I fact, we ourselves are made up of carbo compouds. The umbers of carbo compouds whose formulae are kow to chemists was recetly estimated to be about three millio! This outumbers by a large margi the compouds formed by all the other elemets put together. Why is it that this property is see i carbo ad o other elemet? The ature of the covalet bod eables carbo to form a large umber of compouds. Two factors oticed i the case of carbo are (i) (ii) Carbo has the uique ability to form bods with other atoms of carbo, givig rise to large molecules. This property is called cateatio. These compouds may have log chais of carbo, brached chais of carbo or eve carbo atoms arraged i rigs. I additio, carbo atoms may be liked by sigle, double or triple bods. Compouds of carbo, which are liked by oly sigle bods betwee the carbo atoms are called saturated compouds. Compouds of carbo havig double or triple bods betwee their carbo atoms are called usaturated compouds. No other elemet exhibits the property of cateatio to the extet see i carbo compouds. Silico forms compouds with hydroge which have chais of upto seve or eight atoms, but these compouds are very reactive. The carbo-carbo bod is very strog ad hece stable. This gives us the large umber of compouds with may carbo atoms liked to each other. Sice carbo has a valecy of four, it is capable of bodig with four other atoms of carbo or atoms of some other moo-valet elemet. Compouds of carbo are formed with oxyge, hydroge, itroge, sulphur, chlorie ad may other elemets givig rise to compouds with specific properties which deped o the elemets other tha carbo preset i the molecule. Agai the bods that carbo forms with most other elemets are very strog makig these compouds exceptioally stable. Oe reaso for the formatio of strog bods by carbo is its small size. This eables the ucleus to hold o to the shared pairs of electros strogly. The bods formed by elemets havig larger atoms are much weaker. 62 Sciece

6 More to Kow! Orgaic compouds The two characteristic features see i carbo, that is, tetravalecy ad cateatio, put together give rise to a large umber of compouds. May have the same o-carbo atom or group of atoms attached to differet carbo chais. These compouds were iitially extracted from atural substaces ad it was thought that these carbo compouds or orgaic compouds could oly be formed withi a livig system. That is, it was postulated that a vital force was ecessary for their sythesis. Friedrich Wöhler disproved this i 1828 by preparig urea from ammoium cyaate. But carbo compouds, except for oxides of carbo, carboate ad hydrogecarboate salts cotiue to be studied uder orgaic chemistry Saturated ad Usaturated Carbo Compouds We have already see the structure of methae. Aother compoud formed betwee carbo ad hydroge is ethae with a formula of C 2 H 6. I order to arrive at the structure of simple carbo compouds, the first step is to lik the carbo atoms together with a sigle bod (Fig. 4.6a) ad the use the hydroge atoms to satisfy the remaiig valecies of carbo (Fig. 4.6b). For example, the structure of ethae is arrived i the followig steps C C Step 1 Figure 4.6 (a) Carbo atoms liked together with a sigle bod Three valecies of each carbo atom remai usatisfied, so each is boded to three hydroge atoms givig: Step 2 Figure 4.6 (b) Each carbo atom boded to three hydroge atoms The electro dot structure of ethae is show i Fig. 4.6(c). Ca you draw the structure of propae, which has the molecular formula C 3 H 8 i a similar maer? You will see that the valecies of all the atoms are satisfied by sigle bods betwee them. Such carbo compouds are called saturated compouds. These compouds are ormally ot very reactive. However, aother compoud of carbo ad hydroge has the formula C 2 H 4 ad is called ethee. How ca this molecule be depicted? We follow the same step-wise approach as above. Each carbo atom gets two hydroge atoms to give We see that oe valecy per carbo atom remais usatisfied. This ca be satisfied oly if there is a double bod betwee the two carbos givig us Figure 4.6 (c) Electro dot structure of ethae C C Step 1 Step 2 Step 3 Carbo ad its Compouds 63

7 Figure 4.7 Structure of ethee The electro dot structure for ethee is give i Fig Yet aother compoud of hydroge ad carbo has the formula C 2 H 2 ad is called ethye. Ca you draw the electro dot structure for ethye? How may bods are ecessary betwee the two carbo atoms i order to satisfy their valecies? Such compouds of carbo havig double or triple bods betwee the carbo atoms are kow as usaturated carbo compouds ad they are more reactive tha the saturated carbo compouds Chais, Braches ad Rigs I the earlier sectio, we metioed the carbo compouds methae, ethae ad propae, cotaiig respectively 1, 2 ad 3 carbo atoms. Such chais of carbo atoms ca cotai tes of carbo atoms. The ames ad structures of six of these are give i Table 4.2. Table 4.2 Formulae ad structures of saturated compouds of carbo ad hydroge No. of C Name Formula Structure atoms 1 Methae CH 4 2 Ethae C 2 H 6 3 Propae C 3 H 8 4 Butae C 4 H 10 5 Petae C 5 H 12 6 Hexae C 6 H Sciece

8 But, let us take aother look at butae. If we make the carbo skeleto with four carbo atoms, we see that two differet skeletos are possible C C C C Figure 4.8 (a) Two possible carbo-skeletos Fillig the remaiig valecies with hydroge gives us Figure 4.8 (b) Complete molecules for two structures with formula C 4 H 10 We see that both these structures have the same formula C 4 H 10. Such compouds with idetical molecular formula but differet structures are called structural isomers. I additio to straight ad brached carbo chais, some compouds have carbo atoms arraged i the form of a rig. For example, cyclohexae has the formula C 6 H 12 ad the followig structure (a) Figure 4.9 Structure of cyclohexae (a) carbo skeleto (b) complete molecule Ca you draw the electro dot structure for cyclohexae? Straight chai, brached chai ad cyclic carbo compouds, all may be saturated or usaturated. For example, bezee, C 6 H 6, has the followig structure Bezee C 6 H 6 (b) Figure 4.10 Structure of bezee All these carbo compouds which cotai just carbo ad hydroge are called hydrocarbos. Amog these, the saturated hydrocarbos are called alkaes. The usaturated hydrocarbos which cotai oe or more double bods are called alkees. Those cotaiig oe or more triple bods are called alkyes Will you be my Fried? Carbo seems to be a very friedly elemet. So far we have bee lookig at compouds of carbo ad hydroge. But carbo also forms bods Carbo ad its Compouds 65

9 with other elemets such as haloges, oxyge, itroge ad sulphur. I a hydrocarbo chai, oe or more hydroges ca be replaced by these elemets, such that the valecy of carbo remais satisfied. I such compouds, the elemet replacig hydroge is referred to as a heteroatom. These heteroatoms are also preset i some Table 4.3 Some fuctioal groups i carbo compouds groups as give i Table Hetero atom Fuctioal group Formula of fuctioal group 4.3. These heteroatoms ad the group cotaiig these cofer specific properties to the compoud, regardless Cl/Br Halo- (Chloro/bromo) Cl, Br of the legth ad ature (substitutes for hydroge atom) of the carbo chai ad hece are called Oxyge 1. Alcohol 2. Aldehyde 3. Ketoe 4. Carboxylic acid OH fuctioal groups. Some importat fuctioal groups are give i the Table 4.3. Free valecy or valecies of the group are show by the sigle lie. The fuctioal group is attached to the carbo chai through this valecy by replacig oe hydroge atom or atoms Homologous Series You have see that carbo atoms ca be liked together to form chais of varyig legths. I additio, hydroge atom or atoms o these carbo chais ca be replaced by ay of the fuctioal groups that we saw above. The presece of a fuctioal group such as alcohol dictates the properties of the carbo compoud, regardless of the legth of the carbo chai. For example, the chemical properties of CH 3 OH, C 2 H 5 OH, C 3 H 7 OH ad C 4 H 9 OH are all very similar. Hece, such a series of compouds i which the same fuctioal group substitutes for hydroge i a carbo chai is called a homologous series. Let us look at the homologous series that we saw earlier i Table 4.2. If we look at the formulae of successive compouds, say CH 4 ad C 2 H 6 these differ by a CH 2 - uit C 2 H 6 ad C 3 H 8 these differ by a CH 2 - uit What is the differece betwee the ext pair propae ad butae (C 4 H 10 )? Ca you fid out the differece i molecular masses betwee these pairs (the atomic mass of carbo is 12 u ad the atomic mass of hydroge is 1 u)? Similarly, take the homologous series for alkees. The first member of the series is ethee which we have already come across i Sectio What is the formula for ethee? The succeedig members have the formula C 3 H 6, C 4 H 8 ad C 5 H 10. Do these also differ by a CH 2 66 Sciece

10 uit? Do you see ay relatio betwee the umber of carbo ad hydroge atoms i these compouds? The geeral formula for alkees ca be writte as C H 2, where = 2, 3, 4. Ca you similarly geerate the geeral formula for alkaes ad alkyes? As the molecular mass icreases i ay homologous series, a gradatio i physical properties is see. This is because the meltig ad boilig poits icrease with icreasig molecular mass. Other physical properties such as solubility i a particular solvet also show a similar gradatio. But the chemical properties, which are determied solely by the fuctioal group, remai similar i a homologous series. Activity 4.2 Calculate the differece i the formulae ad molecular masses for (a) CH 3 OH ad C 2 H 5 OH (b) C 2 H 5 OH ad C 3 H 7 OH, ad (c) C 3 H 7 OH ad C 4 H 9 OH. Is there ay similarity i these three? Arrage these alcohols i the order of icreasig carbo atoms to get a family. Ca we call this family a homologous series? Geerate the homologous series for compouds cotaiig up to four carbos for the other fuctioal groups give i Table Nomeclature of Carbo Compouds The ames of compouds i a homologous series are based o the ame of the basic carbo chai modified by a prefix phrase before or suffix phrase after idicatig the ature of the fuctioal group. For example, the ames of the alcohols take i Activity 4.2 are methaol, ethaol, propaol ad butaol. Namig a carbo compoud ca be doe by the followig method (i) Idetify the umber of carbo atoms i the compoud. A compoud havig three carbo atoms would have the ame propae. (ii) I case a fuctioal group is preset, it is idicated i the ame of the compoud with either a prefix or a suffix (as give i Table 4.4). (iii) If the ame of the fuctioal group is to be give as a suffix, the ame of the carbo chai is modified by deletig the fial e ad addig the appropriate suffix. For example, a three-carbo chai with a ketoe group would be amed i the followig maer Propae e = propa + oe = propaoe. (iv) If the carbo chai is usaturated, the the fial ae i the ame of the carbo chai is substituted by ee or ye as give i Table 4.4. For example, a three-carbo chai with a double bod would be called propee ad if it has a triple bod, it would be called propye. Carbo ad its Compouds 67

11 Table 4.4 Nomeclature of fuctioal groups Fuctioal Prefix/Suffix Example group 1. Haloge Prefix-chloro, Chloropropae bromo, etc. Q U E S T I O N S 1. How may structural isomers ca you draw for petae? 2. What are the two properties of carbo which lead to the huge umber of carbo compouds we see aroud us? 3. What will be the formula ad electro dot structure of cyclopetae? Bromopropae 2. Alcohol Suffix - ol Propaol 3. Aldehyde Suffix - al Propaal 4. Ketoe Suffix - oe Propaoe 5. Carboxylic acid Suffix - oic acid Propaoic acid 6. Double bod (alkees) Suffix - ee Propee 7. Triple bod (alkyes) Suffix - ye Propye 68 Sciece

12 4. Draw the structures for the followig compouds. (i) Ethaoic acid (ii) Bromopetae* (iii) Butaoe (iv) Hexaal. *Are structural isomers possible for bromopetae? 5. How would you ame the followig compouds? (i) CH 3 CH 2 Br (iii) 4.3 CHEMICAL PROPERTIES OF CARBON COMPOUNDS I this sectio we will be studyig about some of the chemical properties of carbo compouds. Sice most of the fuels we use are either carbo or its compouds, we shall first study combustio Combustio Carbo, i all its allotropic forms, burs i oxyge to give carbo dioxide alog with the release of heat ad light. Most carbo compouds also release a large amout of heat ad light o burig. These are the oxidatio reactios that you leart about i the first Chapter (i) C + O 2 CO 2 + heat ad light (ii) CH 4 + O 2 CO 2 + H 2 O + heat ad light (iii) CH 3 CH 2 OH + O 2 CO 2 + H 2 O + heat ad light Balace the latter two reactios like you leart i the first Chapter. Activity 4.3 CAUTION: This Activity eeds the teacher s assistace. Take some carbo compouds (aphthalee, camphor, alcohol) oe by oe o a spatula ad bur them. Observe the ature of the flame ad ote whether smoke is produced. Place a metal plate above the flame. Is there a depositio o the plate i case of ay of the compouds? (ii) Activity 4.4? Light a buse burer ad adjust the air hole at the base to get differet types of flames/presece of smoke. Whe do you get a yellow, sooty flame? Whe do you get a blue flame? Saturated hydrocarbos will geerally give a clea flame while usaturated carbo compouds will give a yellow flame with lots of black smoke. This results i a sooty deposit o the metal plate i Activity 4.3. However, limitig the supply of air results i icomplete combustio of eve saturated hydrocarbos givig a sooty flame. The gas/kerosee stove used at home has ilets for air so that a sufficietly oxyge-rich Carbo ad its Compouds 69

13 Do You Kow? More to Kow! mixture is burt to give a clea blue flame. If you observe the bottoms of cookig vessels gettig blackeed, it meas that the air holes are blocked ad fuel is gettig wasted. Fuels such as coal ad petroleum have some amout of itroge ad sulphur i them. Their combustio results i the formatio of oxides of sulphur ad itroge which are major pollutats i the eviromet. Why do substaces bur with or without a flame? Have you ever observed either a coal or a wood fire? If ot, the ext time you get a chace, take close ote of what happes whe the wood or coal starts to bur. You have see above that a cadle or the LPG i the gas stove burs with a flame. However, you will observe the coal or charcoal i a agithi sometimes just glows red ad gives out heat without a flame. This is because a flame is oly produced whe gaseous substaces bur. Whe wood or charcoal is igited, the volatile substaces preset vapourise ad bur with a flame i the begiig. A lumious flame is see whe the atoms of the gaseous substace are heated ad start to glow. The colour produced by each elemet is a characteristic property of that elemet. Try ad heat a copper wire i the flame of a gas stove ad observe its colour. You have see that icomplete combustio gives soot which is carbo. O this basis, what will you attribute the yellow colour of a cadle flame to? Formatio of coal ad petroleum Coal ad petroleum have bee formed from biomass which has bee subjected to various biological ad geological processes. Coal is the remais of trees, fers, ad other plats that lived millios of years ago. These were crushed ito the earth, perhaps by earthquakes or volcaic eruptios. They were pressed dow by layers of earth ad rock. They slowly decayed ito coal. Oil ad gas are the remais of millios of tiy plats ad aimals that lived i the sea. Whe they died, their bodies sak to the sea bed ad were covered by silt. Bacteria attacked the dead remais, turig them ito oil ad gas uder the high pressures they were beig subjected to. Meawhile, the silt was slowly compressed ito rock. The oil ad gas seeped ito the porous parts of the rock, ad got trapped like water i a spoge. Ca you guess why coal ad petroleum are called fossil fuels? Activity Oxidatio Take about 3 ml of ethaol i a test tube ad warm it getly i a water bath. Add a 5% solutio of alkalie potassium permagaate drop by drop to this solutio. Does the colour of potassium permagaate persist whe it is added iitially? Why does the colour of potassium permagaate ot disappear whe excess is added? You have leart about oxidatio reactios i the first Chapter. Carbo compouds ca be easily oxidised o combustio. I additio to this complete oxidatio, we have reactios i which alcohols are coverted to carboxylic acids 70 Sciece

14 We see that some substaces are capable of addig oxyge to others. These substaces are kow as oxidisig agets. Alkalie potassium permagaate or acidified potassium dichromate are oxidisig alcohols to acids, that is, addig oxyge to the startig material. Hece they are kow as oxidisig agets Additio Reactio Usaturated hydrocarbos add hydroge i the presece of catalysts such as palladium or ickel to give saturated hydrocarbos. Catalysts are substaces that cause a reactio to occur or proceed at a differet rate without the reactio itself beig affected. This reactio is commoly used i the hydrogeatio of vegetable oils usig a ickel catalyst. Vegetable oils geerally have log usaturated carbo chais while aimal fats have saturated carbo chais. You must have see advertisemets statig that some vegetable oils are healthy. Aimal fats geerally cotai saturated fatty acids which are said to be harmful for health. Oils cotaiig usaturated fatty acids should be chose for cookig Substitutio Reactio Saturated hydrocarbos are fairly ureactive ad are iert i the presece of most reagets. However, i the presece of sulight, chlorie is added to hydrocarbos i a very fast reactio. Chlorie ca replace the hydroge atoms oe by oe. It is called a substitutio reactio because oe type of atom or a group of atoms takes the place of aother. A umber of products are usually formed with the higher homologues of alkaes. CH 4 + Cl 2 CH 3 Cl + HCl (i the presece of sulight) Q U E S T I O N S 1. Why is the coversio of ethaol to ethaoic acid a oxidatio reactio? 2. A mixture of oxyge ad ethye is burt for weldig. Ca you tell why a mixture of ethye ad air is ot used? 4.4 SOME IMPORTANT CARBON COMPOUNDS ETHANOL AND ETHANOIC ACID May carbo compouds are ivaluable to us. But here we shall study the properties of two commercially importat compouds ethaol ad ethaoic acid.? Carbo ad its Compouds 71

15 Do You Kow? Activity Properties of Ethaol Ethaol is a liquid at room temperature (refer to Table 4.1 for the meltig ad boilig poits of ethaol). Ethaol is commoly called alcohol ad is the active igrediet of all alcoholic driks. I additio, because it is a good solvet, it is also used i medicies such as ticture iodie, cough syrups, ad may toics. Ethaol is also soluble i water i all proportios. Cosumptio of small quatities of dilute ethaol causes drukeess. Eve though this practice is codemed, it is a socially widespread practice. However, itake of eve a small quatity of pure ethaol (called absolute alcohol) ca be lethal. Also, log-term cosumptio of alcohol leads to may health problems. Reactios of Ethaol Teacher s demostratio Drop a small piece of sodium, about the size of a couple of grais of rice, ito ethaol (absolute alcohol). What do you observe? How will you test the gas evolved? (i) Reactio with sodium 2Na + 2CH 3 CH 2 OH 2CH 3 CH 2 O Na + + H 2 (Sodium ethoxide) Alcohols react with sodium leadig to the evolutio of hydroge. With ethaol, the other product is sodium ethoxide. Ca you recall which other substaces produce hydroge o reactig with metals? (ii) Reactio to give usaturated hydrocarbo: Heatig ethaol at 443 K with excess cocetrated sulphuric acid results i the dehydratio of ethaol to give ethee Hot coc. CH3 CH2OH H SO CH 2 = CH 2 + H2O 2 4 The cocetrated sulphuric acid ca be regarded as a dehydratig aget which removes water from ethaol. How do alcohols affect livig beigs? Whe large quatities of ethaol are cosumed, it teds to slow metabolic processes ad to depress the cetral ervous system. This results i lack of coordiatio, metal cofusio, drowsiess, lowerig of the ormal ihibitios, ad fially stupour. The idividual may feel relaxed but does ot realise that his sese of judgemet, sese of timig, ad muscular coordiatio have bee seriously impaired. Ulike ethaol, itake of methaol i very small quatities ca cause death. Methaol is oxidised to methaal i the liver. Methaal reacts rapidly with the compoets of cells. It causes the protoplasm to get coagulated, i much the same way a egg is coagulated by cookig. Methaol also affects the optic erve, causig blidess. Ethaol is a importat idustrial solvet. To prevet the misuse of ethaol produced for idustrial use, it is made ufit for drikig by addig poisoous substaces like methaol to it. Dyes are also added to colour the alcohol blue so that it ca be idetified easily. This is called deatured alcohol. NCERT 72 Sciece

16 More to Kow! Alcohol as a fuel Sugarcae plats are oe of the most efficiet covertors of sulight ito chemical eergy. Sugarcae juice ca be used to prepare molasses which is fermeted to give alcohol (ethaol). Some coutries ow use alcohol as a additive i petrol sice it is a cleaer fuel which gives rise to oly carbo dioxide ad water o burig i sufficiet air (oxyge) Properties of Ethaoic Acid Ethaoic acid is commoly called acetic acid ad belogs to a group of acids called carboxylic acids. 5-8% solutio of acetic acid i water is called viegar ad is used widely as a preservative i pickles. The meltig poit of pure ethaoic acid is 290 K ad hece it ofte freezes durig witer i cold climates. This gave rise to its ame glacial acetic acid. The group of orgaic compouds called carboxylic acids are obviously characterised by a special acidity. However, ulike mieral acids like HCl, which are completely ioised, carboxylic acids are weak acids. Activity 4.8 Take 1 ml ethaol (absolute alcohol) ad 1 ml glacial acetic acid alog with a few drops of cocetrated sulphuric acid i a test tube. Warm i a water-bath for at least five miutes as show i Fig Pour ito a beaker cotaiig ml of water ad smell the resultig mixture. Reactios of ethaoic acid: (i) Esterificatio reactio: Esters are most commoly formed by reactio of a acid ad a alcohol. Ethaoic acid reacts with absolute ethaol i the presece of a acid catalyst to give a ester CH COOH + CH CH OH Acid CH C O CH CH O (Ethaoic acid) (Ethaol) (Ester) Activity 4.7 Compare the ph of dilute acetic acid ad dilute hydrochloric acid usig both litmus paper ad uiversal idicator. Are both acids idicated by the litmus test? Does the uiversal idicator show them as equally strog acids? Carbo ad its Compouds Esters are sweet-smellig substaces. These are used i makig perfumes ad as flavourig agets. O treatig with sodium hydroxide, which is a alkali, the ester is coverted back to alcohol ad sodium salt of carboxylic acid. This reactio is kow as sapoificatio because it is used i the preparatio of soap. Figure 4.11 Formatio of ester

17 CH3COOC2H5 NaOH C2H5OH+CH3COONa (ii) Reactio with a base: Like mieral acids, ethaoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethaoate or commoly called sodium acetate) ad water: NaOH + CH3COOH CH3COONa + H2O How does ethaoic acid react with carboates ad hydrogecarboates? ed Let us perform a activity to fid out. Activity 4.9 Set up the apparatus as show i Chapter 2, Activity 2.5. Take a spatula full of sodium carboate i a test tube ad add 2 ml of dilute ethaoic acid. What do you observe? Pass the gas produced through freshly prepared lime-water. What do you observe? Ca the gas produced by the reactio betwee ethaoic acid ad sodium carboate be idetified by this test? Repeat this Activity with sodium hydrogecarboate istead of sodium carboate. o N be C E re R pu T bl is h (iii) Reactio with carboates ad hydrogecarboates: Ethaoic acid reacts with carboates ad hydrogecarboates to give rise to a salt, carbo dioxide ad water. The salt produced is commoly called sodium acetate. 2CH3COOH + Na2CO3 2CH 3COONa + H2O + CO2 CH 3COOH + NaHCO 3 CH3COONa + H2O + CO2 Q U E S T I O N S How would you distiguish experimetally betwee a alcohol ad a carboxylic acid? What are oxidisig agets?? 4.5 SOAPS AND DETERGENTS Activity 4.10 o tt Figure 4.12 Formatio of micelles 74 Take about 10 ml of water each i two test tubes. Add a drop of oil (cookig oil) to both the test tubes ad label them as A ad B. To test tube B, add a few drops of soap solutio. Now shake both the test tubes vigourously for the same period of time. Ca you see the oil ad water layers separately i both the test tubes immediately after you stop shakig them? Leave the test tubes udisturbed for some time ad observe. Does the oil layer separate out? I which test tube does this happe first? Sciece

18 ed This activity demostrates the effect of soap i cleaig. Most dirt is oily i ature ad as you kow, oil does ot dissolve i water. The molecules of soap are sodium or potassium salts of log-chai carboxylic acids. The ioic-ed of soap dissolves i water while the carbo chai dissolves i oil. The soap molecules, thus form structures called micelles (see Fig. 4.12) where oe ed of the molecules is towards the oil droplet while the ioic-ed faces outside. This forms a emulsio i water. The soap micelle thus helps i dissolvig the dirt i water ad we ca wash our clothes clea (Fig. 4.13). Ca you draw the structure of the micelle that would be formed if you dissolve soap i a hydrocarbo? o N be C E re R pu T bl is h o tt More to Kow! Micelles Soaps are molecules i which the two eds have differig properties, oe is hydrophilic, that is, it dissolves i water, while the other ed is hydrophobic, that is, it dissolves i hydrocarbos. Whe soap is at the surface of water, the hydrophobic tail of soap will ot be soluble i water ad the soap will alig alog the surface of water with the ioic ed i water ad the hydrocarbo tail protrudig out of water. Iside water, Carbo ad its Compouds these molecules have a uique orietatio that keeps the hydrocarbo portio out of the water. This is achieved by formig clusters of molecules i which the hydrophobic tails are i the iterior of the cluster ad the ioic eds are o the surface of the cluster. This formatio is called a micelle. Soap i the form of a micelle is able to clea, sice the oily dirt will be collected i the cetre of the micelle. The micelles stay i solutio as a colloid ad will ot come together to precipitate because of io-io repulsio. Thus, the dirt suspeded i the micelles is also easily rised away. The soap micelles are large eough to scatter light. Hece a soap solutio appears cloudy. Figure 4.13 Effect of soap i cleaig 75

19 Activity 4.11 Take about 10 ml of distilled water (or rai water) ad 10 ml of hard water (from a tubewell or had-pump) i separate test tubes. Add a couple of drops of soap solutio to both. Shake the test tubes vigorously for a equal period of time ad observe the amout of foam formed. I which test tube do you get more foam? I which test tube do you observe a white curdy precipitate? Note for the teacher: If hard water is ot available i your locality, prepare some hard water by dissolvig hydrogecarboates/ sulphates/chlorides of calcium or magesium i water. Activity 4.12 Take two test tubes with about 10 ml of hard water i each. Add five drops of soap solutio to oe ad five drops of deterget solutio to the other. Shake both test tubes for the same period. Do both test tubes have the same amout of foam? I which test tube is a curdy solid formed? Have you ever observed while bathig that foam is formed with difficulty ad a isoluble substace (scum) remais after washig with water? This is caused by the reactio of soap with the calcium ad magesium salts, which cause the hardess of water. Hece you eed to use a larger amout of soap. This problem is overcome by usig aother class of compouds called detergets as cleasig agets. Detergets are geerally ammoium or sulphoate salts of log chai carboxylic acids. The charged eds of these compouds do ot form isoluble precipitates with the calcium ad magesium ios i hard water. Thus, they remai effective i hard water. Detergets are usually used to make shampoos ad products for cleaig clothes. Q U E S T I O N S 1. Would you be able to check if water is hard by usig a deterget? 2. People use a variety of methods to wash clothes. Usually after addig the soap, they beat the clothes o a stoe, or beat it with a paddle, scrub with a brush or the mixture is agitated i a washig machie. Why is agitatio ecessary to get clea clothes?? 76 Sciece

20 What you have leart Carbo is a versatile elemet that forms the basis for all livig orgaisms ad may of the thigs we use. This large variety of compouds is formed by carbo because of its tetravalecy ad the property of cateatio that it exhibits. Covalet bods are formed by the sharig of electros betwee two atoms so that both ca achieve a completely filled outermost shell. Carbo forms covalet bods with itself ad other elemets such as hydroge, oxyge, sulphur, itroge ad chlorie. Carbo also forms compouds cotaiig double ad triple bods betwee carbo atoms. These carbo chais may be i the form of straight chais, brached chais or rigs. The ability of carbo to form chais gives rise to a homologous series of compouds i which the same fuctioal group is attached to carbo chais of differet legths. The fuctioal groups such as alcohols, aldehydes, ketoes ad carboxylic acids bestow characteristic properties to the carbo compouds that cotai them. Carbo ad its compouds are some of our major sources of fuels. Ethaol ad ethaoic acid are carbo compouds of importace i our daily lives. The actio of soaps ad detergets is based o the presece of both hydrophobic ad hydrophilic groups i the molecule ad this helps to emulsify the oily dirt ad hece its removal. E X E R C I S E S 1. Ethae, with the molecular formula C 2 H 6 has (a) (b) (c) 6 covalet bods. 7 covalet bods. 8 covalet bods. (d) 9 covalet bods. 2. Butaoe is a four-carbo compoud with the fuctioal group (a) (b) (c) carboxylic acid. aldehyde. ketoe. (d) alcohol. 3. While cookig, if the bottom of the vessel is gettig blackeed o the outside, it meas that (a) (b) (c) (d) the food is ot cooked completely. the fuel is ot burig completely. the fuel is wet. the fuel is burig completely. Carbo ad its Compouds 77

21 4. Explai the ature of the covalet bod usig the bod formatio i CH 3 Cl. 5. Draw the electro dot structures for (a) ethaoic acid. (b) H 2 S. (c) propaoe. (d) F What is a homologous series? Explai with a example. 7. How ca ethaol ad ethaoic acid be differetiated o the basis of their physical ad chemical properties? 8. Why does micelle formatio take place whe soap is added to water? Will a micelle be formed i other solvets such as ethaol also? 9. Why are carbo ad its compouds used as fuels for most applicatios? 10. Explai the formatio of scum whe hard water is treated with soap. 11. What chage will you observe if you test soap with litmus paper (red ad blue)? 12. What is hydrogeatio? What is its idustrial applicatio? 13. Which of the followig hydrocarbos udergo additio reactios: C 2 H 6, C 3 H 8, C 3 H 6, C 2 H 2 ad CH Give a test that ca be used to differetiate chemically betwee butter ad cookig oil. 15. Explai the mechaism of the cleaig actio of soaps. I Group Activity Use molecular model kits to make models of the compouds you have leart i this Chapter. II Take about 20 ml of castor oil/cotto seed oil/liseed oil/soyabea oil i a beaker. Add 30 ml of 20 % sodium hydroxide solutio. Heat the mixture with cotiuous stirrig for a few miutes till the mixture thickes. Add 5-10 g of commo salt to this. Stir the mixture well ad allow it to cool. You ca cut out the soap i facy shapes. You ca also add perfume to the soap before it sets. 78 Sciece