5.7 - Properties of Ionic Compounds and Covalent Compounds

Experiment 5.1: Comparing Ionic and Covalent Compounds

Aim: To study differences in properties between ionic and covalent compounds.

Problem statement: What are the differences in properties between ionic and covalent compounds?

Materials: Solid lead(II) bromide, $\mathrm{PbB}{\mathrm{r}}_2$; naphthalene, ${\mathrm{C}}_{10}{\mathrm{H}}_8$; magnesium chloride, $\mathrm{MgC}{\mathrm{l}}_2$; cyclohexane, ${\mathrm{C}}_6{\mathrm{H}}_{12}$; and distilled water.

Apparatus: Test tubes, spatula, evaporating dish, Bunsen burner, pipeclay triangle, wire gauze, beaker, $10\mathrm{c}{\mathrm{m}}^3$ measuring cylinder, tripod stand, battery, switch, bulb and carbon electrodes.

A. Electrical Conductivity

Hypothesis: Ionic compounds conduct electricity when molten but not when solid; covalent compounds do not conduct in either state.

• Manipulated variable: type of compound
• Responding variable: electrical conductivity
• Fixed variable: carbon electrode

Procedure:

1. Fill half a crucible with lead(II) bromide powder.
2. Connect the crucible contents to a circuit containing carbon electrodes, a bulb, a switch and a battery.
3. Switch on and observe the bulb, then switch off.
4. Heat until all lead(II) bromide melts; switch on again and observe the bulb.
5. Repeat using naphthalene and record the bulb condition for each substance in solid and molten states.

Safety precautions: Perform the experiment in a fume chamber. Naphthalene is flammable, bromine produced while heating lead(II) bromide is poisonous, and excessive naphthalene exposure can cause haemolytic anaemia, liver and nervous system failure, cataract and retinal bleeding.

B. Solubility in Water and an Organic Solvent

1. Place half a spatula of magnesium chloride in a test tube.
2. Add $5\mathrm{c}{\mathrm{m}}^3$ distilled water, shake gently and observe solubility.
3. Repeat with cyclohexane as solvent.
4. Repeat both tests using naphthalene instead of magnesium chloride and record the results.

C. Melting and Boiling Points

1. Put half a spatula each of magnesium chloride and naphthalene into separate test tubes.
2. Heat both test tubes in a water bath.
3. Observe physical-state changes and infer their relative melting and boiling points.

Electrical Conductivity

Ionic compounds	Covalent compounds
Do not conduct when solid because ions are fixed in a lattice by strong electrostatic forces.	Do not conduct in any state because their neutral molecules carry no charge.
Conduct when molten or in aqueous solution because the ions can move freely and carry charge.	—

Solubility

• Most ionic compounds dissolve in water but not in organic solvents.
• A lattice is an orderly arrangement of atoms, ions or molecules in a solid crystal.
• Oxygen attracts shared electrons more strongly than hydrogen, producing unequal sharing and partial charges in water.
• Water is polar. Its partially negative oxygen end attracts cations, while its partially positive hydrogen end attracts anions.
• These attractions can overcome the electrostatic forces in an ionic lattice, allowing the ions to separate and move in water.
• Organic solvents cannot overcome the ionic lattice forces.
• Most covalent compounds do not dissolve in water but dissolve in organic solvents because their molecules are neutral.

Melting Point, Boiling Point and Volatility

• Ionic compounds have high melting and boiling points and are not easily volatile because much heat is needed to overcome strong electrostatic attraction forces between oppositely charged ions.
• Simple molecular covalent compounds such as methane, $\mathrm{C}{\mathrm{H}}_4$, have low melting and boiling points and are easily volatile because little heat is needed to overcome weak Van der Waals attraction forces between molecules.
• The covalent bonds inside each simple molecule remain strong; melting or boiling overcomes attractions between molecules.

• Van der Waals attraction between fine hairs on a lizard’s feet and the wall lets it cling to the surface.

Structures of Covalent Compounds

Simple molecular structure	Giant molecular structure
Small structures; may be solids, liquids or gases. Examples: ${\mathrm{H}}_2\mathrm{O}$ and $\mathrm{C}{\mathrm{O}}_2$.	Very large structures, usually solids. Example: $\mathrm{Si}{\mathrm{O}}_2$.
Strong covalent bonds within molecules but weak Van der Waals forces between molecules.	Strong covalent bonds throughout the structure; no intermolecular Van der Waals forces within the giant structure.
Low melting and boiling points.	High melting and boiling points because many strong covalent bonds must be broken.

Uses in Daily Life

Field	Examples
Industry	Ionic lithium iodide, $\mathrm{LiI}$, is used in batteries; covalent pigments and turpentine solvent occur in paint.
Agriculture	Ionic ammonium nitrate, $\mathrm{N}{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_3$, and potassium chloride, $\mathrm{KCl}$, are fertilisers; covalent bromoethane, ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Br}$, and chloropicrin, $\mathrm{CC}{\mathrm{l}}_3\mathrm{N}{\mathrm{O}}_2$, occur in pesticides.
Medicine	Ionic sodium bicarbonate, $\mathrm{NaHC}{\mathrm{O}}_3$, is used in antacids to relieve gastric pain; covalent paracetamol, ${\mathrm{C}}_8{\mathrm{H}}_9\mathrm{N}{\mathrm{O}}_2$, treats fever and irritation.
Domestic use	Ionic sodium chlorate(V), $\mathrm{NaCl}{\mathrm{O}}_3$, occurs in detergents; covalent glycerol, ${\mathrm{C}}_3{\mathrm{H}}_5(\mathrm{OH}{)}_3$, moisturises skin and helps prevent dry skin in skincare products.

Plastic Pollution

• Marine plastic harms aquatic animals through ingestion and absorption of its chemicals into their tissues.