Alkanes — KCSE Chemistry

Alkanes undergo two main types of reactions: combustion and substitution.

1. Combustion: This is a reaction between an alkane and oxygen, producing carbon dioxide and water. It releases energy, making it an exothermic reaction.

   • General equation:
     [ C_nH_{2n+2} + O_2 \rightarrow CO_2 + H_2O ]
   • Example:
     [ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O ]

2. Substitution: In this reaction, one hydrogen atom in an alkane is replaced by a halogen (like chlorine or bromine). This typically occurs in the presence of UV light.

   • General equation:
     [ C_nH_{2n+2} + X_2 \rightarrow C_nH_{2n+1}X + HX ]
   • Example:
     [ CH_4 + Cl_2 \rightarrow CH_3Cl + HCl ]

Understanding these reactions is crucial for grasping the behavior of alkanes in various chemical processes.

Alkanes are saturated hydrocarbons with single bonds between carbon atoms. Their physical properties vary with molecular size due to changes in intermolecular forces. Here are key properties:

• Boiling and Melting Points: As the molecular size increases, the boiling and melting points of alkanes increase. This is due to stronger London dispersion forces in larger molecules.
• Solubility: Alkanes are non-polar and insoluble in water but soluble in organic solvents. This is because they cannot form hydrogen bonds with water.
• Density: Alkanes have lower densities than water. As molecular size increases, density also increases, but they remain less dense than water.
• State at Room Temperature: Smaller alkanes (C1 to C4) are gases, while larger alkanes (C5 and above) are liquids or solids.

Understanding these properties helps explain the behavior of alkanes in various conditions, making them significant in both natural and industrial processes.

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They consist only of carbon and hydrogen atoms, with single bonds between the carbon atoms. Alkanes are also known as paraffins and are characterized by their relatively low reactivity due to the stability of their C-C and C-H bonds.

Key characteristics of alkanes:

• Saturated: They contain the maximum number of hydrogen atoms.
• Structure: Their carbon atoms can be arranged in straight chains or branched structures.
• Physical properties: Alkanes are typically colorless, odorless gases or liquids at room temperature, with increasing boiling points as the carbon chain lengthens.
• Combustion: Alkanes burn in oxygen to produce carbon dioxide and water, releasing energy.

In the laboratory, alkanes can be distinguished from other hydrocarbons by their lack of unsaturation, which means they do not react with bromine water or potassium permanganate, unlike alkenes and alkynes.