Problem 121 - General Chemistry: Principles and Modern Applications

Question

The most likely of the following mixtures to be an ideal solution is (a) \(\mathrm{NaCl}-\mathrm{H}_{2} \mathrm{O} ;\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}-\mathrm{C}_{6} \mathrm{H}_{6}\) (c) \(\mathrm{C}_{7} \mathrm{H}_{16}-\mathrm{H}_{2} \mathrm{O} ;\) (d) \(\mathrm{C}_{7} \mathrm{H}_{16}-\mathrm{C}_{8} \mathrm{H}_{18}\)

Step-by-Step Solution

Answer

The most likely mixture to form an ideal solution is (d) C7H16 - C8H18.

Step 1: Understand A Basic Principle

The principle being referred to is 'Like dissolves like'. This principle is based on the fact that polar substances dissolve best in polar solvents and non-polar substances dissolve best in non-polar solvents.

Step 2: Identifying Characteristics of Substances

Now apply this principle to the options given and identify the characteristics of the substances: \n(a) NaCl (an ionic compound) and H2O (a polar compound).\n(b) CH3CH2OH (an alcohol, polar) and C6H6 (Benzene, nonpolar).\n(c) C7H16 (Heptane, nonpolar) and H2O (a polar compound). \n(d) C7H16 (Heptane, nonpolar) - C8H18 (Octane, nonpolar).

Step 3: Identifiying The Ideal Solution

From the previous step, it can be seen that in option (d), both the solute and the solvent are non-polar and similar in nature. Hence, they comply with the 'like dissolves like' principle, which makes this mixture the most likely to form an ideal solution.

Key Concepts

Like Dissolves Like

When it comes to understanding why certain substances mix well while others do not, a fundamental principle to remember is 'like dissolves like.' This catchy phrase encapsulates a concept essential in the field of chemistry and has practical applications in everything from creating pharmaceuticals to concocting culinary delights.

What the principle suggests is rather intuitive: substances that share similar molecular characteristics will more likely dissolve in one another. For example, polar solvents, such as water, are adept at dissolving polar substances like salt (sodium chloride). This is because the slightly positive and negative charges in polar molecules can interact well with other polar molecules, leading to a stable solution.

On the other hand, non-polar substances, which lack these charge differences, mix well with other non-polar compounds. They do this by a process known as dispersion forces, contributing to their mutual solubility. A classic example of 'like dissolves like' in action is when oil (non-polar) is added to vinegar (polar), and separation occurs; however, if you mix oil with something non-polar like hexane, they mix thoroughly, conforming to the principle.

Polarity of Substances

The concept of polarity is pivotal in understanding solubility and solution behaviors in chemistry. Polarity refers to the distribution of electrical charge over the atoms in a molecule. In a polar molecule, there is an uneven distribution of electrons between the atoms, leading to regions of partial positive and negative charge.

Water is the 'universal solvent' largely because it is polar, with the oxygen atom having a slight negative charge and the hydrogen atoms a slight positive charge. This polarity allows water to form hydrogen bonds with other polar substances, thereby facilitating dissolving them.

Identifying Polar and Non-Polar Substances

A quick way to determine if a substance is polar or non-polar is by looking at its molecular structure and the difference in electronegativity between its atoms. Molecules with significant electronegativity differences, and that are asymmetric in shape, are often polar. For instance, ethanol (CH3CH2OH) is polar because of the electronegative oxygen atom that draws electrons towards itself. In contrast, benzene (C6H6) is non-polar; it has a symmetrical ring structure with equal sharing of electrons.

Solubility Principles

Understanding solubility is key to predicting how substances will behave when mixed. Solubility is the property of a substance to dissolve in a solvent, which is governed by both physical forces and thermodynamic principles.

For the dissolution to occur, molecules of the solute (the substance being dissolved) must interact with the molecules of the solvent (the substance doing the dissolving), overcoming both the solute's own intermolecular forces and the solvent's intermolecular forces. This process is driven by factors like temperature, pressure, and most importantly, the intermolecular forces discussed in the concept of polarity.

Let's apply these principles to our earlier exercise. Between NaCl-H2O and CH3CH2OH-C6H6, the former combination involves a strong ionic bond, and the latter a polar-nonpolar mix, neither of which would result in significant solubility based on these principles. In contrast, the similar non-polar characteristics of C7H16 (Heptane) and C8H18 (Octane) mean they can mix seamlessly, leading to a high degree of solubility, consistent with the 'like dissolves like' principle and forming what could be an 'ideal solution.'