Problem 2 - Chemical Principles in the Laboratory

Question

If we classify substances as ionic, molecular, macromolecular, or metallic, in which if any categories are all the members a. soluble in water? b. electrical conductors in the melt? c. insoluble in all common solvents? d. solids at room temperature?

Step-by-Step Solution

Answer

a. Ionic b. Ionic and Metallic c. Macromolecular d. Ionic, Macromolecular, Metallic

Step 1: Understanding Solubility in Water

Substances that are generally soluble in water are ionic compounds and some molecular compounds. Ionic compounds dissolve in water due to their ability to dissociate into positive and negative ions, interacting with polar water molecules.

Step 2: Electrical Conductivity in Melt

In the melt and aqueous solution, ionic compounds conduct electricity due to the free movement of ions. Metallic substances can also conduct electricity in the solid and liquid state because of the sea of delocalized electrons.

Step 3: Insolubility in Common Solvents

Macromolecular (or network covalent) substances, such as diamond and graphite, are generally insoluble in all common solvents due to their extensive network of covalent bonds, which make them very stable and resistant to dissolving.

Step 4: Solid State at Room Temperature

Ionic, macromolecular, and metallic substances are usually solid at room temperature. Ionic compounds have high melting points due to the strong electrostatic forces, macromolecular compounds have strong covalent bonds throughout the structure, and metallic substances have strong metallic bonds which keep them solid at room temperature.

Key Concepts

Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between oppositely charged ions. These generally involve a metal and a non-metal coming together to transfer electrons, creating a strong bond. A common characteristic of ionic compounds is their high solubility in water. This is because water molecules are polar, which enables them to break the ionic bonds and dissolve the substance.

• They are typically solid at room temperature, with high melting and boiling points due to the strong attraction between ions.
• In a molten state or when dissolved in water, they are excellent conductors of electricity, as the ions are free to move and carry charge.
• Ionic compounds are usually brittle due to the rigid lattice structure, which can break under stress.

Molecular Compounds

Molecular compounds, also known as covalent compounds, form when atoms share electrons to achieve stability. These compounds are typically made up of non-metal elements bonded covalently. This leads to the formation of discrete molecules.

• They often have lower melting and boiling points compared to ionic compounds, as the forces holding molecules together (intermolecular forces) are weaker.
• Unlike ionic compounds, molecular compounds are poor conductors of electricity as they do not have free-moving charged particles.
• Solubility in water varies across molecular compounds; some are soluble while others are not, depending on their polarity and the ability to form hydrogen bonds with water.

Metallic Substances

Metallic substances consist of metal atoms held together in a lattice of positive ions surrounded by a "sea of electrons." This unique arrangement allows metals to exhibit properties that are distinct from ionic and molecular compounds.

• They are generally good electrical and thermal conductors in both solid and liquid states, due to freely moving delocalized electrons.
• Metallic substances are often ductile and malleable, allowing them to be shaped without breaking.
• They usually have high melting points and remain solid at room temperature, as strong metallic bonds hold the lattice together.

Macromolecular Substances

Macromolecular substances, also known as network covalent substances, consist of a massivly bonded network of atoms. This category includes materials like diamond and graphite, where atoms are bonded in a continuous network.

• They are not soluble in any common solvents, because the strong covalent bonds require a large amount of energy to break.
• These substances tend to have very high melting and boiling points due to the extensive bonding throughout the structure.
• Electrical conductivity is variable: graphite can conduct electricity due to delocalized electrons, while diamond cannot.
• Macromolecular substances are always solid at room temperature because of their rigid structure.