4. Carbon and its Compounds

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Questions :

1. What would be the electron dot structure of carbon dioxide which has the formula CO₂?

Answer: E.C of C is 2,4 and E.C of O is 2,6 as there are 2 Oxygen atoms which need 2e each and carbon needs 4e. So, C will share 2 electrons to each O.

2. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint – The eight atoms of sulphur are joined together in the form of a ring.)

Answer: E.C of S is 2,8,6 hence it needs 2 electrons, if it forms a ring it will share 1e to the each 2S atoms present beside him.

Questions :

1. How many structural isomers can you draw for pentane?

Answer: We can draw 3 structural isomers for pentane(C₅H₁₂).
i. n-pentane

ii. 2-methylbutane

iii. 2,2 dimethylpropane

2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Answer: The 2 properties are:
i. Catenation: The ability to form covalent bond easily, with other atoms of C, especially, by single bond forming compounds having long but stable chains.
As, if long chains can be formed by an atom there are a wide range of combination of compounds it can form and thereby, number of carbon compounds increase.
ii. Tetravalency: The ability of carbon to bond strongly at a time with 4 carbon atoms or 4 mono-valent element due to its valency of 4.
As, 1 carbon atom can form bond with 4 atoms of C or other 4 mono-valent element,etc which can even bond with other 3 atoms. So, it have a wide range of elements to bond and large number of atoms to bond. Also, due to its small nucleus it can hold the shared pair of electrons strongly, thus increasing number of stable compounds. Finally, increasing number of carbon compounds.

3. What will be the formula and electron dot structure of cyclopentane?

Answer: The formula of cyclopentane is C₅H₁₀.
Electron-dot structure:

4. Draw the structures for the following compounds.
(i) Ethanoic acid
(ii) Bromopentane*
(iii) Butanone
(iv) Hexanal.
*Are structural isomers possible for bromopentane?

Answer: (i) Ethanoic acid(CH₃COOH)

(ii) Bromopentane(C₅H₁₁Br)

(iii) Butanone(C₄H₈O)

(iv) Hexanal(C₆H₁₂O)

Yes, structural isomers are possible for bromopentane, those are:

5. How would you name the following compounds?

Answer: (i) Bromoethane
(ii) Methanal
(iii) 1-Hexyne
Reason: (i) has 2 C atoms and 1 Br atom and all single bonds, (ii) has 1 C atom and aldehyde group and (iii) has 6 C atoms and one triple bond of C atom with another C atom.

Questions :

1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Answer: It is an oxidation reaction, because the reactant, ethanol(CH₃CH₂OH) contains 6 hydrogen atoms and only 1 oxygen atom but the product ethanoic acid(CH₃COOH) contains 4 hydrogen atoms and 2 oxygen atoms. It means during the reaction the product gained 1 oxygen atom and losed 2 hydrogen atoms, and hence according to the definition of oxidation, ethanol is oxidised in the reaction and so, the conversion of ethanol to ethanoic acid is an oxidation reaction.

2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Answer: There are 2 reasons behind it:
i. In, the reaction of ethyne during welding, 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O only oxygen is most important in the air, but as the reaction needs more amount of O₂ than available in air to give a clean flame we use a mixture with more content of O₂. And if it is not done it leads to incomplete combustion. As, after incomplete combustion, yellow flame release harmful gases like CO and unburnt carbon particles polluting the environment, so it is used to avoid it.
ii. But, the main reason why a common welder use it because due to incomplete combustion, the efficiency of fuel decreases and a part of it is wasted in form of unburnt C particles, leading to economic loss. In addition to it, as welding is a work which needs a large amount of heat. But on incomplete combustion, the desired amount of heat is not produced decreasing the quality of weld and increasing the time needed, ultimately leading to loss of monetary gains. So, it is done to avoid it.
Hence, a mixture of oxygen and ethyne is burnt for welding.

Questions :

1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer: We can distinguish between them by two methods:
i. Litmus test method: We will take a sample 'A' of alcohol and sample 'B' of a carboxylic acid and will test both the samples by a litmus paper, the sample which will turn blue litmus red is a carboxylic acid, as it is a weak acid and which doesn't is an alcohol as it doesn't dissociate its H⁺ ion.
ii. Reaction with carbonates and hydrogencarbonates: Carboxylic acid will give out effervescence after reaction due to production of CO₂ gas.
E.g: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
But, as alcohol's pH is near to 7 and it doesn't act as an acid it will not react with the carbonate and hydrogencarbonates leading to absence of CO₂ and effervescence.
Thus, by the difference in the results of this two substances we can distinguish them experimentally.

2. What are oxidising agents?

Answer: The substances that has the ability to oxidize other substances and reduce itself by giving oxygen atom to them or by removing hydrogen atom from them or by accepting their electrons are known as oxidising agents. E.g: Oxygen, hydrogen peroxide, alkaline potassium permanganate, nitric acid, etc.

Questions :

1. Would you be able to check if water is hard by using a detergent?

Answer: No, as detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, we are not able to see the insoluble substance (scum). And, so as detergent shows no different behavior in hard water to in soft water we would not be able to check if water is hard by using a detergent

2. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Answer: The two reasons behind it:
i. As, soap cleans the dirt from clothes by collecting dirt in center of a micelle, by agitation the soap spreads uniformly in the cloth and thus the micelle gets lots of dirt to collect in its center leading to increase in micelle formation and soap is also properly utilised, also, dirt in every part of cloth is cleaned resulting in clean clothes.
ii. The main reason why clothes are beaten is as the micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion. So, in order to remove the micelle with the dirt in its center, we have to agitate it so that the micelle gets removed from the cloth along with the dirt so to get clean clothes.

Exercises :

1. Ethane, with the molecular formula C₂H₆ has

(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.

Answer: (b) 7 covalent bonds. Reason:

2. Butanone is a four-carbon compound with the functional group

(a) carboxylic acid.
(b) aldehyde.
(c) ketone.
(d) alcohol.

Answer: (c) ketone. Reason: As it has suffix "one" which is used in case of ketone as the functional group.

3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.

Answer: (b) the fuel is not burning completely. Reason: These sooty deposit is due to the unburnt carbon particles, which indicates that the fuel is not burning completely due to lack of oxygen or the fuel is an unsaturated carbon compound.

4. Explain the nature of the covalent bond using the bond formation in CH₃Cl.

Answer: Covalent bond is a bond in which electrons are shared mutually to gain stability by completing the octet configuration. As, in CH₃Cl the carbon atom contains 4 electrons in valence shell and thus it is not easy to lose electrons and hold the gained 4 electrons, so C atom shares its 1-1 electron to 3 H atom and one Cl atom, thus they form a covalent bond. In it C has shared its 4 electrons and the 3 H atoms and one Cl atom has shared their 1-1 electron to C atom and thus octet of all 5 atoms is completed gaining stability and forming chloromethane.

5. Draw the electron dot structures for

(a) ethanoic acid.
(b) H₂S.
(c) propanone.
(d) F₂ .

Answer: (a) ethanoic acid(CH₃COOH)

(b) H₂S(hydrogen sulphide)

(c) propanone(C₃H₆O)

(d) F₂(fluorine)

6. What is an homologous series? Explain with an example.

Answer: A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series. It will have a general formula, also as change in molecular mass will have a uniform pattern in a homologous series so, the change in physical properties like solubility and melting point and boiling point will have a uniform pattern. But, the chemical properties remain same as they mostly depend on the functional group, which is same in each case. E.g: The alcohol homologous series: CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH in which there is a general formulae "C_nH_2n+1OH" also, there is a uniform difference in molecular mass of 14u and also, their chemical properties are very similar.

7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Answer: They can be differentiated on the basis of these physical and chemical properties.

Ethanol(C₂H₅OH)	Ethanoic acid(CH₃COOH)
1. It has smell like alcohol.	1. It has smell like vinegar.
2. It doesn't change the color of litmus.	2. It changes the color of litmus from blue to red.
3. It doesn't reacts with base.	3. It reacts with base.
4. It doesn't reacts with carbonates and hydrogencarbonates.	4. It reacts with carbonates and hydrogencarbonates.
5. It produces ester on reacting with an acid like, CH₃COOH in presence of an acid catalyst.	5. It produces ester on reacting with an alcohol like, C₂H₅OH in presence of an acid catalyst.

8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?

Answer: It takes place because when the molecules of soap are added to water the to two tails/ends in the molecule of soap, one hydrophobic/carbon chain dissolves in oil/hydrocarbons and the hydrophilic/ionic-end dissolves in water. As, after this the soap molecules, orient in a form of cluster where hydrophobic end of the molecules is towards the oil droplet while the ionic-end facing outside, ultimately resulting in formation of structures known as micelle. No, a micelle be formed in other solvents such as ethanol and the reason is the solubility of the hydrophobic tail in solvents like ethanol, as we know that hydrophobic tail dissolves in oil/hydrocarbon and sticks to it with other side attached with water, leading to formation of micelle. But, as here the solvent is only hydrocarbon so after adding it to ethanol it will dissolve in it and thereby the hydrophobic tail will not be able to trap dirt in centre as it will be already dissolved in solvent, resulting in absence of micelle. Also, as here is absence of water then the unique orientation of molecules will not take place, resulting in absence of micelle.

9. Why are carbon and its compounds used as fuels for most applications?

Answer: They are used as fuels for most applications, mainly due to their high calorific value as then the transport and storage cost reduces, with increased efficiency. Then is their ideal ignition temperature as they don't burn at very low temperature so danger of fire reduces and it also don't burn at very high temperature and therefore, energy is not wasted in just starting combustion and engines are not needed to be very efficient and costly. Rest, is that saturated hydrocarbons burn with a clean flame and doesn't produce the soot. And more pure we will make that carbon compound from impurities present in it, more it will become an ideal fuel and due to this all qualities of carbon and its compounds, they are used as fuels for most applications.

10. Explain the formation of scum when hard water is treated with soap.

Answer: We know that hard water contains lot of dissolved minerals, especially Ca and Mg salts. And soap is sodium or potassium salts of long-chain carboxylic acids/ sodium or potassium salt of stearic acid. So, when these are treated with each other, the Ca²⁺ ions and Mg²⁺ions from their salts like chlorides, sulfides, hydrogen carbonates react with sodium stearate/potassium stearate to form calcium stearate and magnesium stearate which we call scum. E.g: Ca(HCO₃)₂ + 2C₁₈H₃₅NaO₂ → C₃₆H₇₀CaO₄ + Na₂CO₃ + H₂O + CO₂. The hard water reduces the cleaning property of soap as soap is wasted. Also, as an insoluble substance scum is formed which later precipitates on clothes and make them worse, resulting in negative cleaning effect of soap.

11. What change will you observe if you test soap with litmus paper (red and blue)?

Answer: We will observe that the litmus paper turns red litmus to blue and blue remains as it is if we will test litmus in soap solution. The reason is that the ionic end means Na and K in soap forms NaOH and K in water which are bases and thus due to presence of these bases the soap solution becomes basic and shows property of a base, also we know that bases are soapy so we can also judge from this property of bases how a soap solution will change color of litmus.

12. What is hydrogenation? What is its industrial application?

Answer: The chemical reaction between molecular hydrogen and other compounds often unsaturated carbon compounds in the presence of catalyst like, Ni, Pd, Pt if at normal atmospheric pressure and temperature is referred to as hydrogenation. It's main industrial application is to saturate unsaturated carbon compounds. Like, in food industry vegetable oil is saturated by this process. In petrochemical industry it is used in production of paraffin and naphthene and also, it have uses in a wide range of sectors.

13. Which of the following hydrocarbons undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄.

Answer: We know that only unsaturated hydrocarbons show addition reaction means compounds with general formula C_nH_2n and C_nH_2n-2 as C₃H₆ and C₂H₂ fits in these formula so, only these hydrocarbons can undergo addition reaction.

14. Give a test that can be used to differentiate chemically between butter and cooking oil.

Answer: We can use two test to differentiate between them but we are using the same property and result for it that is saturation and fading of color. That is, butter contains high amount of saturated fats about 60-65% so it is saturated carbon compound and oil is unsaturated carbon compound.
Test No:-1(Baeyer's test)

i. Take 2 test tubes, dilute alkaline KMNO₄ solution and samples of butter and oil.
ii. Put the samples in the test tubes and add the solution to it.
iii. We observe that in one sample A the pink to purple color of solution has faded and it had became colorless, while in another sample B there is no effect on solution's color.
iv. We know that unsaturated compounds make that solution colorless hence the sample B is butter as butter has lots of saturated compounds and sample A is cooking oil as it is unsaturated compound.

Test No:-2(Bromine water test)

i. Take 2 test tubes, bromine water means diluted bromine and samples of butter and oil.
ii. Put the samples in the test tubes and add the bromine water to it. iii. We observe that the brown color of the solution has faded away in one test tube label as, sample A and in another test tube label as sample B there is no effect on solution's color. iv. We know that unsaturated compounds make that solution colorless hence the sample A is cooking oil as it is unsaturated compound and sample B is butter as butter has lots of saturated compounds.

15. Explain the mechanism of the cleaning action of soaps.

Answer: Soap are sodium or potassium salts of long-chain carboxylic acids which have two ends with differing properties, one is hydrophilic/ionic end, that is, it dissolves in water, while the other end is hydrophobic, that is, it dissolves in hydrocarbons. Also, lot of dirt is of the form of hydrocarbon like oil and is insoluble in water. So, when the soap is added to water the soap molecules reach the clothes due to agitation and then the process starts. As, the hydrophobic end will be attracted towards the oil and will dissolve in it, so it sticks to it and so, many molecules of soap stick their hydrophobic end to the dirt, at the same time hydrophilic/ionic end protrudes out due to earlier phenomenon which will form a unique orientation of the molecules with the ionic end opposite to dirt's side and carbon chain towards the dirt and ultimately will form a structure known as micelle. As, the molecules of soap trap the dirt inside their cluster so the dirt is now removed from cloth, then the ion-ion repulsion in ionic end of various soap molecules of a micelle comes into play as due to this the micelle stay suspended in water as a colloid(emulsion) and is easily removed away along with the trapped dirt from the cloth and water by agitation. And so is the entire mechanism of the cleaning action of soaps from very start to very end. Also, we have to remember that the water plays an important part as solvent in these process without which it is very tough and purity of water is necessary for it.

Best of luck and keep learning and be growing.