METALS AND THEIR COMPOUNDS

METALS AND THEIR COMPOUNDS Metals are elements whose atoms ionize by electron loss, while non-metals are elements whose atoms ionize by electron gain. Metals are in groups 1, 2 and 3 of the periodic table. Their valence electrons enter the s-and/or partially fill the p-orbitals. Physical properties of metals Metals are: (1) Malleable (2) Ductile (3) Sonorous (4) Hard but not brittle (5) Good conductors of heat and electricity. (6) Lustrous They also have; (7) Relatively high densities (8) High melting and boiling points. Some metals do not exhibit these properties. For instance; Mercury is a liquid with a melting point of 39 0 c. Na and k are light soft metals with low melting points of 97 0 c and 63 0 c respectively. Chemical properties of metals Metals react when the ionize by loss of electron to form cations such as Na +, Ca 2+ and Al 3+ ; hence they are reducing agents. ACTIVITY SERIES: is the arrangement of elements in order of their reactivity. It starts with the most reactive element and ends with the least reactive. The activity series of common metals including hydrogen is as follows: k - Most reactive (strongest reducing agent) Na Ca Mg Al Zn Fe Sn Ph (H) Cu Hg 1

Ag pt Au - least reactive (weakest reducing agent) The activity series arranges the metals in the order of their ability to react and displace the ions of one another from the solution of their salts. Generally, a metal higher up in the series will displace the ions of metal lower in the series from the solutions of their salts. For instance: Zinc metal will displace or replace ions of copper in a solution of a copper(ii) salt i.e. Zn (s) + CuSO 4 (aq) Cu (s) + ZnSO 4(aq) Reactions of Metals and their compounds 1. Reactions of metals with oxygen or air: k, Na, Ca burn vigorously in air to form oxides 4Na (s) + O 2(g) 2Na 2 O (s) (iv) (v) Mg burns in air, when heated to produce the oxide and nitride: (a) 2Mg (s) + O 2(g) 2MgO (s) (b) 3Mg (s) + N 2 (g) Mg 3 N 2 (s) Al, Zn and Fe burn in air when strongly heated and finely divided e.g. 4Al (s) + 3O 2 (g) 2Al 2 O 3 (s) Pb, Cu and Hg form oxides when strongly heated in air, but do not burn in air. 2Hg (s) + O 2 (g) 2HgO (s) Ag, pt and Au do not burn or form oxides when heated in air. 2. Reaction with water: (iv) K and Na react vigorously with cold water to liberate hydrogen gas. 2Na (s) + 2H 2 O (s) 2NaOH (aq) + H 2 (g) Ca reacts moderately with water to liberate hydrogen. Ca + 2H 2 O Ca(OH) 2 + H 2 (g) Mg, Al, Zn, Fe and Sn react with steam to liberate hydrogen. Zn (s) + H 2 O (g) ZnO (s) + H 2 (g) pb, Cu, Hg, Ag, pt and Au do not react with water or steam. This is because they are below hydrogen in the activity series. 3. Reaction of metals with acids. k, Na, and Ca react explosively with dilute acids to liberate hydrogen gas. e.g. Ca (s) + 2HCl (aq) CaCl 2 (aq) + H 2 (q) Mg, Al, Zn, Fe, Sn and Pb displace hydrogen from strong acids with decreasing reactivity. Zn (s) + H 2 SO 4 (aq) ZnSO 4(aq) + H 2 (q) 2

Cu, Hg, Ag, pt and Au do not displace hydrogen from dilute acids, since they are below hydrogen in the reactivity series. 4. Reaction with chlorine: All metals react with chlorine to form the corresponding chloride Mg (s) + Cl 2 (q) MgCl 2 (s) 5. Reaction with aqueous solutions of other salts: A metal higher up in the series will displace another metal lower in the series from the aqueous solution of its salt. e.g. Zn (s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s) METHODS OF EXTRACTION OF METALS The three reduction methods by which metals are extracted from their ores are; Electrolysis for highly reactive metals like Na, k, Ca, Mg and Al. Reduction by coke: for metals in the middle of the activity series e.g. Zn, Fe, Sn and pb. Thermal reduction (Roasting in air) for metals lower down in the activity series e.g. Cu, Hg and pb. Gold (Au) is a unique metal, it is unreactive. It occurs naturally as a free metal. ALKALI METALS The alkali metals are the group I elements in the periodic table: Each element has only one valence electron in the highest occupied orbital. The members include Li, Na, k, Ca and Fr. Cs and Fr are not common. NOTE: Hydrogen is not a group 1 element because it does not follow the inert gasses directly on the periodic table. Group 1 elements are usually stored under paraffin oil, air tight bottles or under any inert liquid like kerosene and petroleum either. This is because they react readily with air and water. SODIUM, Na It occurs naturally as rocksalt, sodium chloride, which is highly soluble in water. It is extracted by electrolytic method. EXTRACTION OF SODIUM IN THE DOWN CELL Fused sodium chloride is mixed with fused calcium chloride at 600 0 c. Fused calcium chloride is added in order for the electrolytic process to be carried out at a much lower temperature of 600 0 c than if pure sodium chloride, which melts at 781 0 c, were used. In the Down cell, the cathode is made of steel while the anode is made of graphite. This is because chlorine gas liberated at the anode will not attack graphite but will attack steel. During the electrolysis, sodium metal is deposited at the cathode while chlorine is liberated at the anode. 3

Molten NaCl Cl 2 gas Molten Na (+) ( ) Molten NaCl Steel gauze cylinder (+) Graphite anode Chemistry of the electrolysis THE DOWN CELL Electrolyte ionizes: NaCl (l) Na + (l) + Cl (l) At the anode (+); chloride ions are oxidized to chlorine. 2Cl (l) Cl 2 (g) + 2e At the cathode ( ): sodium ions are reduced to sodium. 2Na + + 2e 2Na (s) Calcium metal is the major contaminant of molten sodium produced at the cathode, but it is removed from the molten sodium by filtration (M.P of Ca is 940 0 c, while the M.p of Na is 98 0 c). Physical properties of Na 1. It is a silvery solid with metallic lustre. 2. Its density is 0.98 (it floats on water). 3. It is soft and can be cut with knife. 4. Its melting point is 97 0 c (low for a metal). 5. It is a good conductor of electricity and heat. Chemical properties 1. Reaction with air: Na tarnishes easily when exposed to air because it is oxidized to sodium oxide 4 Na (s) + O 2 (g) 2 Na 2 O (s) 4

2. Reaction with water: Na reacts explosively with water to form sodium hydroxide and hydrogen is liberated. 2 Na + 2 H 2 O 2 Na OH (aq) + H 2 (g) 3. Reaction with acids: Na liberated hydrogen from dilute acids. 2 Na (s) + 2HCl (aq) 2NaCl + H 2 (g) 4. Reaction with ammonia: Na reacts with ammonia to for sodamide and hydrogen. 2 Na (s) + 2NH 3 (g) 2 NaNH 2 (s) + H 2 (g) Test for Sodium ions Flame test: Sodium compounds give a golden-yellow colour to a non-luminous flame. USES OF Na 1. Liquid sodium is used as a coolant in nuclear reactors. 2. Sodium vapour lamps are used to lighting highways and air ports. 3. Sodium is used in the extraction of titanium. 4. Na is used in the production of lead(iv) tetraethyl which serves as antiknock agent in petrol. Assignment Explain the solvay process ALUMINIUM mica. The main ore of aluminium is bauxite. Others include: kaolin, cryolite, corundum and Extraction of Al. Two stages are involved which are; purification of bauxite and electrolysis of alumina Purification of bauxite: The ore is purified as follows: Bauxite is first roasted in air to make it anhydrous then, ground into powder. The powdered bauxite is dissolved in hot concentrated NaOH (aq) under pressure to form soluble sodium aluminate (III), NaAl(OH)4 Al 2 O 3 (s) + 2 NaOH (aq) + 3H 2 O (s) 2NaAl (OH) 4 (aq) The mixture is filtered to remove the insoluble trioxosilicate(iv) in purities. The filtrate which contains Na Al(OH) 4 (aq) is then seeded with Aluminium hydroxide crystals to induce the precipitation of aluminium hydroxide. Na Al(OH) 4 (aq) Al(OH) 3 (s) + NaOH (aq) 5

The aluminium hydroxide precipitate is then filtered off, washed, dried and heated strongly to yield pure aluminium oxide or alumina, Al 2 O 3. 2 Al (OH) 3 (s) Al 2 O 3 (s) + 3 H 2 O (s) 2. Electrolysis of alumina: pure aluminium metal is produced by electrolysis of alumina dissolved in molten cryolite at about 950 0 c. The cryolite serves as the solvent for the alumina and helps to reduce the melting point from 2050 0 c to 950 0 c. The extraction is carried out in an electrolytic cell composed of a rectangular steel tank lined with graphite (carbon) as the cathode, and graphite rods, which serve as the anode dipping into the solution of alumina in molten cryolite as the electrolyte. Aluminium in Cryolite Molten Aluminium Graphite Cathode Steel Case Outlet Chemistry of the process Electrolyte Ionizes thus: Al 2 O 3(s) 2Al 3+ (l) + 3O 2 (s) At anode (+); oxide ion is discharge to liberate oxygen gas. 60 2 (l) 30 2 (g) + 12 e At the cathode ( - ); Aluminium ion is discharged to deposit aluminium. 4 Al 3+ + 12 e 4 Al (s) The molten aluminium is tapped at the bottom of the cell. The oxygen liberated at the anode burn away the carbon anodes to produce CO. Hence, there is the need to replace the graphite anodes regularly during the electrolytic process. 6