A Level Chemistry OCR Practice Exam

How is a homologous series defined?

1. A series of compounds without functional groups.

2. A set of compounds differing by CH2 with the same functional group.

3. A collection of elements arranged by increasing atomic number.

4. A group of compounds that share identical structural formulas.

The correct answer is: A set of compounds differing by CH2 with the same functional group.

A homologous series is defined as a set of compounds that differ by a constant unit, typically a –CH2 group, while sharing the same functional group. This structural similarity leads to predictable trends in physical and chemical properties, making them easier to study and categorize. For example, in the alkane series, each successive compound differs by the addition of a –CH2 group, resulting in sequential changes in boiling point, melting point, and other properties due to the increasing molecular size. The definition emphasizes the importance of the functional group shared among the compounds, which greatly influences their reactivity and chemical behavior.