Chemistry 2A/B Topic 2 Ionic Compounds and Analysis

Ionic bonding is the transfer of electrons. Atoms lose or gain electrons to form positively or negatively charged atoms called ions.

Shell with just one electron wants to get rid of it. Atoms on the left side of the periodic table have one or two electrons in their outer shell. They will react quickly with an ion with an opposite charge to get rid of it/them....Nearly full shell wants to get the extra electron. They react with the atoms on the left side of the table to become an ion with a full outer shell.

Elements most likely to form ions are in groups 1,2,6 and 7 because they can lose or gain one or two electrons easily to become ions.

Cations are atoms in groups 1 or 2 and end up with a positive charge because they have given away a negative electron and therefore have more positive protons.

Anions are atoms in groups 6 or 7 and end up with a negative charge because they have gained a negative electron and therefore have more negative electrons.

Ionic Compounds have a regular lattice structure. Ionic bonds produce compounds with similar structures. The ions form a closely packed arrangement because the ionic bonds are very strong electrostatic forces of attraction between oppositively charged ions.

They have similar properties:

1) High melting and boiling points because of the very strong bonds which it takes a lot of energy to break them.

2) Can conduct electricity when aqueous or molten because when dissolved or melted the ions separate and are all free to move so they'll carry electric current. When solid the ions are held in solids and so they aren't free to move and conduct electricity.

2 simple rules:

1) when 2 different elements combine the compounds name is 'something-IDE'

2) when 3 or more different elements combine and one of them is oxygen the compounds name is often 'something-ATE'

FORMULAS

The charges of the ions need to balance. How many of one ion do you need to give the other a full shell.

Eg. Oxygen has 6 electrons in its outer shell (needs to gain 2) and Lithium has 1 electron in its outer shell (needs to lose 1). With 2 lithiums oxygen can have a full outer shell and so can both of the lithiums.

Li₂O Lithium Oxide (metal always comes first)

Common salts of sodium, potassium and ammonium are soluble

Nitrates are all soluble

Common chlorides are soluble except Silver and Lead chloride

Common sulfates are soluble except Lead, Barium and Calcium sulfate

Common carbonates and hydroxides are insoluble except sodium, potassium and ammonium ones

To make a pure, dry sample of an insoluble salt you can use a precipitation reaction. You need to pick the right two soluble salts and when they react you get the insoluble salt.

Eg. Lead nitrate (soluble) + Sodium chloride (soluble) --> Lead chloride (insoluble) + Sodium nitrate (soluble)

Method:

Dissolve both soluble salts in distilled water and mix together

Pour the solution through filter paper into a beaker

Wash it with distilled water.

Insoluble salt is left on the filter paper

Barium Sulfate is an insoluble salt. It can be used for x-rays. Barium sulfate is opaque to x-rays so when drunk it can show up the gut and any problems can be seen.

Barium salts are toxic however it can be safely drunk because it's insoluble so it cant be absorbed into the bloodstream - it just passes through the body. This is called a barium meal.

FLAME TESTS

Sodium ions give a yellow/orange flame , Potassium ions give a lilac flame , Calcium ions give a brick-red flame ,

Copper ions give a blue-green flame

Flame tests work when you've got a sample of a pure element or when it's in a compound

Testing for carbonates - check for CO₂

To test to see if a gas is carbon dioxide you need to bubble it through limewater. If it is carbon dioxide the limewater will go milky

TEST FOR SULFATES AND CHLORIDES

To test for sulfate ions, add dilute hydrochloric acid, followed by barium chloride solution. A white precipitate of barium sulfate means the original compound was a sulfate.

To test for chloride ions, add dilute nitric acid, followed by silver nitrate solution. A chloride gives a white precipitate of silver chloride.

Spectroscopy can be used to identify elements by analysing the patterns of light emitted by the elements in a heated sample. This is fast and reliable and can be used to detect even very small amounts of elements.