Question

Which of the substances in each of the following sets would be expected to have the highest boiling point? Explain why. a. Ga, KBr, O \(_{2}\) b. Hg, NaCl, He c. \(\mathrm{H}_{2}, \mathrm{O}_{2}, \mathrm{H}_{2} \mathrm{O}\)

Short answer

The substances with the highest boiling points in each set are: a. KBr, due to its ionic bonding. b. NaCl, also due to its ionic bonding. c. H2O, because of its strong hydrogen bonding.

Step-by-step solution

Identify the substances in Set A

In Set A, we have Ga (gallium), KBr (potassium bromide), and O2 (oxygen).

Analyze the substances in Set A

- Ga is a metallic element and has metallic bonding. - KBr is an ionic compound. It has ionic bonding between the potassium (K+) and bromide (Br-) ions. - O2 is a covalent (molecular) compound. It has London dispersion forces and very weak dipole-dipole forces.

Compare the boiling points of substances in Set A

Ionic compounds generally have higher boiling points than covalent compounds due to stronger ionic bonds. Metallic bonding is also generally stronger than the forces in covalent compounds. So, the expected order of boiling points is KBr > Ga > O2. Therefore, KBr would have the highest boiling point in Set A.

Identify the substances in Set B

In Set B, we have Hg (mercury), NaCl (sodium chloride), and He (helium).

Analyze the substances in Set B

- Hg is a metallic element and has metallic bonding. - NaCl is an ionic compound. It has ionic bonding between the sodium (Na+) and chloride (Cl-) ions. - He is a noble gas and has only London dispersion forces.

Compare the boiling points of substances in Set B

Similar to Set A, ionic compounds generally have higher boiling points than covalent compounds, and metallic bonding is generally stronger than the forces in covalent compounds. So, the expected order of boiling points is NaCl > Hg > He. Therefore, NaCl would have the highest boiling point in Set B.

Identify the substances in Set C

In Set C, we have H2 (hydrogen), O2 (oxygen), and H2O (water).

Analyze the substances in Set C

- H2 is a diatomic molecule with weak London dispersion forces. - O2 is a diatomic molecule with weak London dispersion forces and very weak dipole-dipole forces. - H2O is a polar covalent compound with strong hydrogen bonding.

Compare the boiling points of substances in Set C

Hydrogen bonding is significantly stronger than the other intermolecular forces present in this set. So, the expected order of boiling points is H2O > O2 > H2. Therefore, H2O would have the highest boiling point in Set C.

Key concepts

Intermolecular Forces

Intermolecular forces are the invisible attractions that hold molecules together. These forces play a crucial role in determining the boiling points of substances. There are several types of intermolecular forces:

• London Dispersion Forces: These are the weakest intermolecular forces. They are present in all molecules, whether polar or nonpolar. Their strength increases with the size and mass of the molecules.
• Dipole-Dipole Forces: These occur between the positive end of one polar molecule and the negative end of another. They are stronger than London dispersion forces.
• Hydrogen Bonding: A special type of dipole-dipole interaction that occurs when hydrogen is directly bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. These are exceptionally strong compared to other intermolecular forces.

When comparing boiling points, remember that the stronger the intermolecular forces, the higher the boiling point of the substance. This is why water (\(H_{2}O\)), with strong hydrogen bonds, has a much higher boiling point than oxygen (\(O_{2}\)), which only has London dispersion forces.

Ionic Compounds

Ionic compounds are formed when metals donate electrons to non-metals, creating oppositely charged ions that attract each other. The strong electrostatic attraction between these ions is known as ionic bonding.

• Strength of Ionic Bonding: The forces holding these ions together are very strong, which leads to high melting and boiling points.
• Examples: Common examples include sodium chloride (\(NaCl\)) and potassium bromide (\(KBr\)).
• State at Room Temperature: Ionic compounds are usually solid at room temperature because of these strong interactions.

In a boiling point comparison, ionic compounds typically have higher boiling points than metals with metallic bonding or molecular compounds with covalent bonds. The strength of ionic bonds makes it significantly more difficult to overcome these attractions compared to the weaker forces found in molecular compounds.

Metallic Bonding

Metallic bonding is a type of chemical bonding found in metal elements. It involves a "sea of electrons" that are free to move around, which gives metals their unique properties.

• Electron Sea Model: In metals, the valence electrons are not bound to any particular atom. Instead, they are shared across the entire structure and can move freely, which is why metals are good conductors of electricity.
• Strength of Metallic Bonds: These bonds are strong, though generally slightly weaker than ionic bonds.
• Properties: Metallic bonding leads to properties such as malleability, ductility, and relatively high melting and boiling points.

While metallic bonding contributes to high boiling points, ionic compounds usually display even higher boiling points due to the very strong ionic interactions present.

Covalent Compounds

Covalent compounds are formed when atoms share electrons. This sharing allows each atom to attain the electron configuration of a noble gas, leading to the formation of a molecule.

• Types of Covalent Compounds: There are many types, ranging from simple diatomic molecules like oxygen (\(O_{2}\)) to complex organic molecules.
• Intermolecular Forces in Covalent Compounds: Typically characterized by relatively low boiling points due to weaker intermolecular forces like London dispersion or dipole-dipole interactions.
• Special Case of Water: Water (\(H_{2}O\)) is a notable exception because of its strong hydrogen bonds, resulting in much higher boiling points compared to other covalent compounds.

In the comparison of boiling points, covalent compounds generally have lower boiling points than ionic or metallic bonded substances, unless strong hydrogen bonding is present as in water.