Problem 97 - Chemistry and Chemical Reactivity

Question

Which of the following substances is likely to dissolve in water, and which is likely to dissolve in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right) ?\) (a) \(\mathrm{NaNO}_{3}\) (b) diethyl ether, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (c) naphthalene, \(\mathrm{C}_{10} \mathrm{H}_{8}\) (d) \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

(a) NaNO₃ and (d) NH₄Cl are likely to dissolve in water. (b) Diethyl ether and (c) naphthalene are likely to dissolve in benzene.

Step 1: Understand Solubility Principles

To determine if a substance will dissolve in water or benzene, we must understand the basic solubility principles. Water is a polar solvent, while benzene is a non-polar solvent. Polar substances are soluble in polar solvents and non-polar substances are soluble in non-polar solvents ('like dissolves like').

Step 2: Evaluate Solubility of NaNO₃

Sodium nitrate (\(\mathrm{NaNO_3}\)) is an ionic compound. Ionic and polar substances are typically soluble in water due to the polarity of water, which helps in dissociating ions.

Step 3: Evaluate Solubility of Diethyl Ether

Diethyl ether (\(\mathrm{CH_3CH_2OCH_2CH_3}\)) is relatively non-polar, so it is more likely to dissolve in benzene (a non-polar solvent) rather than in water.

Step 4: Evaluate Solubility of Naphthalene

Naphthalene (\(\mathrm{C_{10}H_8}\)) is a non-polar aromatic hydrocarbon. Non-polar substances like naphthalene are more likely to dissolve in non-polar solvents like benzene.

Step 5: Evaluate Solubility of NH₄Cl

Ammonium chloride (\(\mathrm{NH_4Cl}\)) is an ionic compound. Like sodium nitrate, the polarity of water makes \(\mathrm{NH_4Cl}\) soluble in water due to ion-dipole interactions.

Key Concepts

Polar and Non-polar Solvents

Solvents are substances that dissolve a solute, forming a solution. In the world of chemistry, understanding the nature of solvents is crucial for predicting and explaining solubility. Solvents are broadly classified into two categories: polar and non-polar.

**Polar Solvents**
Water is a prime example of a polar solvent. It has a positive and a negative end, much like a magnet. This polarity allows water to interact with other polar substances and ionic compounds. This interaction often involves breaking apart the solute into its individual ions or polar molecules, effectively dissolving it.

**Non-polar Solvents**
On the other hand, non-polar solvents like benzene have no such positive or negative ends. Their electrons are distributed more evenly throughout the molecule, making them non-polar. These solvents are typically better at dissolving non-polar solutes because similarly uncharged molecules mix together more readily. The lack of charge means that non-polar solvents cannot dissolve ionic or polar solutes effectively.

Ionic Compounds

Ionic compounds are made up of charged particles called ions. These ions are formed when atoms gain or lose electrons. For instance, in sodium nitrate (\(\mathrm{NaNO_3}\)), the compound is made up of sodium ions (\(\mathrm{Na^{+}}\)) and nitrate ions (\(\mathrm{NO_3^{-}}\)).

**Dissolving in Water**
Ionic compounds dissolve well in polar solvents because of the unique interactions that occur. Water, being polar, has partial positive charges on its hydrogen atoms and partial negative charges on its oxygen atom. When ionic compounds are introduced, the positive end of water can attract negative ions, while the negative end attracts positive ions. This attraction helps to pull the ions away from their lattice structures, leading to dissolution.

**Example**
Ammonium chloride (\(\mathrm{NH_4Cl}\)) in water is an example where the water's polar nature helps to dissolve the ionic solid by surrounding its ions and breaking them apart from the crystal structure.

Like Dissolves Like

The phrase "like dissolves like" is a simple yet powerful rule in chemistry that helps us predict solubility. This principle states that a solute will tend to dissolve best in a solvent that has a similar polarity.

**Polar with Polar**
Polar solvents like water are effective at dissolving polar solutes and ionic compounds. This happens because the polar molecules can interact with and stabilize each other through dipole-dipole interactions or ion-dipole interactions.

**Non-polar with Non-polar**
Similarly, non-polar solvents are best at dissolving non-polar substances. Non-polar compounds like naphthalene (\(\mathrm{C_{10}H_8}\)) share the same lack of charge as non-polar solvents like benzene, allowing them to mix together well without any charge repulsion or attractions that could inhibit dissolution.

**Exceptions to the Rule**
While "like dissolves like" is a handy guideline, there are exceptions. Solubility can also be influenced by temperature, pressure, and the presence of other substances. But understanding the polar and non-polar nature of solutes and solvents gives a strong foundation in predicting solubility outcomes.