Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Hydrocarbons
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Hydrocarbons
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 6
Theme: Chemistry Of Life
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Explains the structure of carbon and its valency Define hydrocarbon Give examples of hydrocarbons and the irstructure Explain is omerism and give examples. Explain homologousseries as it relates to the physical and chemical properties of hydrocarbons Distinguish betweenaliphatic and aromatichydrocarbon
This section provides a detailed explanation of the core concepts related to hydrocarbons. 2.
1. Structure of Carbon and its Valency Atomic Structure: Carbon is a Group 14 element with atomic number
6. Its electron configuration is 1s2 2s2 2p
2. Valency: In its ground state, carbon has two unpaired electrons in the 2p orbital, suggesting a valency of
2. However, carbon almost always exhibits a valency of 4 (tetravalency) in organic compounds. This is explained by the concept of hybridization. One electron from the 2s orbital is promoted to the empty 2p orbital, resulting in four unpaired electrons (1s2 2s1 2px1 2py1 2pz1). These four orbitals (one 2s and three 2p) then mix or hybridize to form four equivalent hybrid orbitals. sp3 hybridization: Occurs when carbon forms four single bonds (e.g., in alkanes). The four sp3 hybrid orbitals are directed towards the corners of a tetrahedron, resulting in bond angles of approximately 109.5°. sp2 hybridization: Occurs when carbon forms one double bond and two single bonds (e.g., in alkenes). Three sp2 hybrid orbitals lie in a plane with 120° bond angles, and the unhybridized p orbital forms a pi (π) bond. sp hybridization: Occurs when carbon forms one triple bond and one single bond, or two double bonds (e.g., in alkynes). Two sp hybrid orbitals are linear with 180° bond angles, and the two unhybridized p orbitals form two pi (π) bonds.
Catenation: Carbon has a unique ability to bond with other carbon atoms to form long chains (straight or branched) and rings. This property, known as catenation, is responsible for the vast diversity of organic compounds, including hydrocarbons. Carbon-carbon bonds are strong and stable, allowing for complex molecular structures. 2.
2. Definition of Hydrocarbons Hydrocarbons are organic compounds that are composed exclusively of carbon (C) and hydrogen (H) atoms. They are the simplest organic compounds and serve as the parent compounds from which all other organic compounds can be considered to be derived.
Sources: The primary natural sources of hydrocarbons are crude oil (petroleum) and natural gas, which are abundant in Nigeria. These fossil fuels were formed from the decomposition of ancient organic matter under heat and pressure over millions of years. 2.
3. Examples of Hydrocarbons and their Structures Hydrocarbons are broadly classified into two main types: Aliphatic and Aromatic. Within aliphatic hydrocarbons, they are further classified based on the types of carbon-carbon bonds present. 2.3.
1. Aliphatic Hydrocarbons These are compounds that do not contain a benzene ring. They can be acyclic (open-chain) or cyclic.
Saturated Hydrocarbons (Alkanes): Contain only single carbon-carbon bonds (C-C) and carbon-hydrogen bonds (C-H).
General formula: CnH2n+2 (where n is the number of carbon atoms). They are relatively unreactive due to the strength of C-C and C-H single bonds.
Examples: Methane (CH4): The simplest alkane, major component of natural gas (used for cooking and power generation in Nigeria). ``` H | H-C-H | H ``` Ethane (C2H6): ``` H H | | H-C-C-H | | H H ``` Propane (C3H8): ``` H H H | | | H-C-C-C-H | | | H H H ``` Butane (C4H10): (n-butane) ``` H H H H | | | | H-C-C-C-C-H | | | | H H H H ``` Propane and Butane are the main components of Liquefied Petroleum Gas (LPG) used widely in Nigeria for domestic cooking.
Unsaturated Hydrocarbons: Contain at least one carbon-carbon double bond (C=C) or triple bond (C≡C). More reactive than alkanes due to the presence of pi (π) bonds.
Alkenes: Contain at least one C=C double bond.
General formula: CnH2n.
Examples: Ethene (C2H4): ``` H H \ / C=C / \ H H ``` Propene (C3H6): ``` H H H \ / | | C=C-C-H / | | H H H ``` Ethene is an important raw material for the production of polyethene plastic, widely used in Nigeria for packaging, pipes, etc.
Alkynes: Contain at least one C≡C triple bond.
General formula: CnH2n-
2. Examples: * Ethyne (C2H2): (also known as Acetylene, used in welding at least one C=C double bond.
General formula: CnH2n.
Examples: Ethene (C2H4): ``` H H \ / C=C / \ H H ``` Propene (C3H6): ``` H H H \ / | | C=C-C-H / | | H H H ``` Ethene is an important raw material for the production of polyethene plastic, widely used in Nigeria for packaging, pipes, etc.
Alkynes: Contain at least one C≡C triple bond.
General formula: CnH2n-
2. Examples: Ethyne (C2H2): (also known as Acetylene, used in welding and cutting) ``` H-C≡C-H ``` Propyne (C3H4): ``` H | H-C≡C-C-H | H ``` Cyclic Aliphatic Hydrocarbons (Cycloalkanes): Alkanes arranged in a ring structure.
General formula: CnH2n (similar to alkenes, but saturated).
Examples: Cyclopropane (C3H6): ``` CH2 / \ CH2—CH2 ``` Cyclohexane (C6H12): ``` CH2 / \ CH2 CH2 | | CH2 CH2 \ / CH2 ``` 2.3.
2. Aromatic Hydrocarbons These are cyclic compounds that contain a special type of conjugated, delocalized pi-electron system, typically a benzene ring. They exhibit unique stability and reactivity (undergo electrophilic substitution rather than addition).
Benzene (C6H6): The simplest and most important aromatic hydrocarbon. It has a hexagonal ring structure with alternating single and double bonds, but all C-C bonds are of intermediate length due to electron delocalization. ``` H / \ C C // \\ HC CH \\ // C C \ / H (Or often represented as a hexagon with an inner circle to denote delocalization) ``` Toluene (Methylbenzene, C7H8): Benzene ring with a methyl group. ``` CH3 | C // \ HC CH // \\ HC CH \ / C C \ / H ``` Aromatic hydrocarbons are found in crude oil and are important precursors for many industrial chemicals, pharmaceuticals, and dyes. 2.
4. Isomerism Definition: Isomerism is the phenomenon where two or more compounds have the same molecular formula but different structural formulas (arrangement of atoms in space). These compounds are called isomers. Types of Structural Isomerism relevant to Hydrocarbons: Chain (or Skeletal)
Isomerism: Occurs when compounds have the same molecular formula but differ in the arrangement of their carbon chain (straight vs. branched).
Example: Butane (C4H10) n-Butane (Butane): Straight chain ``` CH3-CH2-CH2-CH3 ``` Isobutane (2-Methylpropane): Branched chain ``` CH3 | CH3-CH-CH3 ``` Positional Isomerism: Occurs when compounds have the same molecular formula and the same carbon skeleton, but differ in the position of a functional group (or in the case of alkenes/alkynes, the position of the multiple bond).
Example: Butene (C4H8)
But-1-ene: Double bond between C1 and C2 ``` CH2=CH-CH2-CH3 ``` But-2-ene: Double bond between C2 and C3 ``` CH3-CH=CH-CH3 ``` 2.
5. Homologous Series Definition: A homologous series is a series of organic compounds that have the same general formula, possess similar chemical properties, and show a gradual change in physical properties as the series is ascended.
Characteristics of a Homologous Series:
1. Same General Formula: All members can be represented by a single general formula (e.g., CnH2n+2 for alkanes, CnH2n for alkenes, CnH2n-2 for alkynes).
2. Gradual Change in Physical Properties: As the number of carbon atoms (and thus molar mass) increases, there is a gradual and predictable change in physical properties such as boiling point, melting point, density, and viscosity. For instance, boiling points generally increase due to stronger intermolecular forces (van der Waals forces).
3. Similar Chemical Properties: Members of a homologous series have the same functional group (or type of bonding), which determines their characteristic chemical reactions.
4. Adjacent Members Differ by -CH2- Unit: Each successive member in the series differs from the previous one by a -CH2- group (a methylene unit), with a corresponding mass difference of 14 amu.
5. Preparation by Similar Methods: Members of a homologous series can often be prepared using similar chemical methods.
Examples: Alkane Series: Methane (CH4), Ethane (C2H6), Propane (C3H8), Butane (C4H10), Pentane (C5H12), etc. * Alkene Series: Ethene (C2H4), Propene (C3H6), Butene (C4H8), Pentene (C5H10), etc. 2.
6. Distinguishing between Aliphatic and Aromatic Hydrocarbons The fundamental difference lies in their structure and characteristic properties. | -CH2- Unit: Each successive member in the series differs from the previous one by a -CH2- group (a methylene unit), with a corresponding mass difference of 14 amu.
5. Preparation by Similar Methods: Members of a homologous series can often be prepared using similar chemical methods.
Examples: Alkane Series: Methane (CH4), Ethane (C2H6), Propane (C3H8), Butane (C4H10), Pentane (C5H12), etc. * Alkene Series: Ethene (C2H4), Propene (C3H6), Butene (C4H8), Pentene (C5H10), etc. 2.
6. Distinguishing between Aliphatic and Aromatic Hydrocarbons The fundamental difference lies in their structure and characteristic properties. | Feature | Aliphatic Hydrocarbons | Aromatic Hydrocarbons | | :------------------ | :--------------------------------------------------------- | :------------------------------------------------------- | | Structure | Open-chain (straight or branched) or cyclic (non-aromatic). Does not contain a benzene ring. | Contains at least one benzene ring or a similar stable conjugated cyclic system. | | Bonding | Saturated (single bonds only - alkanes) or unsaturated (double/triple bonds - alkenes/alkynes). Localized electrons. | Characterized by a delocalized pi-electron system within the ring (e.g., benzene's 6 pi electrons spread over 6 carbons). | | Stability | Generally less stable than aromatic compounds, especially unsaturated ones. | Highly stable due to resonance (electron delocalization). | | Reactivity | Alkanes: Undergo substitution reactions (e.g., with halogens in UV light).
Alkenes/Alkynes: Undergo addition reactions (e.g., with H2, Br2). | Primarily undergo electrophilic substitution reactions (e.g., nitration, sulfonation, halogenation). They resist addition reactions that would disrupt their stable aromatic system. | | Smell | Often have a faint, sometimes unpleasant, 'petrol-like' smell (especially lower members). | Often have distinct, pleasant smells (hence 'aromatic'). Benzene has a sweet, strong smell. | | Examples | Methane, Ethane, Propane, Butane, Ethene, Propene, Ethyne, Cyclohexane. | Benzene, Toluene, Naphthalene, Anthracene. | | Test (e.g., with Bromine water) | Unsaturated aliphatic hydrocarbons (alkenes, alkynes) decolorize bromine water rapidly. Saturated alkanes do not. | Do not readily decolorize bromine water under normal conditions (resist addition). | | Sooty Flame* | Burn with a less sooty or non-sooty flame (higher H/C ratio). | Burn with a very sooty flame due to high carbon content and incomplete combustion. | This section outlines the step-by-step activities for the teacher and students. 3.
1. Introduction (10 minutes)
Teacher Activity: Begins by displaying images or actual samples of substances like LPG cylinders, plastic items, petrol, crude oil. Asks students to identify these items and discusses their origin and uses in Nigeria (e.g., cooking, transportation, packaging).
Poses a question: "What are these substances fundamentally made of?" Introduces the term "hydrocarbons" as the answer, linking it to carbon and hydrogen. Clearly states the objectives for the lesson.
Student Activity: Observe the displayed items and participate in identifying them and discussing their uses. Attempt to answer the teacher's question, drawing on prior knowledge. Listen attentively to the introduction of hydrocarbons and the lesson objectives. 3.
2. Explanation of Carbon Structure and Valency (15 minutes)
Teacher Activity: Explains the atomic structure of carbon (electron configuration). Discusses carbon's valency of 4 and the concept of hybridization (sp3, sp2, sp) without going into complex orbital theory but emphasizing the resulting bond angles and shapes (tetrahedral, trigonal planar, linear). Emphasizes catenation, using analogies like Lego blocks to show how carbon atoms can link to form diverse structures. Draws simple diagrams for methane (sp3), ethene (sp2), and ethyne (sp) to illustrate the different bonding types and shapes.
Student Activity: Take notes on carbon's electron configuration, valency, and hybridization types. Actively listen and ask clarifying questions about catenation. Sketch the basic shapes of methane, ethene, and ethyne as guided by the teacher. 3.
3. Defining Hydrocarbons and Examples (20 minutes)
Teacher Activity: Provides a clear definition of hydrocarbons. Classifies hydrocarbons into aliphatic (saturated and unsaturated) and aromatic types. For each major class (alkanes, alkenes, alkynes, benzene), provides the general formula, drawing detailed structures for the first few members (e.g., methane, ethane, ethene, propene, ethyne, benzene). Uses models (if available, e.g., molecular model kits or improvised models from clay/play-doh and toothpicks) to demonstrate 3D structures. Highlights Nigerian relevance (LPG, petrol, plastics).
Student Activity: Write down the definition of hydrocarbons and their classifications. Copy and draw the structures of example hydrocarbons (e.g., methane, ethane, ethene, propene, ethyne, benzene). Participate in identifying types of bonds (single, double, triple). Handle and observe molecular models if provided, discussing the arrangement of atoms. 3.
4. Explaining Isomerism (15 minutes)
Teacher Activity: Defines isomerism, emphasizing "same molecular formula, different structural formula." Focuses on chain isomerism and positional isomerism relevant to hydrocarbons. Uses butane (C4H10) to clearly illustrate chain isomerism (n-butane vs. isobutane). Draws both structures. Uses butene (C4H8) to illustrate positional isomerism (but-1-ene vs. but-2-ene). Draws both structures. Encourages students to verify the molecular formulas are the same for each set of isomers.
Student Activity: Define isomerism in their notes. Draw and name the isomers of butane and butene as guided by the teacher, confirming the molecular formulas. Engage in a brief discussion on why these different arrangements lead to different compounds. 3.
5. Explaining Homologous Series (10 minutes)
Teacher Activity: Defines homologous series and lists its five key characteristics (general formula, -CH2- difference, similar chemical properties, gradual change in physical properties, similar preparation methods). Uses alkanes and alkenes as prime examples to demonstrate these characteristics. Discusses how boiling points of alkanes increase with molecular size, linking it to the concept of fractional distillation of crude oil in Nigeria.
Student Activity: Write down the definition and characteristics of a homologous series. Listen to the examples and the explanation of how physical properties change, connecting it to real-world processes like refining. 3.
6. Distinguishing Aliphatic and Aromatic Hydrocarbons (10 minutes)
Teacher Activity: Recap the definitions and examples of aliphatic and aromatic hydrocarbons. Presents a comparison table highlighting key differences (structure, bonding, stability, reactivity, flame type, smell). Emphasizes the benzene ring as the defining feature of aromatic compounds. Discusses the environmental impact of aromatic compounds (e.g., components of crude oil pollution in the Niger Delta).
Student Activity: Create their own comparison table for aliphatic
Energy and Fuels (Nigerian Economy): Context: Nigeria is a major crude oil and natural gas producer. Hydrocarbons are the primary components of these fossil fuels.
Application: Students learn that petrol (gasoline), diesel, kerosene, and aviation fuel are all mixtures of different hydrocarbons, separated from crude oil by fractional distillation at refineries (e.g., Kaduna Refinery, Warri Refinery).
LPG (Liquefied Petroleum Gas): A mixture of propane and butane, widely used as cooking gas in Nigerian homes. Understanding hydrocarbons explains its properties, storage, and combustion.
Natural Gas: Methane is the main component of natural gas, used for electricity generation and industrial heating in Nigeria. Students can relate to gas flaring issues and efforts to utilize gas more effectively.
Petrochemical Industry and Plastics: Context: Hydrocarbons are crucial raw materials for the petrochemical industry.
Application: Ethene, a simple alkene, is polymerized to produce polyethene, a plastic used for making water bottles, shopping bags, pipes, and other everyday items found in every Nigerian market and household. Other hydrocarbons are used to produce synthetic rubber, detergents, and fertilizers. This knowledge connects chemistry to manufacturing and consumer goods.
Environmental Impact: Context: Oil exploration and production in the Niger Delta have significant environmental consequences.
Application: The study of hydrocarbons helps students understand the chemical nature of oil spills, which are mixtures of various hydrocarbons (e.g., alkanes, aromatics like benzene and toluene). This explains why oil spills are difficult to clean up and why they are toxic to marine life and soil. The concept of incomplete combustion of hydrocarbons leading to sooty flames (as seen with aromatic compounds) can be linked to air pollution from vehicle emissions and gas flaring.