Tiêu đề 1. The Language of Chyemistry.pdf ✅

➢ VARIABLE VALENCY Some elements have variable valency. For example, iron has valencies of 2 (e.g., in FeCl2) and 3 (e.g., in FeCl3). FeCl2 is named iron(II) chloride, and FeCl3 is named iron(III) chloride. This method of naming a compound is adopted if it contains an element with variable valency. Table: Some elements with variable valencies Metal Nonmetal Copper —1, 2 Phosphorus—3, 5 Iron —2, 3 Sulphur —2, 4, 6 Tin—2, 4 Lead—2, 4 Let us look at a few examples of compounds containing elements of variable valency. Copper (I) chloride 1 1 Cu Cl  CuCl Copper (II) chloride 2 1 Cu Cl  CuCl2 Iron (II) oxide 2 2 Fe O  FeO Iron (III) oxide 3 2 Fe O  Fe2O3 Can you guess the valencies of phosphorus in PCl3, and of sulphur in H2S, SO2 and SO3 ? ➢ WHAT ARE COMPOUND RADICALS Certain groups of atoms of different elements remain intact in many chemical reactions. In fact, they behave like single atoms and have a valency. They are called compound radicals. They do not exist independently, but only as parts of compounds. Common examples of monovalent radicals are hydroxide (OH) and nitrate (NO3). Carbonate (CO3) and sulphate (SO4) are examples of divalent radicals. The phosphate (PO4) radical is trivalent. The formulae of compounds containing compound radicals are also obtained by transposing valencies (Table) Table: Formulae of some compounds containing compound radicals Elements or radicals with valencies Compound formed Formula Name 1 Na 1 OH NaOH Sodiumhydroxide 1 K 1 NO3 KNO3 Potassium nitrate 1 NH4 2 SO4 (NH4)2SO4 Ammonium sulphate 1 Na 2 CO3 Na2CO3 Sodium carbonate 1 H 1 NO3 HNO3 Nitric acid 1 H 2 SO4 H2SO4 Sulphuric acid ➢ CHEMICAL EQUATIONS You know that an element is represented by a symbol and a compound, by a formula. A chemical change is represented by an equation called a chemical equation. For example, the burning of carbon in a sufficient supply of oxygen to form carbon dioxide is represented by the following equation. product 2 reac tan ts C+ O2 → CO  If the supply of oxygen is insufficient, carbon monoxide (CO) is formed. Let us express the reaction thus: C + O2 → CO The substances that react among themselves are called reactants and those that are formed are called products. Remember that no atoms are lost or gained in a chemical reaction. So the number of atoms of each element on the reactant side must be the same as that on the product side.