Isomers are molecules that have the same chemical formula but different arrangements of atoms, as shown here for two isomers of pentane, C₅H₁₂.
(b) Which isomer do you expect to have the higher standard molar entropy? Explain.

Verified step by step guidance

1. Identify the structural differences between the two isomers of pentane, C₅H₁₂. Note how the atoms are arranged differently in each isomer.
2. Understand the concept of standard molar entropy, which is a measure of the randomness or disorder in a system. More complex molecular structures with greater freedom of movement typically have higher entropy.
3. Compare the complexity and branching of the isomers. Isomers with more branching generally have less freedom of movement because the branches restrict the rotation of the molecule's bonds.
4. Determine which isomer has less branching. This isomer will likely have higher standard molar entropy because its molecular structure allows for greater freedom of movement and more possible microstates.
5. Conclude that the isomer with the simpler, less branched structure typically has a higher standard molar entropy due to its increased molecular freedom compared to a more branched isomer.

Key Concepts

Isomerism

Isomerism refers to the phenomenon where two or more compounds have the same molecular formula but differ in the arrangement of atoms. This can lead to distinct physical and chemical properties, as seen in structural isomers like the different forms of pentane. Understanding isomerism is crucial for predicting how these compounds will behave in various chemical contexts.

Standard Molar Entropy

Standard molar entropy is a measure of the amount of disorder or randomness in a system at standard conditions (1 bar and 25°C). It is an important thermodynamic property that reflects the number of accessible microstates for a given substance. Generally, more complex molecules with greater degrees of freedom and larger surface areas have higher entropies.

Effect of Molecular Structure on Entropy

The molecular structure significantly influences the entropy of isomers. For instance, branched isomers tend to have lower entropy than their straight-chain counterparts due to reduced molecular motion and fewer conformations. In the case of pentane isomers, the straight-chain form typically exhibits higher entropy because it has more possible arrangements and interactions compared to its branched isomer.