-> CA + B (Element A has replaced B in the compound CB to form a new compound CA and the free element B.) Some examples are: Cl2 + 2NaBr -> 2NaCl + Br2(v) Br2 + 2KI -> 2KBr + I2(v) Cl2 + H2S -> 2HCl + S(v) Again, the less reactive halogen cannot replace the more reactive halogen: I2 + 2KBr -> no reaction Common reactions Metal-acid reaction Metals react with acids to form salts and hydrogen gas. Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2 ^ However less reactive metals can not displace the hydrogen from acids. (They may react with oxidizing acids though.) Cu + HCl -> No reaction Reaction between metal and water Metals react with water to form metal oxides and hydrogen gas. The metal oxides further dissolve in water to form alkalies. Fe(s) + H2O (g) -> FeO(s) + H2 ^ Ca(s) + 2H2O (l) -> Ca(OH)2(aq) + H2 ^ The reaction can be extremely violent with alkali metals as the hydrogen gas catches fire. Metals like gold and silver, which are below hydrogen in the reactivity series, do not react with water. Metal extraction Coke or more reactive metals are used to reduce metals by carbon from their metal oxides, such as in the carbothermic reaction of zinc oxide (zincite) to produce zinc metal: ZnO + C -> Zn + CO and the use of aluminium to produce manganese from manganese dioxide: 3MnO2 + 4Al -> 3Mn + 2Al2O3 Even for reactions that run in the direction opposite of their intrinsic reactivities, displacement can be driven to occur, as in the Acheson process for displacing silicon from silicon dioxide using carbon: SiO2 + 2C -> Si + 2CO Thermite reaction Using highly reactive metals as reducing agents leads to exothermic reactions that melt the metal produced. This is used for welding railway tracks. Fe2O3(s) + 2 Al(s) -> 2 Fe(l) + Al2O3(s) (Haematite) 3CuO + 2Al -> 3Cu + Al2O3 Silver tarnish Silver tarnishes due to the presence of hydrogen sulfide, leading to formation of silver sulfide. 4Ag + 2H2S + O2 -> 2Ag2S + 2H2O 3Ag2S + 2Al -> 6Ag + Al2S3 Extraction of halogens Chlorine is manufactured industrially by the Deacon's process. The reaction takes place at about 400 to 450 °C in the presence of a variety of catalysts such as CuCl2. 4HCl + O2 -> 2 Cl2 + 2H2O Bromine