Carbon and its Compounds Class 10 Science Notes

Bonding in Carbon – The Covalent Bond

Carbon, with atomic number 6, has an electronic configuration of $2, 4$ . This means it has 4 valence electrons in its outermost shell (L-shell). To achieve a stable noble gas configuration (a completely filled outermost shell), carbon needs to either gain 4 electrons to become $C^{4 -}$ or lose 4 electrons to become $C^{4 +}$ .

Gaining 4 electrons ( $C^{4 -}$ ): This is energetically unfavorable because the nucleus with 6 protons would struggle to hold onto 10 electrons.
Losing 4 electrons ( $C^{4 +}$ ): This requires a very large amount of energy to remove 4 electrons, leaving a cation with only 2 electrons.

Carbon overcomes this challenge by sharing its valence electrons with other atoms. This sharing forms a strong bond called a covalent bond. In a covalent bond, the shared electrons are attracted by the nuclei of both atoms, effectively belonging to both and helping both achieve a stable electron configuration.

Formation of Covalent Bonds

Covalent bonds are formed by the sharing of electron pairs. The number of electron pairs shared determines the type of covalent bond:

Single Covalent Bond: Formed by the sharing of one pair of electrons between two atoms.
Example: Hydrogen molecule ( $H_{2}$ )
Hydrogen has 1 valence electron and needs 1 more to fill its K-shell (like Helium).
Two hydrogen atoms share their single electrons, forming one shared pair.
Representation: $H : H$ or $H - H$
Example: Chlorine molecule ( $C l_{2}$ )
Chlorine (atomic number 17, configuration 2, 8, 7) has 7 valence electrons and needs 1 more to complete its octet.
Two chlorine atoms share one electron each, forming one shared pair.
Representation: $C l - C l$
Double Covalent Bond: Formed by the sharing of two pairs of electrons between two atoms.
Example: Oxygen molecule ( $O_{2}$ )
Oxygen (atomic number 8, configuration 2, 6) has 6 valence electrons and needs 2 more to complete its octet.
Each oxygen atom shares two of its electrons with the other, forming two shared pairs.
Representation: $O = O$
Triple Covalent Bond: Formed by the sharing of three pairs of electrons between two atoms.
Example: Nitrogen molecule ( $N_{2}$ )
Nitrogen (atomic number 7, configuration 2, 5) has 5 valence electrons and needs 3 more to complete its octet.
Each nitrogen atom shares three of its electrons with the other, forming three shared pairs.
Representation: $N \equiv N$

Covalent Compounds

Compounds formed by covalent bonds are called covalent compounds.

Properties of Covalent Compounds

Property	Observation	Reason
Melting and Boiling Points	Generally low.	The intermolecular forces between covalent molecules are weak, requiring less energy to overcome.
Electrical Conductivity	Poor conductors of electricity in both solid and molten states.	Covalent compounds do not form ions; electrons are localized in shared bonds, so there are no mobile charged particles.
Solubility in Water	Many are insoluble in water (which is a polar solvent).	Covalent molecules are generally non-polar or weakly polar, and "like dissolves like".
Solubility in Organic Solvents	Many are soluble in organic solvents (which are non-polar).	Covalent molecules can interact with other non-polar molecules.

Example: Methane ( $C H_{4}$ )

Methane is a simple compound of carbon, widely used as a fuel. Carbon has 4 valence electrons, and each hydrogen atom has 1. Carbon shares one electron with each of the four hydrogen atoms, and each hydrogen atom shares its electron with carbon. This results in four single covalent bonds ( $C - H$ ) in the methane molecule, allowing all atoms to achieve a stable electron configuration.

Remember
Carbon forms covalent bonds because gaining or losing 4 electrons is energetically very difficult. Sharing electrons allows carbon to achieve a stable octet.

Exam-smart
A question might show the electronic configuration of an atom and ask how it achieves stability. You would need to identify if it's likely to gain, lose, or share electrons based on its valence electrons, and then describe the type of bond formed (ionic or covalent) and the resulting compound's properties. For example, if given data on melting points and conductivity of a carbon compound, you'd infer covalent bonding due to low melting points and poor conductivity.

Allotropes of Carbon

Allotropes are different physical forms in which an element can exist. Carbon exhibits allotropy, meaning it can be found in nature in several distinct forms with vastly different physical properties. This is because the carbon atoms arrange themselves differently in each allotrope, leading to variations in their structure and bonding.

The primary allotropes of carbon discussed are diamond, graphite, and fullerenes.

Diamond

Structure: In diamond, each carbon atom is covalently bonded to four other carbon atoms. This bonding forms a very strong, rigid, three-dimensional tetrahedral network.
Properties: Due to this strong, rigid structure, diamond is:
The hardest naturally occurring substance.
A poor conductor of heat and electricity because electrons are held tightly in the covalent bonds and are not free to move.
Has a very high melting and boiling point.
Uses: Its extreme hardness makes it useful for cutting tools, drills, and jewelry.
Formation: Diamond can be synthesized under high pressure and high temperature conditions.

Graphite

Structure: In graphite, each carbon atom is covalently bonded to three other carbon atoms within the same plane. These atoms form hexagonal rings, creating flat layers. Within each layer, one of the bonds is a double bond, satisfying carbon's valency. These layers are stacked on top of each other and held together by weak intermolecular forces.
Properties: The layered structure gives graphite distinct properties:
It is smooth and slippery because the layers can slide over each other easily due to the weak forces between them.
It is a good conductor of electricity because the delocalized electrons in the double bonds within each layer are free to move along the plane.
It has a high melting and boiling point.
Uses: Its lubricating property makes it useful as a dry lubricant, and its electrical conductivity is used in electrodes for batteries and industrial processes. It is also used in pencil leads.

Fullerenes

Structure: Fullerenes are a class of allotropes where carbon atoms are bonded together in a closed cage-like structure. The most famous fullerene is Buckminsterfullerene (C-60), which consists of 60 carbon atoms arranged in a structure resembling a football or a geodesic dome.
Properties: Fullerenes have properties that differ from both diamond and graphite, depending on their specific structure and size. C-60 is relatively stable.
Uses: They have potential applications in areas like nanotechnology, drug delivery, and superconductors.

Comparison of Diamond and Graphite

Feature	Diamond	Graphite
Bonding	Each C atom bonded to 4 other C atoms (tetrahedral)	Each C atom bonded to 3 other C atoms (hexagonal layers)
Structure	Rigid 3D network	Layered structure
Hardness	Extremely hard	Soft and slippery
Electrical Conductivity	Poor conductor	Good conductor
Thermal Conductivity	Good conductor	Moderate conductor
Lubrication	Not a lubricant	Excellent lubricant

Remember
The physical properties of carbon allotropes are dramatically different due to variations in their structure and bonding arrangement, even though they are made of the same element (carbon).

Exam-smart
You might be given a description of a substance's properties (e.g., extremely hard, poor electrical conductor) and asked to identify which allotrope of carbon it is, or vice-versa. You could also be asked to explain why graphite conducts electricity but diamond does not, relating it to their electron structures.

Versatile Nature of Carbon

Carbon's unique ability to form a vast number of compounds stems from two key properties: catenation and tetravalency.

Catenation: This is the unique property of carbon to form bonds with other carbon atoms. This allows carbon to create long chains, branched chains, and even ring structures.
Tetravalency: Carbon has a valency of four, meaning it can form four covalent bonds with other atoms. This allows it to bond with other carbon atoms or with atoms of various other elements like hydrogen, oxygen, nitrogen, sulfur, and halogens.

The combination of these two properties results in an enormous diversity of carbon compounds, far exceeding those formed by any other element.

Saturated and Unsaturated Carbon Compounds

Carbon compounds can be classified based on the type of bonds between carbon atoms:

Saturated Carbon Compounds: These compounds contain only single bonds between carbon atoms. They are generally less reactive.
Example: Ethane ( $C_{2} H_{6}$ ), Propane ( $C_{3} H_{8}$ ).
Unsaturated Carbon Compounds: These compounds contain at least one double bond or triple bond between carbon atoms. They are generally more reactive than saturated compounds.
Double Bond: Alkenes (e.g., Ethene, $C_{2} H_{4}$ ).
Triple Bond: Alkynes (e.g., Ethyne, $C_{2} H_{2}$ ).

Compound Type	Bond Type	Reactivity	General Formula (Hydrocarbons)
Saturated	Single bonds only	Less reactive	Alkanes: $C_{n} H_{2 n + 2}$
Unsaturated	Double or triple	More reactive	Alkenes: $C_{n} H_{2 n}$
			Alkynes: $C_{n} H_{2 n - 2}$

Chains, Branches and Rings

Carbon atoms can arrange themselves in various structural forms:

Chains: Carbon atoms form long, continuous chains. These can be straight or branched.
Straight Chain: Atoms linked linearly, e.g., Butane ( $C_{4} H_{10}$ ) as $C H_{3} - C H_{2} - C H_{2} - C H_{3}$ .
Branched Chain: The carbon chain has side branches, e.g., Isobutane (also $C_{4} H_{10}$ ) as $C H_{3} - C H (C H_{3}) - C H_{3}$ . Compounds with the same molecular formula but different structures are called structural isomers.
Rings: Carbon atoms form closed rings.
Example: Cyclohexane ( $C_{6} H_{12}$ ), Benzene ( $C_{6} H_{6}$ ).

Carbon compounds containing only carbon and hydrogen are called hydrocarbons. Saturated hydrocarbons are alkanes, those with double bonds are alkenes, and those with triple bonds are alkynes.

Will you be my Friend?

Carbon's ability to bond with other elements is crucial for creating diverse compounds.

Heteroatoms: In hydrocarbon chains, hydrogen atoms can be replaced by other elements like halogens (Cl, Br), oxygen (O), or nitrogen (N). These elements are called heteroatoms.
Functional Groups: A heteroatom or a group of atoms that confers specific properties to a carbon compound, regardless of the rest of the carbon chain, is called a functional group.

Class of Compound	Functional Group	Formula of Group	Example
Haloalkane	Halogen	—Cl, —Br	Chloropropane
Alcohol	Hydroxyl	—OH	Propanol
Aldehyde	Aldehyde	—CHO	Propanal
Ketone	Carbonyl	—CO—	Propanone
Carboxylic Acid	Carboxyl	—COOH	Propanoic acid

Remember
Functional groups determine the chemical properties of a carbon compound.

Homologous Series

A homologous series is a series of carbon compounds with the same functional group, differing from each other by a repeating unit of — $C H_{2}$ —.

Characteristics:
All members have the same functional group.
Successive members differ by a — $C H_{2}$ — unit (or 14 atomic mass units).
Gradation in physical properties (like melting/boiling points) with increasing molecular mass.
Similar chemical properties due to the same functional group.
Examples:
Alkanes: Methane ( $C H_{4}$ ), Ethane ( $C_{2} H_{6}$ ), Propane ( $C_{3} H_{8}$ ), Butane ( $C_{4} H_{10}$ ). Each successive member differs by — $C H_{2}$ —.
Alcohols: Methanol ( $C H_{3} O H$ ), Ethanol ( $C_{2} H_{5} O H$ ), Propanol ( $C_{3} H_{7} O H$ ).

Exam-smart
Questions might ask to identify the difference between successive members of a series or predict properties based on a given homologous series. You may be given a list of compounds and asked to identify which belong to the same homologous series.

Nomenclature of Carbon Compounds

The naming of carbon compounds follows a systematic method, often based on the IUPAC (International Union of Pure and Applied Chemistry) system.

Identify the parent carbon chain: Determine the longest chain of carbon atoms.
Identify functional groups: Recognize any functional groups present.
Determine the base name: Based on the number of carbon atoms in the parent chain (e.g., meth- for 1, eth- for 2, prop- for 3, but- for 4).
Add prefixes/suffixes:

For saturated chains (alkanes), add "-ane".
For unsaturated chains (alkenes), replace "-ane" with "-ene" and indicate the position of the double bond.
For unsaturated chains (alkynes), replace "-ane" with "-yne" and indicate the position of the triple bond.
For functional groups, add appropriate suffixes (e.g., "-ol" for alcohol, "-al" for aldehyde, "-one" for ketone, "-oic acid" for carboxylic acid). If the suffix starts with a vowel, the final 'e' of the parent alkane name is dropped.
For halogens, use prefixes like "chloro-" or "bromo-".

Class of Compound	Prefix/Suffix	Example
Haloalkane	Prefix	Chloropropane
Alcohol	Suffix -ol	Propanol
Aldehyde	Suffix -al	Propanal
Ketone	Suffix -one	Propanone
Carboxylic Acid	Suffix -oic acid	Propanoic acid
Alkenes	Suffix -ene	Propene
Alkynes	Suffix -yne	Propyne

Remember
When a suffix starting with a vowel is added to a parent alkane name, the terminal 'e' of the alkane name is dropped (e.g., Propane + -ol = Propanol).

Exam-smart
You will be given a chemical formula or structure and asked to name it, or given a name and asked to draw its structure. Pay close attention to the number of carbon atoms, type of bonds, and functional groups.

Chemical Properties of Carbon Compounds

Carbon compounds exhibit several key chemical properties, including combustion, oxidation, addition reactions, and substitution reactions. These reactions are fundamental to understanding how carbon compounds behave and are used in various applications.

Combustion

Combustion is a chemical process that involves the rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light. Carbon and most of its compounds, when burned in oxygen, undergo combustion.

Combustion of Carbon: Carbon (in its allotropic forms) burns in oxygen to produce carbon dioxide ( $C O_{2}$ ), releasing significant heat and light.

C_{(s)} + O_{2 (g)} \to C O_{2 (g)} + heat and light

Combustion of Hydrocarbons: Hydrocarbons, like methane ( $C H_{4}$ ), burn in sufficient oxygen to form carbon dioxide ( $C O_{2}$ ) and water ( $H_{2} O$ ), along with heat and light. This is called complete combustion.

C H_{4 (g)} + 2 O_{2 (g)} \to C O_{2 (g)} + 2 H_{2} O_{(g)} + heat and light

For ethanol ( $C_{2} H_{5} O H$ ):

C_{2} H_{5} O H_{(l)} + 3 O_{2 (g)} \to 2 C O_{2 (g)} + 3 H_{2} O_{(g)} + heat and light

Incomplete Combustion: When the supply of oxygen is limited, carbon compounds undergo incomplete combustion. This produces carbon monoxide ( $C O$ ) or even soot (elemental carbon, C), along with water and less heat.

2 C H_{4 (g)} + 3 O_{2 (g)} \to 2 C O_{(g)} + 4 H_{2} O_{(g)} + heat and light

C H_{4 (g)} + O_{2 (g)} \to C_{(s)} (soot) + 2 H_{2} O_{(g)} + heat and light

Flame: Combustion typically produces a flame, which is visible light emitted from the combustion of gaseous substances.
Blue Flame: Indicative of complete combustion, often seen with fuels like LPG when sufficient air is supplied.
Yellow, Sooty Flame: Occurs during incomplete combustion, producing soot. This is characteristic of unsaturated hydrocarbons.
Fossil Fuels: Fuels like coal and petroleum are called fossil fuels because they are formed from the remains of ancient plants and animals subjected to geological processes over millions of years. The combustion of fossil fuels containing sulphur and nitrogen can produce oxides of sulphur ( $S O_{x}$ ) and nitrogen ( $N O_{x}$ ), which are significant environmental pollutants.

Remember
Saturated hydrocarbons generally burn with a clean, blue flame, while unsaturated hydrocarbons produce a yellow, sooty flame. Incomplete combustion, even of saturated hydrocarbons, leads to soot formation.

Exam-smart
A question might describe a flame (e.g., yellow and sooty) or a deposit on a plate above a flame and ask to identify the type of hydrocarbon or the condition of combustion. You would relate the flame colour and soot to incomplete combustion and unsaturated compounds.

Oxidation

Oxidation is a reaction where a substance gains oxygen or loses hydrogen. In the context of carbon compounds, oxidation can occur during combustion or through specific oxidizing agents.

Complete Oxidation: This is the same as complete combustion, where carbon compounds are fully converted to $C O_{2}$ and $H_{2} O$ .
Partial Oxidation: Alcohols can be oxidized to carboxylic acids. This involves the addition of oxygen.
For example, ethanol ( $C_{2} H_{5} O H$ ) can be oxidized to ethanoic acid ( $C H_{3} C O O H$ ).

C_{2} H_{5} O H + [O] \to C H_{3} C O O H

(where $[O]$ represents the oxygen from the oxidizing agent).

Oxidizing Agents: Substances that cause oxidation in other compounds are called oxidizing agents. They themselves get reduced.
Common oxidizing agents used in the lab include alkaline potassium permanganate ( $K M n O_{4}$ ) and acidified potassium dichromate ( $K_{2} C r_{2} O_{7}$ ).
When ethanol is treated with alkaline $K M n O_{4}$ and heated, the persistent colour of $K M n O_{4}$ indicates that it is reacting, and the ethanol is being oxidized.

Remember
Oxidation can be seen as adding oxygen. Ethanol to ethanoic acid is a classic example of partial oxidation.

Exam-smart
You might be given a scenario where ethanol is heated with an oxidizing agent like $K M n O_{4}$ , and asked to explain the colour change of the $K M n O_{4}$ and the type of reaction occurring. The answer involves the ethanol gaining oxygen.

Addition Reaction

Addition reactions are characteristic of unsaturated hydrocarbons (those containing double or triple bonds between carbon atoms). In these reactions, an atom or group of atoms is added across the multiple bond.

Hydrogenation: Unsaturated hydrocarbons add hydrogen ( $H_{2}$ ) in the presence of a catalyst to form saturated hydrocarbons.
Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. Common catalysts for hydrogenation are nickel (Ni) or palladium (Pd).
Example: Ethene ( $C_{2} H_{4}$ , an alkene) reacts with hydrogen to form ethane ( $C_{2} H_{6}$ , an alkane).

C_{2} H_{4} + H_{2} \overset{Ni or Pd}{\to} C_{2} H_{6}

Industrial and Biological Significance:
Hydrogenation of Vegetable Oils: This process is used to convert liquid vegetable oils (which have long unsaturated carbon chains) into solid or semi-solid fats (like vanaspati ghee, which have saturated carbon chains). This increases their melting point and shelf life.
Dietary Health: Vegetable oils containing unsaturated fatty acids are generally considered healthier than animal fats, which tend to contain saturated fatty acids.

Remember
Addition reactions break a double or triple bond and add atoms across it, turning unsaturated compounds into saturated ones. Catalysts are crucial.

Exam-smart
A question might ask why vegetable oils are hydrogenated to make vanaspati ghee or discuss the difference between saturated and unsaturated fats in terms of health and chemical structure. You would explain the addition of hydrogen and the change from unsaturated to saturated chains.

Substitution Reaction

Substitution reactions are typical of saturated hydrocarbons (like alkanes). In these reactions, one atom or a group of atoms in the hydrocarbon is replaced by another atom or group of atoms.

Reactivity: Saturated hydrocarbons are relatively unreactive and do not readily undergo addition reactions because they lack multiple bonds.
Reaction with Halogens: In the presence of sunlight, saturated hydrocarbons like methane react with halogens such as chlorine ( $C l_{2}$ ) or bromine ( $B r_{2}$ ) through a substitution reaction.
Sunlight acts as an initiator for the reaction.
One or more hydrogen atoms can be substituted by halogen atoms, one by one.
Example: Methane reacts with chlorine in the presence of sunlight to form chloromethane ( $C H_{3} C l$ ) and hydrogen chloride ( $H C l$ ).

C H_{4 (g)} + C l_{2 (g)} \overset{Sunlight}{\to} C H_{3} C l_{(l)} + H C l_{(g)}

If excess chlorine is used and the reaction continues, further substitution can occur, leading to dichloromethane ( $C H_{2} C l_{2}$ ), trichloromethane ( $C H C l_{3}$ ), and tetrachloromethane ( $C C l_{4}$ ).
Mechanism: These reactions proceed via a free radical mechanism, where highly reactive species called free radicals are involved in replacing hydrogen atoms.

Remember
Substitution reactions involve replacing one atom with another, often in saturated compounds under specific conditions like sunlight.

Exam-smart
You might be asked to identify a substitution reaction or explain why alkanes are less reactive than alkenes. The key is the replacement of hydrogen atoms by other atoms, often initiated by light.

Some Important Carbon Compounds – Ethanol and Ethanoic Acid

Properties of Ethanol

Ethanol is a liquid at room temperature. It is commonly known as alcohol and is the primary active ingredient in all alcoholic beverages. Its chemical formula is $C_{2} H_{5} O H$ .

Uses of Ethanol:

Active ingredient in alcoholic drinks: Responsible for the intoxicating effect.
Good solvent: Used in medicines like tincture of iodine (an antiseptic), cough syrups, and tonics.
Soluble in water: Mixes with water in all proportions.

Effects of Ethanol Consumption:

Small quantities of dilute ethanol: Cause drunkenness.
Pure ethanol (absolute alcohol): Even a small intake can be lethal.
Long-term consumption: Leads to numerous health problems.

Remember
Ethanol is soluble in water in all proportions, which is a key physical property.

Exam-smart
A question might describe a scenario involving alcoholic drinks or medicines and ask about the properties or effects of ethanol, testing understanding of its solubility and toxicity.

Reactions of Ethanol

Ethanol undergoes several important chemical reactions that showcase its functional group and its properties as an alcohol.

Reaction with Sodium

Alcohols react with active metals like sodium to produce hydrogen gas and an alkoxide salt. When ethanol reacts with sodium, it forms sodium ethoxide.

Observation: Effervescence (bubbling) due to the evolution of a gas.
Reaction:

2 N a_{(s)} + 2 C H_{3} C H_{2} O H_{(l)} \to 2 C H_{3} C H_{2} O^{-} N a_{(a q)}^{+} + H_{2 (g)}

Here, $C H_{3} C H_{2} O H$ is ethanol, $N a$ is sodium, $C H_{3} C H_{2} O^{-} N a^{+}$ is sodium ethoxide, and $H_{2}$ is hydrogen gas.

Testing the gas: The gas evolved is hydrogen. Hydrogen gas burns with a pop sound.
Why it happens: Sodium is a highly reactive metal. It readily donates an electron to the hydroxyl (-OH) group of ethanol. The hydrogen atom from the -OH group is displaced as hydrogen gas. This reaction is a good way to confirm the presence of the acidic hydrogen in the hydroxyl group of alcohols, though they are much weaker acids than mineral acids.

Reaction to form Unsaturated Hydrocarbon (Dehydration)

Ethanol can be converted into an unsaturated hydrocarbon called ethene through a dehydration reaction. This involves the removal of a molecule of water from ethanol.

Conditions: Ethanol is heated at 443 K (170 °C) in the presence of excess concentrated sulphuric acid ( $H_{2} S O_{4}$ ).
Role of Sulphuric Acid: Concentrated sulphuric acid acts as a dehydrating agent. It absorbs the water molecule formed during the reaction, driving the equilibrium towards the formation of ethene.
Reaction:

C H_{3} C H_{2} O H \to_{443 K}^{C o n c . H_{2} S O_{4}} C H_{2} = C H_{2} + H_{2} O

Here, $C H_{3} C H_{2} O H$ is ethanol, $C H_{2} = C H_{2}$ is ethene (an alkene), and $H_{2} O$ is water.

Significance: This reaction demonstrates how alcohols can be transformed into alkenes, which are important building blocks in the petrochemical industry.

Denatured Alcohol

Ethanol is an important industrial solvent. To prevent its misuse for drinking, it is made unfit for consumption.

Process: Small amounts of poisonous substances, such as methanol, are added to ethanol.
Identification: Dyes are also added to give it a distinct colour (often blue).
Result: This specially treated ethanol is called denatured alcohol. It can be used for industrial purposes without the risk of being consumed.

Alcohol as a Fuel

Ethanol can be used as a fuel.

Source: It is produced by the fermentation of molasses, which is derived from sugarcane.
Advantage: Ethanol is considered a cleaner fuel compared to petrol. When burnt in sufficient air, it produces only carbon dioxide and water.
Application: Some countries use ethanol as an additive in petrol.

Properties of Ethanoic Acid

Ethanoic acid, commonly known as acetic acid, is a carboxylic acid. A 5-8% solution of acetic acid in water is called vinegar, widely used as a preservative in pickles.

Physical Properties:
Pure ethanoic acid has a melting point of 290 K. In cold climates, it can freeze, earning it the name glacial acetic acid.
It has a characteristic sour taste and a pungent smell.
Chemical Properties (Acidity):
Ethanoic acid is a weak acid. Unlike strong mineral acids like HCl, which ionise completely in water, carboxylic acids like ethanoic acid ionise partially.
Comparison with HCl:
Litmus test: Both dilute acetic acid and dilute hydrochloric acid turn blue litmus red.
Universal indicator: Dilute HCl shows a lower pH (stronger acid) than dilute acetic acid.
Reaction with Ethanol (Esterification): When heated with ethanol in the presence of concentrated sulphuric acid, ethanoic acid reacts to form an ester called ethyl ethanoate. Esters have a sweet smell and are used in perfumes and as flavouring agents.

CH_3COOH_{(l)} + CH_3CH_2OH_{(l)} \xrightarrow[Conc. H_2SO_4]{Heat} CH_3COOCH_2CH_3_{(l)} + H_2O_{(l)}

Here, $C H_{3} C O O H$ is ethanoic acid, $C H_{3} C H_{2} O H$ is ethanol, $C H_{3} C O O C H_{2} C H_{3}$ is ethyl ethanoate, and $H_{2} O$ is water.

Remember
While alcohols react with sodium to produce hydrogen, they are very weak acids compared to mineral acids. Concentrated sulphuric acid is a powerful dehydrating agent.

Exam-smart
A question might present a scenario where ethanol is heated with concentrated sulphuric acid and ask to identify the product and the type of reaction. You would need to recall the dehydration reaction and identify ethene as the product. Similarly, a case study involving the use of vinegar as a preservative would relate to the properties of ethanoic acid.

Reactions of Ethanoic Acid

Ethanoic acid, a carboxylic acid, undergoes several characteristic reactions. These reactions highlight its acidic nature and the reactivity of the carboxyl group (-COOH).

Esterification Reaction

Ethanoic acid reacts with alcohols in the presence of an acid catalyst (typically concentrated sulfuric acid) to form esters. This process is called esterification. Esters are compounds that are generally sweet-smelling and are used in perfumes and as flavouring agents.

The general reaction is:

R - C O O H + R^{'} - O H \overset{A c i d C a t a l y s t}{\to} R - C O O - R^{'} + H_{2} O

Specifically, ethanoic acid reacting with ethanol:

C H_{3} C O O H + C H_{3} C H_{2} O H \overset{C o n c . H_{2} S O_{4}}{\to} C H_{3} C O O C H_{2} C H_{3} + H_{2} O

(Ethanoic acid) + (Ethanol) $\to$ (Ethyl ethanoate / Ethyl acetate) + (Water)

The ester formed, ethyl ethanoate (also known as ethyl acetate), has a characteristic sweet smell.

Saponification: When an ester is treated with sodium hydroxide (a base), it hydrolyzes to form an alcohol and the sodium salt of the carboxylic acid. This reaction is called saponification because it is the basis for soap making. Soaps are sodium or potassium salts of long-chain carboxylic acids.

C H_{3} C O O C H_{2} C H_{3} + N a O H \to C H_{3} C O O N a + C H_{3} C H_{2} O H

(Ethyl ethanoate) + (Sodium hydroxide) $\to$ (Sodium ethanoate) + (Ethanol)

Remember
Esterification is a reversible reaction. The presence of an acid catalyst speeds up the reaction.

Exam-smart
A question might describe a sweet-smelling liquid and ask to identify it or the reaction that produced it. You might also be given a reaction with NaOH and asked to identify the products as alcohol and soap precursor.

Reaction with a Base

Like other acids, ethanoic acid reacts with bases to form a salt and water. This is a typical neutralization reaction. When ethanoic acid reacts with sodium hydroxide, it forms sodium ethanoate (commonly known as sodium acetate) and water.

C H_{3} C O O H + N a O H \to C H_{3} C O O N a + H_{2} O

(Ethanoic acid) + (Sodium hydroxide) $\to$ (Sodium ethanoate) + (Water)

Sodium ethanoate is the sodium salt of ethanoic acid.

Reaction with Carbonates and Hydrogencarbonates

Ethanoic acid reacts with carbonates (like sodium carbonate, $N a_{2} C O_{3}$ ) and hydrogencarbonates (like sodium hydrogencarbonate or sodium bicarbonate, $N a H C O_{3}$ ) to produce a salt, carbon dioxide ( $C O_{2}$ ), and water ( $H_{2} O$ ).

The gas produced, carbon dioxide, turns lime water (calcium hydroxide solution) milky, which is a characteristic test for $C O_{2}$ .

Reaction with sodium carbonate:

2 C H_{3} C O O H + N a_{2} C O_{3} \to 2 C H_{3} C O O N a + H_{2} O + C O_{2} ↑

(Ethanoic acid) + (Sodium carbonate) $\to$ (Sodium ethanoate) + (Water) + (Carbon dioxide)

Reaction with sodium hydrogencarbonate:

C H_{3} C O O H + N a H C O_{3} \to C H_{3} C O O N a + H_{2} O + C O_{2} ↑

(Ethanoic acid) + (Sodium hydrogencarbonate) $\to$ (Sodium ethanoate) + (Water) + (Carbon dioxide)

Remember
The reaction with carbonates and hydrogencarbonates produces effervescence (fizzing) due to the release of carbon dioxide gas.

Exam-smart
You might be given observations from an experiment where a gas is passed through lime water, turning it milky. You would need to identify the reactants (e.g., an acid and a carbonate) and the gas ( $C O_{2}$ ) produced by ethanoic acid.

Soaps and Detergents

Soaps and detergents are cleansing agents that work on the principle of emulsification to remove oily dirt. They are effective because their molecules have a unique structure with two distinct ends.

Structure of Soap and Detergent Molecules

Soap and detergent molecules are surfactants, meaning they have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

Hydrophilic Head: This part of the molecule is ionic or polar and readily dissolves in water.
Hydrophobic Tail: This part consists of a long hydrocarbon chain, which is non-polar and dissolves in oils and grease.

This dual nature allows them to bridge the gap between water and oil.

Micelle Formation

When soap or detergent is added to water containing oily dirt, the molecules arrange themselves to form structures called micelles.

The hydrophobic tails of the surfactant molecules cluster together in the centre, away from the water.
The hydrophilic heads face outwards, towards the water molecules.
The oily dirt gets trapped within the hydrophobic core of the micelle.
The micelle, with the dirt enclosed, remains suspended in the water as a colloid due to repulsion between the charged heads.
When the water is rinsed away, the dirt is carried along with it, leaving the surface clean.

The formation of micelles is crucial for cleaning because it allows insoluble oily dirt to be suspended in water and washed away. This process is why soap solutions appear cloudy, as the micelles are large enough to scatter light.

Difference Between Soaps and Detergents

Soaps are typically sodium or potassium salts of long-chain carboxylic acids. Detergents are generally sodium salts of long-chain sulphonic acids or ammonium salts with halide ions.

A key difference arises when used with hard water, which contains dissolved calcium ( $C a^{2 +}$ ) and magnesium ( $M g^{2 +}$ ) ions.

Soaps react with these ions to form an insoluble precipitate called scum. This scum reduces the cleaning efficiency of the soap and can leave a residue. Activity 4.11 demonstrates this by showing less foam and a white curdy precipitate when soap is added to hard water compared to distilled water.
Detergents do not readily form insoluble precipitates with $C a^{2 +}$ and $M g^{2 +}$ ions. Their charged ends are designed to remain soluble even in the presence of these ions. This makes detergents effective even in hard water, as seen in Activity 4.12 where detergents produce more foam and less scum than soap in hard water.

Feature	Soap	Detergent
Chemical Nature	Sodium/Potassium salts of long-chain carboxylic acids	Sodium salts of sulphonic acids or ammonium salts with halide ions
Effectiveness in Hard Water	Forms insoluble scum with $C a^{2 +}$ and $M g^{2 +}$ ions, reducing cleaning power	Remain effective; do not form insoluble precipitates with $C a^{2 +}$ and $M g^{2 +}$ ions
Examples of Use	Washing clothes, bathing	Shampoos, laundry detergents, dishwashing liquids

Remember
The hydrophobic tail interacts with oil/grease, and the hydrophilic head interacts with water. This is the fundamental principle behind how both soaps and detergents work.

Exam-smart
A question might describe a scenario where clothes are not getting clean in hard water. Students would need to explain why soap is ineffective (scum formation) and suggest that a detergent would be a better alternative due to its ability to work in hard water. They might also be asked to draw a diagram illustrating micelle formation.

Carbon and its Compounds — Notes

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