Which of the following statements concerning homologous series is INCORRECT?
A. Members of a series have identical chemical properties.
B. Members of a series have the same general formula.
C. Each member differs from the next by a CH2 group.
D. Members of a series show a gradual change of physical properties.
The correct answer and explanation is :
The incorrect statement concerning homologous series is:
A. Members of a series have identical chemical properties.
Explanation:
A homologous series refers to a group of organic compounds that share a similar general formula, exhibit a consistent pattern of chemical behavior, and differ by a constant structural unit (usually a CH₂ group). Let’s break down each statement:
- B. Members of a series have the same general formula.
- This statement is correct. All compounds in a homologous series share a general formula, which can be represented by a common formula for the series. For example, alkanes have the general formula CₙH₂ₙ₊₂, alkenes have CₙH₂ₙ, and alcohols have CₙH₂ₙ₊₁OH. This common formula helps define the family of compounds.
- C. Each member differs from the next by a CH₂ group.
- This statement is correct. In a homologous series, each successive compound differs from the previous one by the addition of a CH₂ unit. This is why the members are said to form a series where each compound’s molecular structure is similar, with only slight differences (a CH₂ group) separating them. For instance, in the alkane series: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and so on.
- D. Members of a series show a gradual change of physical properties.
- This statement is correct. In a homologous series, physical properties such as boiling point, melting point, solubility, and density tend to change gradually. For example, as the number of carbon atoms increases in the alkane series, the boiling point increases steadily. This change occurs because larger molecules have stronger intermolecular forces, leading to higher melting and boiling points.
- A. Members of a series have identical chemical properties.
- This statement is incorrect. While members of a homologous series have similar chemical properties due to their similar functional groups and molecular structures, they do not have identical chemical properties. As the size and structure of molecules increase (i.e., the chain length grows), certain chemical properties, such as reactivity, may change. For example, the reactivity of alkanes changes with chain length, and larger molecules might undergo different reactions due to their size and structure, even though they all share the same functional group (saturated carbon-carbon bonds in the case of alkanes).
In summary, while homologous series members share similarities in chemical structure and general trends in physical properties, they do not exhibit identical chemical properties.