Question

Directions: Each question contains Statement-1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (c) Statement-1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Statement 1 Boiling point of n-pentane is higher than that of neopentane while the melting point of \(n\) -pentane is lower than that of neopentane. and Statement 2 Boiling point of alkanes depend upon the surface area while melting point depends upon the packing of molecules in the solid state. Neopentane fits into the crystal lattice more readily and it has minimum surface area compared to n-pentane.

Short answer

(a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is not a correct explanation for Statement- 1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Answer: (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1

Step-by-step solution

To begin with, let's confirm the properties of n-pentane and neopentane. n-Pentane is a straight-chain alkane, while neopentane is a branched alkane. Straight-chain alkanes generally have a larger surface area compared to their branched counterparts. #step2#: Examine the effect of surface area on boiling point

As per Statement 2, the boiling point of alkanes depends upon the surface area. Generally, alkanes with larger surface area have a higher boiling point due to increased van der Waals forces. Since n-pentane has a greater surface area than neopentane, its boiling point should be higher than neopentane. This is in agreement with Statement 1. #step3#: Examine the effect of molecular packing on melting point

Statement 2 also states that the melting point depends on the packing of molecules in the solid state. Branched alkanes typically fit into the crystal lattice more readily because of their more symmetrical structure, and hence, have higher melting points. In this case, neopentane, being a branched alkane, fits into the crystal lattice better than n-pentane and thus should have a higher melting point. This also agrees with the Statement 1. #step4#: Determine the correct option

Since both Statement 1 and Statement 2 are true, and Statement 2 provides a correct explanation for Statement 1, the correct option is: (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1

Key concepts

Understanding Alkanes

Alkanes are a fundamental type of hydrocarbon, consisting exclusively of carbon and hydrogen atoms. These molecules are saturated, meaning they only contain single bonds between carbon atoms. This structure results in a relatively simple chemical makeup that is highly stable. Alkanes can vary considerably in size and complexity:

• Straight-chain alkanes: These have a linear arrangement of carbon atoms, like n-pentane.
• Branched alkanes: These have carbon chains that include side branches, such as neopentane.

This difference in molecular structure can profoundly affect their physical properties, including boiling and melting points.
Because of their saturated nature, alkanes do not readily react with many substances, which is one reason they are often used as solvents or fuels.

Boiling Point of Alkanes

The boiling point of alkanes is crucially influenced by their molecular structure. It is primarily determined by the strength of van der Waals forces between molecules. These forces become stronger when there is more surface area for molecules to interact. Therefore, several factors influence the boiling point:

• Surface Area: Straight-chain alkanes like n-pentane have a larger surface area compared to branched alkanes, leading to stronger van der Waals attractions and a higher boiling point.
• Molecular Weight: Generally, as the molecular weight of an alkane increases, so does its boiling point.

Branched alkanes like neopentane have fewer points of contact due to their more compact shapes, resulting in lower boiling points despite having the same molecular formula as their straight-chain isomers.

Melting Point and Molecular Packing

Melting point is another important physical property of alkanes, heavily influenced by how well molecules fit into a crystalline structure. The arrangement and packing of molecules in the solid state determine how easily they can transition into the liquid state:

• Molecular Symmetry: Branched alkanes like neopentane are typically more symmetrical, allowing them to pack more efficiently into a solid crystalline form. This efficient packing yields a higher melting point.
• Straight Chains vs. Branches: Straight-chain alkanes like n-pentane tend to pack less efficiently, leading to weaker intermolecular forces in the solid state and thus a lower melting point.

Understanding these properties enlightens why neopentane, although having similar molecular weight to n-pentane, exhibits a higher melting point.

Molecular Structure of Alkanes

The molecular structure of alkanes plays a crucial role in determining their physical properties. The differences between straight-chain and branched alkanes highlight the importance of structural configuration:

• Straight-Chain Alkanes: Such as n-pentane, feature carbon atoms in a linear sequence, maximizing potential surface area for intermolecular interactions.
• Branched Alkanes: Like neopentane, have a core of carbon atoms surrounded by carbon branches. This branching typically reduces surface area but allows for more compact and efficient molecular packing.

The molecular structure affects both boiling and melting points, showcasing why structural isomers like n-pentane and neopentane can have vastly different properties despite sharing the same molecular formula.