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Chemistry - Alkanes

Grade 9IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Alkanes are a homologous series of saturated hydrocarbons, meaning they contain only single covalent bonds (C−CC-C) between carbon atoms.

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The general formula for any alkane is CnH2n+2C_nH_{2n+2}, where nn represents the number of carbon atoms.

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As the chain length (nn) increases, the boiling point and viscosity increase, while the volatility and flammability decrease.

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Alkanes are generally unreactive due to the strength of the C−CC-C and C−HC-H bonds, but they undergo combustion and substitution reactions.

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Substitution reactions occur when an atom (like Hydrogen) is replaced by another atom (like a Halogen). This requires Ultraviolet (UV) light to provide the activation energy.

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Complete combustion occurs in excess oxygen, producing carbon dioxide (CO2CO_2) and water (H2OH_2O). Incomplete combustion occurs in limited oxygen, producing carbon monoxide (COCO) or soot (CC) and water.

📐Formulae

CnH2n+2C_nH_{2n+2}

CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

CH4+Cl2→UVCH3Cl+HClCH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl

2C2H6+7O2→4CO2+6H2O2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O

💡Examples

Problem 1:

Determine the molecular formula of an alkane that contains 88 carbon atoms.

Solution:

C8H(2×8)+2=C8H18C_8H_{(2 \times 8) + 2} = C_8H_{18}

Explanation:

Using the general formula CnH2n+2C_nH_{2n+2}, we substitute n=8n = 8 to find the number of hydrogen atoms.

Problem 2:

Write the balanced chemical equation for the complete combustion of Butane (C4H10C_4H_{10}).

Solution:

2C4H10+13O2→8CO2+10H2O2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O

Explanation:

In complete combustion, the hydrocarbon reacts with oxygen to form carbon dioxide and water. The coefficients are adjusted to balance the number of Carbon, Hydrogen, and Oxygen atoms on both sides.

Problem 3:

Describe the reaction between Ethane (C2H6C_2H_6) and Bromine (Br2Br_2) in the presence of UV light.

Solution:

C2H6+Br2→UVC2H5Br+HBrC_2H_6 + Br_2 \xrightarrow{UV} C_2H_5Br + HBr

Explanation:

This is a photochemical substitution reaction. One hydrogen atom in the ethane molecule is replaced by one bromine atom, forming bromoethane and hydrogen bromide. The UV light is essential to break the Br−BrBr-Br bond.