Analysing properties of ionic and covalent compounds
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Malaysia SPM Form 4 Chemistry, Chapter 4: Chemical Bond.
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[edit] Properties of Ionic Compounds
[edit] The Structure of Ionic Compounds--Crystal Lattices
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- The alternate positive and negative ions in an ionic solid are arranged in an orderly way in a giant ionic lattice structure shown on the left.
- The ionic bond is the strong electrical attraction between the positive and negative ions next to each other in the lattice.
- The bonding extends throughout the crystal in all directions.
- Salts and metal oxides are typical ionic compounds.
- Some of these compounds, like magnesia (MgO) and alumina (Al2O3), are so stable that they are used as refractory material, to line the inside of furnaces. Such substances must be stable up to at least 1500 °C.
- Another property of crystal lattices is that they are non-conductors of electricity. This is because the ions are in fixed positions and are unable to move.
[edit] Properties of Ionic Compounds
- This strong bonding force makes the structure hard (if brittle) and has high melting and boiling points, so they are not very volatile!
- The bigger the charges on the ions the stronger the bonding attraction eg magnesium oxide Mg2+O2- has a higher melting point than sodium chloride Na+Cl-.
- Unlike covalent molecules, ALL ionic compounds are crystalline solids at room temperature.
- They are hard but brittle, when stressed the bonds are broken along planes of ions which shear away. They are NOT malleable like metal.
- Many ionic compounds are soluble in water but not all, so don't make assumptions.
- The solid crystals DO NOT conduct electricity because the ions are not free to move to carry an electric current.
- However, if the ionic compound is melted or dissolved in water, the liquid will now conduct electricity, as the ion particles are now free.
[edit] Properties of Covalent Compounds
- Covalent compounds can be divided into those which form small (simple) independent molecules and those which form giant molecular lattices.
[edit] Simple Molecule
[edit] Structure
- These are made up of independent molecular units, as shown in Figure 6.7.
- As there are no ions formed, the attractive forces between molecules in solid, covalent compounds like iodine and sulphur are much weaker.
- They are called van der Waals' forces and produce a weak, molecular lattice with low melting points.
- n covalent liquids like water, the molecules are even further apart, so the van der Waals' forces are weaker still, and in covalent gases like ammonia and methane, these forces are almost non-existent.
- However, in water, there are other attractive forces between molecules. These forces are called hydrogen bonds and they give water much higher melting and boiling points than expected with such weak van der Waals' forces.
[edit] Properties Of Simple Covalent Molecular Substances - Small Molecules!
- The electrical forces of attraction, that is the chemical bond*, between atoms in any molecule are strong and most molecules do not change chemically on moderate heating.(* sometimes referred to as the intramolecular bond)
- However, the electrical forces** between molecules are weak and easily weakened further on heating.
- These weak attractions are known as **intermolecular forces and consequently the bulk material is not usually very strong.
- Consequently small covalent molecules tend to be volatile liquids, easily vapourised, or low melting point solids.
- On heating the inter-molecular forces are easily overcome with the increased kinetic energy gain of the particles and so have low melting and boiling points.
- They are also poor conductors of electricity because there are no free electrons or ions in any state to carry electric charge.
- Most small molecules will dissolve in a solvent to form a solution.
[edit] Macromolecular compounds
- These have giant, covalent molecules with extremely large molecular lattices.
- They are very stable, as all the atoms are joined together by strong covalent bonds to give a giant three-dimensional lattice.
- Often the lattice is tetrahedral in shape, as every atom is covalently linked to four others.
- Examples of such macromolecules are diamond and sand (see Figure 6.8).
[edit] Diamond and Silica(Sand)
- A diamond crystal or a grain of sand is just one giant molecule. Such molecules, because they are so rigid and strong, have very high melting points.
Large Covalent Molecules And Their Properties
- This type of structure is thermally very stable and they have high melting and boiling points.
- They are usually poor conductors of electricity because the electrons are not usually free to move as they can in metallic structures.
- Also because of the strength of the bonding in all directions in the structure, they are often very hard, strong and will not dissolve in solvents like water.
- Silicon dioxide (silica, SiO2) has a similar 3D structure and properties, shown below diamond.
- The hardness of diamond enables it to be used as the 'leading edge' on cutting tools.
[edit] Graphite
- Diamond is an allotrope of carbon. Allotropes are different forms of the same element in the same physical state
- Oxygen O2 (dioxygen) and ozone O3 (trioxygen) are two gaseous allotropes of the element oxygen.
- Carbon also occurs in the form of graphite. The carbon atoms form joined hexagonal rings forming layers 1 atom thick.
- There are three strong covalent bonds per carbon (3 C-C bonds in a planar arrangement from 3 of its 4 outer electrons), BUT, the fourth outer electron is 'delocalised' or shared between the carbon atoms to form the equivalent of a 4th bond per carbon atom.
- The layers are only held together by weak intermolecular forces shown by the dotted lines NOT by strong covalent bonds.
- Like diamond and silica (above) the large molecules of the layer ensure graphite has typically very high melting point because of the strong 2D bonding network (note: NOT 3D network)..
- Graphite will not dissolve in solvents because of the strong bonding but there are two crucial differences compared to diamond ...
- Electrons, from the 'shared bond', can move freely through each layer, so graphite is a conductor like a metal (diamond is an electrical insulator and a poor heat conductor). Graphite is used in electrical contacts eg electrodes in electrolysis.
- The weak forces enable the layers to slip over each other so where as diamond is hard material graphite is a 'soft' crystal, it feels slippery. Graphite is used as a lubricant.
- These two different characteristics described above are put to a common use with the electrical contacts in electric motors and dynamos. These contacts (called brushes) are made of graphite sprung onto the spinning brass contacts of the armature. The graphite brushes provide good electrical contact and are self-lubricating as the carbon layers slide over each other.
