Chemistry 2A/B Topic 3 Covalent Compounds and Separation Techniques

Covalent Bonding is when atoms share one or more pairs of electrons. This gives them both a full outer shell. Each bond provides an extra shared electron for each atom.

When they make covalent bonds they form a molecule.

- is a single bond   ,   = is a double bond

IMPORTANT EXAMPLES:

1) Hydrogen, H₂ - Hydrogen atoms have just one electron in their outer shell so they need one more to complete it. They form a single covalent bond with another hydrogen.    H - H

2) Hydrogen Chloride, HCl - Both hydrogen and chlorine need one more electron to complete their outer shell so they share one pair.    H - Cl

3) Methane, CH₄ - Carbon needs 4 electrons to fill up its outer shell so it shares a pair with 4 hydrogen atoms which all need 1.

      H

H - C - C

      H

4) Oxygen, O₂ - Oxygen needs two more electrons to fill its outer shell so it shares a pair with another oxygen atom.    O - O

5) Water, H₂O - Oxygen need two more electrons so it shares a pair with 2 hydrogen atoms which both need 1 more.    H - O - H

6) Carbon Dioxide, CO₂ - Carbon needs 4 more electrons so it shares 2 pairs with 2 oxygen atoms which both need 2 more.    O = C = O

SIMPLE MOLECULAR

The atoms in these substances make very strong covalent bonds to form small molecules of two or more atoms. However, the forces of attraction between these molecules are very weak. Result - very low melting and boiling points. Most are gases or liquids at room temperature. They don't conduct electricity because there are no ions.

GIANT MOLECULAR

Similar to ionic lattices except there are no charged ions. All atoms are bonded to each other by strong covalent bonds and have high melting and boiling points. They don't conduct electricity (except graphite) and are insoluble in water. Main examples are diamond and graphite, made entirely from carbon atoms.

DIAMOND

...hardest natural substance. Each carbon atom forms four covalent bonds in a very rigid giant covalent structure. Makes diamonds great cutting tools. It doesn't conduct electricity because there are no free electrons.

GRAPHITE

Each carbon atom forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other. This makes graphite useful as a lubricant. The layers are held together so loosely that they can be rubbed off onto paper - which is how pencil works. As only three out of each carbon's four electrons are used in bonds, there are lots of delocalised electrons.  These electrons can move which makes graphite a good conductor of electricity. 

Identifying the bonding in a substance can be done by observing the physical properties. They will be either an ionic lattice, giant molecular or simple molecular.

IONIC LATTICE - Conducts electricity when dissolved but not when solid. Have high melting and boiling points.

GIANT MOLECULAR - Very high melting and boiling points, except for graphite. Don't conduct electricity. Insoluble in water.

SIMPLE MOLECULAR - Very low melting and boiling points. Don't conduct electricity.

Miscible: when liquids mix together

Immiscible: when liquids won't mix together

Use a separating funnel to separate immiscible liquids. When mixed they will separate into two layers, the denser liquid at the bottom. The tap of the separating funnel can be opened to drain the bottom layer.

Use fractional distillation to separate miscible liquids. The different liquids must have different boiling points so that when the mixture is heated the different liquids will condense at different temperatures and can be collected.

Air is filtered to remove dust. It is cooled to around -200 degrees and during this cooling water vapour condenses and is removed. Carbon dioxide freezes and then is removed. The liquefied air enters the fractioning colum and is heated slowly. Remaining gases are separated out. Oxygen and argon come out together so another colum is used to separate them.

Used to identify substances in a mixture. It uses the fact that different substances wash through wet filter paper at different rates.

Put spots of each mixture on a pencil baseline on filter paper. Roll up the paper and put into a beaker containing a solvent. The solvent seeps up the paper, taking the samples with it. The different chemicals in the sample form separate spots on the paper.

You can calculate the R_f for each chemical. An R_f is the ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent.