Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Alkanes are saturated hydrocarbons containing only single covalent bonds, represented by the general formula .
Alkanes are generally unreactive due to the strength of the and bonds, but they undergo substitution reactions with halogens.
A substitution reaction occurs when one atom (or group of atoms) is replaced by another atom (or group of atoms).
Photochemical Reaction: The reaction between alkanes and halogens (such as or ) requires Ultraviolet (UV) light to proceed.
The UV light provides the activation energy necessary to break the covalent bond in the halogen molecule ( or ).
In the reaction, one hydrogen atom in the alkane is replaced by one halogen atom, producing a haloalkane and a hydrogen halide (e.g., or ).
Further substitution can occur if the halogen is in excess, eventually replacing all hydrogen atoms in the molecule.
📐Formulae
💡Examples
Problem 1:
Write the chemical equation for the photochemical reaction between Methane () and Bromine (), and state the necessary condition.
Solution:
Explanation:
One hydrogen atom from the methane molecule is substituted by one bromine atom. The reaction produces bromomethane and hydrogen bromide. The essential condition is the presence of Ultraviolet (UV) light.
Problem 2:
What observation would be made when Ethane () is mixed with Bromine water () and placed in direct sunlight?
Solution:
The reddish-brown color of the bromine water will slowly disappear (decolourise).
Explanation:
In the presence of UV light from the sunlight, a substitution reaction occurs where and are formed. Since the molecular bromine () is consumed, the characteristic reddish-brown color fades.
Problem 3:
Explain why alkanes do not react with chlorine in the dark.
Solution:
The bond requires energy to break to initiate the reaction.
Explanation:
Alkanes are relatively inert. The reaction with halogens is a photochemical radical process; without UV light to provide the energy for homolytic fission of the halogen bond, the reaction cannot start.