Problem 53 - Chemistry: An Atoms-Focused Approach

Question

In each of the following pairs of compounds, which compound is likely to be more soluble in \(\mathrm{CCl}_{4} ?\) a. \(C C 1_{4}\) or \(C H C l_{3}\) b. \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{2} \mathrm{OH}\) c. \(\mathrm{NaF}\) or \(\mathrm{MgO}\) d. \(\mathrm{CaF}_{2}\) or \(\mathrm{BaF}_{2}\)

Step-by-Step Solution

Answer

Question: Which compound is likely to be more soluble in carbon tetrachloride (CCl4) among the following pairs: a. CCl4 or CHCl3 b. CH3OH or CH3(CH2)4CH2OH c. NaF or MgO d. CaF2 or BaF2 Answer: a. CCl4 b. CH3(CH2)4CH2OH c. NaF d. BaF2

Step 1: a. Comparing solubility of CCl4 and CHCl3 in CCl4

Carbon tetrachloride (CCl4) is a nonpolar molecule, so it will better dissolve nonpolar compounds. CCl4 is itself nonpolar, while chloroform (CHCl3) is slightly polar due to the presence of the hydrogen atom. Therefore, CCl4 is likely to be more soluble in CCl4.

Step 2: b. Comparing solubility of CH3OH and CH3(CH2)4CH2OH in CCl4

Methanol (CH3OH) is a polar molecule due to the presence of the polar -OH (hydroxyl) group. On the other hand, hexanol (CH3(CH2)4CH2OH) is less polar than methanol because it has a longer nonpolar hydrocarbon chain, but it still contains a polar -OH group. Since CCl4 is a nonpolar solvent, neither of them are highly soluble in it. However, hexanol is likely to be more soluble than methanol, as its nonpolar region is larger.

Step 3: c. Comparing solubility of NaF and MgO in CCl4

Sodium fluoride (NaF) and magnesium oxide (MgO) are both ionic compounds, which means they are made of charged particles and will be more soluble in polar solvents like water rather than nonpolar solvents like CCl4. Both compounds have very limited solubility in nonpolar solvents. However, since NaF has a smaller charge density than MgO, it is likely to have a slightly higher solubility in CCl4.

Step 4: d. Comparing solubility of CaF2 and BaF2 in CCl4

Calcium fluoride (CaF2) and barium fluoride (BaF2) are both ionic compounds, containing cations (Ca2+ and Ba2+) and anions (F-). These compounds are more soluble in polar solvents. Considering the similar structures and charge densities of these two compounds, it is difficult to accurately predict their relative solubility in CCl4. Nonetheless, due to the relatively larger size of the barium cation, BaF2 might have a slightly lower lattice energy, making it more soluble in CCl4.

Key Concepts

Nonpolar solvents

Nonpolar solvents, like carbon tetrachloride (\( \mathrm{CCl_4} \)), are characterized by their lack of polarity, which means they do not have distinct positive and negative ends. These types of solvents are effective at dissolving other nonpolar compounds due to the principle of "like dissolves like."

• A molecule is considered nonpolar if it has a symmetrical distribution of electrical charge, resulting in no overall dipole moment.


• Nonpolar solvents are usually made of atoms or molecules that have similar electronegativities.

Since \( \mathrm{CCl_4} \) is nonpolar, it serves as an excellent solvent for dissolving other nonpolar substances. This property helps explain why \( \mathrm{CCl_4} \) itself is more soluble in \( \mathrm{CCl_4} \) than chloroform (\( \mathrm{CHCl_3} \)).

Ionic compounds

Ionic compounds consist of metals and nonmetals that exchange electrons, creating charged particles called ions. The positive ions are cations, and the negative ones are anions, leading to the formation of a stable ionic lattice due to strong electrostatic interactions.

• Common examples include sodium fluoride (\( \mathrm{NaF} \)) and magnesium oxide (\( \mathrm{MgO} \)).


• These compounds typically have high melting and boiling points due to the energy required to break the ionic bonds.

Ionic compounds are generally soluble in polar solvents like water because the solvent molecules can stabilize the ions, allowing them to dissociate. However, their solubility in nonpolar solvents like \( \mathrm{CCl_4} \) is very limited. For example, between \( \mathrm{NaF} \) and \( \mathrm{MgO} \), sodium fluoride might have slightly better solubility in \( \mathrm{CCl_4} \) due to its lower charge density.

Polarity

Polarity is a fundamental concept in chemistry that explains the distribution of electrical charge around different atoms, molecules, or compounds. The polarity of a substance significantly affects its solubility properties.

• A polar molecule has unequal sharing of electrons, resulting in a positive and a negative pole, or dipole, within the molecule.


• Polarity is determined by the difference in electronegativity between atoms in a chemical bond.

For instance, in the case of \( \mathrm{CH_3OH} \) (methanol), the polar -OH group makes it a polar molecule. This contrasts with \( \mathrm{CH_3(CH_2)_4CH_2OH} \) (hexanol), which though contains a polar -OH group, has a longer nonpolar tail that reduces its polarity overall. In a nonpolar solvent like \( \mathrm{CCl_4} \), a less polar compound like hexanol has a higher likelihood of dissolving.

Solubility prediction

Solubility prediction involves assessing which substances will dissolve in which solvents based on chemical principles. One key factor is the nature of the solvent and solute: the general guideline is that "like dissolves like."

• Nonpolar solvents dissolve nonpolar substances effectively, while polar solvents dissolve polar and ionic substances.


• This rule can guide predictions about unknown solubilities based on known properties of substances.

In solubility prediction, consider the solvent and the effect of Molecular Size, Polarity, and Charge Density.
Take, for example, \( \mathrm{CaF_2} \) and \( \mathrm{BaF_2} \) in \( \mathrm{CCl_4} \): Both are ionic, but due to subtle differences like lattice energy, \( \mathrm{BaF_2} \) might unexpectedly have a bit higher solubility. Understanding these nuances and applying these principles helps chemists predict and explain solubility outcomes.