Question

For each pair of substances, choose the one that you expect to have the higher standard molar entropy (S°) at 25 °C.

a. NO₃(s); NaNO₃(aq)
b. CH₄(g); CH₃CH₃(g)
c. Br₂(l); Br₂(g)
d. Br₂(g); F₂(g)
e. PCl₃(g); PCl₅(g)

Answer 1

To predict which substance has the higher standard molar entropy at 25 °C, consider factors such as the phase, molecular complexity, size, and dispersal of particles. In general, substances in aqueous or gaseous states, with more atoms and weaker intermolecular forces, have greater entropy.

Step-by-step explanation:

The standard molar entropy (S°) at 25 °C for each pair of substances can be predicted based on the number of atoms involved, phase, complexity of the molecules, and the dispersal of particles. For each pair:

• NaNO₃(aq) will have a higher S° than NO₃(s) because solutions generally have higher entropy than solids due to the increased dispersal of particles.
• CH₄(g) will have a lower S° than CH₃CH₃(g) because larger molecules with more atoms and bonds have more possible microstates and thus higher entropy.
• Br₂(g) will have a higher S° than Br₂(l) because gases have more freedom of motion and more microstates available than liquids.
• F₂(g) has a higher S° than Br₂(g) as lighter gases with weaker intermolecular forces tend to have higher entropy due to their higher speeds and more accessible microstates.
• PCl₅(g) will have a higher S° than PCl₃(g) because molecules with more atoms have more vibrational modes, leading to a larger number of microstates and higher entropy.