Advanced Higher Chemistry Unit 1 The oxides, chlorides and hydrides

Metal oxides n Oxygen is highly electronegative and readily reacts with metals to form ionic metal oxides. n Ionic metal oxides tend to be basic i.e. they dissolve in water to form alkalis. n The reaction of sodium oxide (sodium hydroxide, NaOH) with water. The reaction of sodium oxide (sodium hydroxide, NaOH) with water. The reaction of sodium oxide (sodium hydroxide, NaOH) with water. n The reaction of magnesium oxide (MgO) with water. The reaction of magnesium oxide (MgO) with water. The reaction of magnesium oxide (MgO) with water.

Non-metal oxides n Oxygen reacts with non-metals to form covalent or polar covalent molecules. n Soluble covalent oxides dissolve in water to form acids. The exception is carbon monoxide which is neutral. n The reaction of phosphorus(V) oxide (P 4 O 10 ) with water. The reaction of phosphorus(V) oxide (P 4 O 10 ) with water. The reaction of phosphorus(V) oxide (P 4 O 10 ) with water. n The reaction of sulphur dioxide (SO 2 ) with water. The reaction of sulphur dioxide (SO 2 ) with water. The reaction of sulphur dioxide (SO 2 ) with water. n Non-metal oxides tend to be gases and exist as discrete molecules.

Silicon oxide n Oxygen reacts with silicon to form silicon dioxide, a giant covalent network. n The ratio of silicon atoms to oxygen atoms is 1: 2 so the formula SiO 2 is the simplest formula rather than the actual molecular formula. n SiO 2 is not soluble in water but will dissolve in alkalis to form the silicate ion and so it can be said to be an acidic oxide.

Melting Points/Boiling Points n In general, the melting and boiling points of the oxides decrease from very high to very low across a period. n Oxides to the left of the Periodic Table are ionic lattices. n Oxides to the right are discrete covalent molecules, the exception being SiO 2.

Electrical Conductivity n Ionic metal oxides will only conduct when molten or in solution when the ionic lattice is broken down and the ions are free to move. n Covalent non-metal oxides do not conduct.

Reaction of Oxides with Acids/Bases n Some oxides react with acids (basic oxides - Li 2 O, Na 2 O) to produce salts. n Acidic oxides (CO 2, Cl 2 O) react with bases to produce salts. n Amphoteric oxides (BeO, Al 2 O 3 ) can react with acids or bases to produce salts.

Chlorides n Chlorine is also a very electronegative element but is less electronegative than oxygen. n The chlorides have very similar periodic properties to the oxides.

Metal chlorides n The metal chlorides tend to be made by combining the metal directly with chlorine of by reacting the metal with hydrogen chloride. n Generally ionic chlorides dissolve in water, but do not react with water, and can be recovered chemically unchanged. AlCl 3 is an exception to this rule. n Sodium chloride crystals dissolving Sodium chloride crystals dissolving Sodium chloride crystals dissolving n AlCl 3 has covalent molecular bonding and reacts with water to form fumes of hydrogen chloride gas.

Non-metal chlorides n Some covalent chlorides react with water to produce fumes of HCl e.g. PCl 5 (s) + 4H 2 O(l)  H 3 PO 4 (aq) + 5HCl(aq) Note this produces a strongly acidic solution. n Hydrolysis of phosphorus trichloride Hydrolysis of phosphorus trichloride Hydrolysis of phosphorus trichloride n Properties of chlorides of third period elements Properties of chlorides of third period elements Properties of chlorides of third period elements

Hydrides n Hydrogen has an electronegativity value of 2.2. It forms ionic metal hydrides with metals in Groups 1 and 2. n Hydrides contain the H - ion. n Hydrides are colourless solids.

n The hydride ion is a strong base and will remove a hydrogen ion from a water molecule i.e. they react with water to produce hydrogen gas and hydroxide ions e.g. 2NaH(s) + 2H 2 O(l)  H 2 (g) + 2NaOH(aq) e.g. 2NaH(s) + 2H 2 O(l)  H 2 (g) + 2NaOH(aq) n Ionic hydrides can be used as drying agents and as reducing agents e.g. LiAlH 4 and NaBH 4.

n Electrolysis of molten hydrides will produce H 2 gas at the positive electrode i.e.2H -  H 2 + 2e - n Because hydrogen is not very electronegative, most hydrides are covalent with typical covalent properties.