C2 Topic 2: Ionic compounds and analysis

Ionic bonds
Ion = atom or group of atoms with a positive or negative charge.
Ionic bonds form between positively and negatively charged ions
Ions form when electrons are transferred.
Ions form when atoms either lose or gain electrons.
Cation
Metal atoms lose electrons to become positively charged ions called cations

Anion
Non-metal atoms gain electrons to become negatively charged ions called anions.

Group / Lose or gain electrons / Charge on group
1 / Lose 1 / +1
2 / Lose 2 / +2
6 / Gain 2 / -2
7 / Gain 1 / -1
Ionic bond = strong electrostatic force of attraction between oppositely charged ions
Ionic compounds
Ionic compounds from between metals and non-metals.
  • Electrons lost by the metal are transferred to the non-metal
  • both positive and negative ions that form end up with stable, full outer shells
  • Oppositely charged ions attract each other stronglyforming an ionic compound held together by ionic bonds
/ Working out ionic formula:
Ionic compounds overall have no charge because they contain equal numbers of positive and negative charges.
E.g for sodium chloride, one Na+ is needed for every Cl- ion
  • formula is NaCl
E.g the ionic compound aluminium oxide:
  • Aluminium oxide contains Al3+ cations and O2- anions
  • For charges to be equal you must have:
  • Two Al3+ cations ( + 6 charge)
  • Three O2- anions ( - 6 charge)
  • formula of aluminium oxide is Al2O3
Names of ionic compounds:
  • Ionic compounds usually end in –ide
  • However, compounds that contain oxygen atoms end in –ate
Structure of ionic compounds
Ionic compounds have a regular arrangement of oppositely charged ions. This is an IONIC LATTICE. There is a strong electrostatic attraction between each of these ions.

Properties of ionic compounds
Conducting electricity:
  • Don’t conduct when solid – no free ions to move and carry charge
  • Do conduct when molten/aqueous – ions are now free to move and carry charge
Melting and boiling points
  • Very high as the strong ionic bonds require a lot of heat energy to break.
Solubility
  • There are rules for the solubility of salts

Precipitates
A precipitate is an insoluble salt formed from the reaction of two soluble salts. It is called a precipitation lesson.
Forming a pure, dry sample of a salt
  1. Mix 2 soluble salts
  2. Precipitate will form
  3. Filter so precipitate stays on filter paper
  4. Wash with water to make salt pure
  5. Dry the salt in an oven or in sun
Barium meals
Used when diagnosing intestinal problems.
Patients drink ‘barium meal’ which contains barium sulphate.
  • Barium sulphate is insoluble
  • Patients swallow the barium meal and it passes through the digestive system
  • Barium absorbs x-rays and so shows up white on any photos taken
  • Barium salts are toxic, however as barium sulphate is insoluble it cannot enter patient’s blood.
Ion tests
Cation tests – Flame tests
Method
  1. Clean flame test loop by dipping in HCl and holding over Bunsen flame until no colour is given off.
  2. Once clean, dip flame test loop into test salt i.e. copper chloride
  3. Hold the loop over the Bunsen burner and observe any colour seen.
  4. The colour will show you which cation is present.

Cation / Colour
Sodium (Na+) / Yellow
Potassium (K+) / Lilac
Calcium (Ca2+) / Red
Copper (Cu2+) / Green/blue
/ Anion tests – precipitation
Anion / Test
Chloride (Cl-) /
  1. Add dilute nitric acid
  2. Add silver nitrate
  3. White precipitate will form

Sulphate (SO42-) /
  1. Add dilute HCl
  2. Add barium chloride
  3. White precipitate will form

Carbonate (CO32-) /
  1. Add dilute acid
  2. If carbonate ions present carbon dioxide will be given off
  3. Test CO2 with limewater