5.7 Properties of Ionic and Covalent Compounds

Short Answer Questions

1. How are ionic compounds formed?
   Answer
   Ionic compounds are formed through the transfer of electrons between a metal and a nonmetal.
2. What type of ions are formed by metals?
   Answer
   Metals form positive ions called cations.
3. What type of ions are formed by nonmetals?
   Answer
   Nonmetals form negative ions called anions.
4. What force holds ionic compounds together?
   Answer
   Strong electrostatic forces hold ionic compounds together.
5. What is the typical structure of ionic compounds?
   Answer
   Ionic compounds typically form a lattice structure.
6. What are the typical melting and boiling points of ionic compounds?
   Answer
   Ionic compounds typically have high melting and boiling points.
7. Are ionic compounds usually soluble in water or organic solvents?
   Answer
   Ionic compounds are usually soluble in water.
8. In what state(s) do ionic compounds conduct electricity?
   Answer
   Ionic compounds conduct electricity when molten or dissolved in water (aqueous state).
9. Are ionic compounds hard or soft?
   Answer
   Ionic compounds are generally hard.
10. Are ionic compounds brittle or flexible?
    Answer
    Ionic compounds are generally brittle.
11. How are covalent compounds formed?
    Answer
    Covalent compounds are formed by the sharing of electrons between two nonmetals.
12. What is the basic structural unit of covalent compounds?
    Answer
    Covalent compounds exist as discrete molecules.
13. What are the typical melting and boiling points of covalent compounds?
    Answer
    Covalent compounds typically have low melting and boiling points.
14. Are covalent compounds usually soluble in water or organic solvents?
    Answer
    Covalent compounds are usually soluble in organic solvents.
15. Do covalent compounds conduct electricity?
    Answer
    Covalent compounds do not conduct electricity in any state.
16. Can covalent compounds exist as solids, liquids, or gases at room temperature?
    Answer
    Covalent compounds can exist as solids, liquids, or gases at room temperature.
17. What type of bond involves the sharing of electron pairs?
    Answer
    A covalent bond involves the sharing of electron pairs.
18. What type of attraction occurs between ions in an ionic bond?
    Answer
    Electrostatic attraction occurs between ions in an ionic bond.
19. Are electrons delocalised in ionic bonds?
    Answer
    Electrons are not delocalised in ionic bonds; they are transferred.
20. What is the name given to the shared electron pairs in a covalent bond?
    Answer
    The shared pairs of electrons are called bonding pairs.

Explanation Questions

1. Explain why ionic compounds have high melting and boiling points.
   Answer
   Ionic compounds have high melting and boiling points because of the strong electrostatic forces between the positive and negative ions in their lattice structures. A large amount of energy is required to overcome these forces and separate the ions, causing a change of state.
2. Describe why ionic compounds conduct electricity when molten or dissolved in water, but not in the solid state.
   Answer
   In the solid state, ions in an ionic compound are held in fixed positions within the lattice structure and cannot move to carry an electrical charge, so it does not conduct electricity. When melted or dissolved in water, the ions become free to move and carry charge, thus enabling electrical conductivity.
3. Explain why covalent compounds typically have low melting and boiling points.
   Answer
   Covalent compounds have low melting and boiling points because the intermolecular forces between the molecules are weak (such as Van der Waals' forces). Only a small amount of energy is needed to overcome these weak forces and separate the molecules, resulting in a low phase transition temperature.
4. Why do covalent compounds not conduct electricity in any state?
   Answer
   Covalent compounds do not conduct electricity because they do not have any charged particles that are free to move. Electrons are shared between atoms and are not free to carry charge.
5. Explain the difference in solubility between ionic and covalent compounds in water and organic solvents.
   Answer
   Ionic compounds are usually soluble in water because the polar water molecules can surround and stabilise the ions through ion-dipole interactions, overcoming the electrostatic attraction of the ionic lattice. Covalent compounds are generally soluble in organic solvents as they are non-polar or have low polarity that has a similar intermolecular force with organic solvents.
6. Describe the formation of a lattice structure in ionic compounds and why this is important to their properties.
   Answer
   In an ionic compound, positive and negative ions are arranged in a regular, repeating three-dimensional lattice structure. This arrangement maximises the electrostatic attraction between ions and is a very stable arrangement, making these structures very hard and brittle. It is also the reason why ionic compounds have high melting and boiling points and conduct electricity when dissolved in water.
7. How do the forces in ionic compounds differ from the forces in covalent compounds?
   Answer
   Ionic compounds have strong electrostatic forces between ions throughout the lattice, which are responsible for their hardness and high melting points. Covalent compounds have much weaker intermolecular forces such as Van der Waals forces between separate molecules, which gives them relatively lower melting and boiling points.
8. How does electron transfer and electron sharing influence the properties of ionic and covalent compounds?
   Answer
   Electron transfer in ionic bonds results in charged ions that are strongly attracted to each other in a lattice, which leads to high melting points, electrical conductivity when molten or in solution, and solubility in water. In covalent bonding, the sharing of electrons results in neutral molecules with relatively weak intermolecular forces, low melting points, no electrical conductivity, and solubility in organic solvents.
9. Compare and contrast the structures of ionic and covalent compounds.
   Answer
   Ionic compounds form extended lattice structures with repeating patterns of positively and negatively charged ions, forming a 3D network. In contrast, covalent compounds exist as discrete molecules with a definite number of atoms bound together, with weak intermolecular forces between them.
10. Explain why ionic compounds are generally brittle.
    Answer
    The ions in the ionic lattice are held in place by strong electrostatic attractions to other ions, which causes a rigid structure. When a force is applied, similarly charged ions can be displaced next to each other, which causes repulsion forces that can fracture the crystal structure, making ionic compounds brittle.