DPS BOKARO, SEC-IV METALLURGY -2 STUDY MATERIAL FOR CLASS-12 CHEMISTRY DEPARTMENT 09/04/2020 Reduction of oxide to the metal Reduction of the metal oxide usually involves heating it with some Other substance acting as a reducing agent (C or CO or even another metal). The method used to extract a metal from its ore depends upon the stability of its compound in the ore, which in turn depends upon the reactivity of the metal: • The oxides of very reactive metals, such as aluminium, form stable oxides and other compounds. A lot of energy is needed to reduce them to extract the metal. • The oxides of less reactive metals, such as iron, form less stable oxides and other compounds. Relatively little energy is needed to reduce them to extract the metal. So, the method of extraction of a metal from its ore depends on the metal's position in the reactivity series. Reactivity and extraction method The table displays some metals in decreasing order of reactivity and the methods used to extract them. Metal Method Potassium Electrolysis Sodium Electrolysis Calcium Electrolysis Magnesium Electrolysis Aluminium Electrolysis (Carbon) (Non-metal) Metal Method Zinc Reduction by carbon or carbon monoxide Iron Reduction by carbon or carbon monoxide Tin Reduction by carbon or carbon monoxide Lead Reduction by carbon or carbon monoxide (Hydrogen) (Non-metal) Copper Various chemical reactions Silver Various chemical reactions Gold Various chemical reactions Platinum Various chemical reactions We can see from the table that reactive metals, such as aluminium, are extracted by electrolysis, while a less reactive metal, such as iron, may be extracted by reduction with carbon. Because gold is so unreactive, it is found as the native metal and not as a compound. It does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal. Some metals can be extracted by heat alone such as mercury and silver, from their corresponding oxides. Most metals are too reactive to be extracted by heat alone. Metal Oxide HEAT → Metal + Oxygen HEAT 2HgO → 2Hg + O 2 Some metals can be extracted by heating with carbon such as zinc, tin, lead, copper and iron from their corresponding oxides. Metals above zinc in the reactivity series are too reactive to be extracted by heating with carbon or carbon monoxide gas. Very reactive metals are strongly bonded in their ores and cannot be extracted using carbon/carbon monoxide. More energy is needed and this is provided by electrolysis. Electrolysis is a process which uses electricity to break down a substance. Metals above zinc in the reactivity series are extracted using electrolysis such as aluminum. Aluminum is extracted from its molten ore, bauxite (Al 2O3). Titanium is produced by reducing titanium (IV) chloride using a more reactive metal such as sodium or magnesium. This is the only way of producing high purity metal. TiCl 4 + 4Na Ti + 4NaCl The more reactive metal sodium releases electron easily. 4Na 4Na + + 4e - These electrons are used to reduce titanium chloride. - - TiCl 4 + 4e Ti + 4Cl Thermodynamic aspect of metallurgy ∆Hr& ∆Sr cannot decide the feasibility of a reaction separately at constant Temperature (T) & Pressure (P) ∆Gr decides the spontaneity of a reaction. ∆G = ∆H - T∆S ---------------- (i) ∆Gr< 0 or negative for a spontaneous or feasible process. For ∆S +ve at high temperature, T∆S value increases, So, -T∆S in eqn. (1) becomes more –ve, ∆G value becomes more – ve, the reaction becomes spontaneous & vice versa If equilibrium constant value, K is large for a reaction, Reactants ⇋ Products, & T increases 0 ∆G r = - RT ln K = -2.303RT log K 0 ∆G rvalues become more –ve with increase in temperature & reaction becomes spontaneous. For a coupled reaction: A → B, ∆G1 > 0 or +ve means non spontaneous reaction and for C → D , ∆G2 < 0 or -ve means spontaneous reaction. Reactions (1) & (2) are coupled i.e. A + C → B + D If ∆G1 + ∆G2 < 0 or –ve, both the reaction becomes spontaneous. Example: (1) 2FeO → 2Fe + O2, ∆G1 > 0 ≈ Non-spontaneous. (2) C + O2 →CO 2, ∆G2 < 0 ≈ Highly Spontaneous (1) + (2): 2FeO + C + O2→ 2Fe + CO 2 + O 2 Overall reaction: 2FeO + C → Fe + CO 2 Here if ∆G1 + ∆G2 < 0 or –ve, so the reaction is spontaneous. This is the basis of metallurgy. Ellingham Diagrams: 0 In Ellingham diagram the ∆G values are taken for per mole of O 2. These diagrams were first constructed by Harold Ellingham in 1944. Ellingham diagram help us in predicting the feasibility of a thermal reduction of an ore. That is, an element will reduce the oxide of other metals which lie above it in Ellingham diagram. =========================================================== .