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Advanced Higher Chemistry. Unit 1 The oxides, chlorides and hydrides. Metal oxides. Oxygen is highly electronegative and readily reacts with metals to form ionic metal oxides. Ionic metal oxides tend to be basic i.e. they dissolve in water to form alkalis.
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Advanced Higher Chemistry Unit 1 The oxides, chlorides and hydrides
Metal oxides • Oxygen is highly electronegative and readily reacts with metals to form ionic metal oxides. • Ionic metal oxides tend to be basic i.e. they dissolve in water to form alkalis. • The reaction of sodium oxide (sodium hydroxide, NaOH) with water. • The reaction of magnesium oxide (MgO) with water.
Non-metal oxides • Oxygen reacts with non-metals to form covalent or polar covalent molecules. • Soluble covalent oxides dissolve in water to form acids. The exception is carbon monoxide which is neutral. • The reaction of phosphorus(V) oxide (P4O10) with water. • The reaction of sulphur dioxide (SO2) with water. • Non-metal oxides tend to be gases and exist as discrete molecules.
Silicon oxide • Oxygen reacts with silicon to form silicon dioxide, a giant covalent network. • The ratio of silicon atoms to oxygen atoms is 1: 2 so the formula SiO2 is the simplest formula rather than the actual molecular formula. • SiO2 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 • In general, the melting and boiling points of the oxides decrease from very high to very low across a period. • Oxides to the left of the Periodic Table are ionic lattices. • Oxides to the right are discrete covalent molecules, the exception being SiO2.
Electrical Conductivity • 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. • Covalent non-metal oxides do not conduct.
Reaction of Oxides with Acids/Bases • Some oxides react with acids (basic oxides - Li2O, Na2O) to produce salts. • Acidic oxides (CO2, Cl2O) react with bases to produce salts. • Amphoteric oxides (BeO, Al2O3) can react with acids or bases to produce salts.
Chlorides • Chlorine is also a very electronegative element but is less electronegative than oxygen. • The chlorides have very similar periodic properties to the oxides.
Metal chlorides • The metal chlorides tend to be made by combining the metal directly with chlorine of by reacting the metal with hydrogen chloride. • Generally ionic chlorides dissolve in water, but do not react with water, and can be recovered chemically unchanged. AlCl3 is an exception to this rule. • Sodium chloride crystals dissolving • AlCl3 has covalent molecular bonding and reacts with water to form fumes of hydrogen chloride gas.
Non-metal chlorides • Some covalent chlorides react with water to produce fumes of HCl e.g. PCl5(s) + 4H2O(l) H3PO4(aq) + 5HCl(aq) Note this produces a strongly acidic solution. • Hydrolysis of phosphorus trichloride • Properties of chlorides of third period elements
Hydrides • Hydrogen has an electronegativity value of 2.2. It forms ionic metal hydrides with metals in Groups 1 and 2. • Hydrides contain the H- ion. • Hydrides are colourless solids.
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) + 2H2O(l) H2(g) + 2NaOH(aq) • Ionic hydrides can be used as drying agents and as reducing agents e.g. LiAlH4 and NaBH4.
Electrolysis of molten hydrides will produce H2 gas at the positive electrode i.e. 2H- H2 + 2e- • Because hydrogen is not very electronegative, most hydrides are covalent with typical covalent properties.
