Problem 21 - Chemistry The Central Science In Si Units

Question

Two nonpolar organic liquids, benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) and toluene \(\left(\mathrm{C}_{7} \mathrm{H}_{8}\right),\) are mixed. (a) Do you expect \(\Delta H_{\text {soln }}\) to be a large positive number, a large negative number, or close to zero? Explain. (b) Benzene and toluene are miscible with each other in all proportions. In making a solution of them, is the entropy of the system increased, decreased, or close to zero, compared to the separate pure liquids?

Step-by-Step Solution

Answer

(a) 9H_{ ext{soln}} is close to zero; (b) Entropy increases.

Step 1: Understanding the Nature of the Solutes

Benzene and toluene are both nonpolar organic liquids, which means their intermolecular forces (primarily dispersion forces) are similar. When two substances with similar intermolecular forces are mixed, the process does not require significant energy change, hence no strong formation or breaking of interactions.

Step 2: Analyzing Enthalpy Change

The enthalpy change for solution formation, 9H_{ ext{soln}}, is influenced by the balance between breaking intermolecular forces in the solute and solvent and forming new intermolecular forces between solute and solvent molecules. In this case, since both substances have similar intermolecular forces, 9H_{ ext{soln}} should be close to zero.

Step 3: Entropy Considerations in Solution Formation

Entropy is a measure of disorder or randomness. Mixing two substances increases the overall disorder of the system. Thus, when benzene and toluene are mixed, the entropy increases as the separate ordered liquids mix into a more disordered solution.

Step 4: Conclusion on Entropy Change

Therefore, when benzene and toluene, which are miscible in all proportions, are mixed, the entropy of the system increases compared to the pure components.

Key Concepts

Enthalpy Change in Solution Formation

During the solution formation process, one major aspect to consider is the enthalpy change, or \(\Delta H_{\text{soln}}\), which represents the heat change associated with the mixing of substances. When discussing two nonpolar liquids like benzene and toluene, we focus on intermolecular forces to understand the energy dynamics. Since both benzene and toluene exhibit primarily dispersion forces due to their nonpolar nature, the energy required to break these forces is nearly matched by the energy released when similar forces form between mixed molecules.

• This energy balance implies that no significant energy input or release occurs.
• Thus, the enthalpy change, \(\Delta H_{\text{soln}}\), is expected to be close to zero.

In scenarios where mixed substances possess vastly different intermolecular attractions, we might see large enthalpic changes. With benzene and toluene, however, the similar nature of their nonpolar characteristics ensures that energy required equals energy released, maintaining thermal balance.

Entropy Considerations in Solution Formation

Entropy, a measure of a system's disorder, plays a crucial role in solution chemistry. When two substances like benzene and toluene mix, the system experiences an increase in entropy. Consider that in the separate states, benzene and toluene molecules have an ordered arrangement due to their nonpolar structure. Mixing disrupts that order, leading to a more random, disordered state.

• This increase in randomness or disorder, results from the blending of distinct molecules.
• Higher entropy indicates a natural tendency for systems to favor states of greater disorder.

Consequently, when benzene and toluene are mixed, the system's entropy increases. This increase in disorder is more favorable energetically, illustrating why these two liquids are miscible in all proportions.

Role of Intermolecular Forces

Intermolecular forces are key players in understanding how solutions form and behave. For benzene and toluene, the intermolecular attractions are primarily London dispersion forces due to their nonpolar characteristics.When two liquids with similar types of intermolecular forces are mixed, as with benzene and toluene, the ease of interaction replicates the environment found within each pure liquid.

• The minimal difference in their forces means mixing doesn't require excessive energy, leading to a small \(\Delta H_{\text{soln}}\).
• Such interactions facilitate miscibility, or the ability of the substances to mix in all proportions.

Understanding these forces helps explain why two entirely nonpolar molecules can mix seamlessly: their similar intermolecular interactions do not disrupt the existing order significantly—they merely blend to form a new yet similar energetic landscape.