Logic of Inorganic Reactions 1 SECTION - I VALENCY/OXIDATION NUMBER/BALANCING EQUATIONS Modern Definition of Valency Valency is the power of an atom of an element to combine with other atoms measured by the number of electrons which an atom or radical will lose, gain or share to form a chemical compound. Types of Valences: (i)Electrovalency( shown in ionic compounds eg. NaCl, K2SO4 etc) (ii)Covalency (shown in covalent compounds eg SO2, NH3, H2SO4 etc) Electrovalency Radicals: Radical is an atom or a group of atoms which form an ion (positive or negative) by loss or gain of electrons. It is part of an ionic compound. Two radicals form a compound. Types of Radicals: (i)Basic Radical: This forms the +ve ion or cation part of the compound. Usually + + 2+ 3+ + 2+ 3+ + + metals e.g Na , K , Ca , Al , Ag , Hg , Fe etc. (exception: NH4 , H ). This part comes from a base. (ii)Acid Radical: This forms the –ve ion or anion part of the compound. Usually it is made up of nonmetals. Some acid radicals contain both metal and nonmetal atoms. (i)monoatomic acid radical: Cl–, Br–, S2-, O2-, N3-, P3-, H– etc 2- 3- 2- 2- – – – (ii)Compound acid radical : SO4 , PO4 , CO3 , SO3 , OH , NO2 , NO3 – 2- – etc. (exceptions: MnO4 , CrO4 , AlO2 etc which contain both metal and non-metal) Charge of Basic Radical = + valency of the radical, Charge of Acid Radical = - velency of the radical Task: Write the ionic representations of the following radicals: Al, Fe(ous), Cu(ic), Sn(ous), NH4, Br, O, SO4, NO2, MnO4, Cr2O7, C, N, PO4 Definition of Electrovalency: Electrovalency is the number of electrons lost or gained (not shared)by a radical. The valency tables(table 1 and 2) given later are electrovalencies. IMPORTANT: In fact all the radicals listed in table 1 under basic radicals are positive ions which are formed by the loss of electron(s). Likewise all the radicals listed in table 2 under acid radicals are negative ions which are formed by the gain of electrons. The basic and acid radicals are never to be considered as neutral species. Na is a metal which is neutral. But Na as a radical is present in a compound like NaCl. Here Na is not neutral although we conventionally write as Na. Actually it is Na+. Similarly iron(Fe) is a metal which is neutral. But Fe as a radical is present in compounds in two forms i.e ferrous(eg. ferrous sulphate) in which it remains as Fe++ or Fe2+ ions and the other ferric( e.g ferric sulphate) in which +++ 3+ it remains as Fe or Fe ions. Similarly chlorine as a gas is neutral and has formula Cl2. But Cl as a radical(chloride) present in any compound like NaCl is actually not Cl, instead it is Cl– (a -ve ion). Conclusion: All basic radicals form +ve ions and all acid radical form -ve ions. The magnitude of charge is equal to the valency of the radical. Dr. S. S.Tripathy 2 Concepts in Chemistry Covalency : The number of electrons which an atom shares in forming a covalent bond with other atoms in a molecule or ion is called the covalency of the atom. There is nothing called covalency of a group of atoms(radical) like that we learnt in electrovalency(sulfate, carbonate etc.). Covalency is determined for an atom in a covalent molecule or ion. This type of valencies are found in binary covalent compounds like PCl5, NH3, CO2 etc. and covalent ions 2- - like SO4 , NO3 etc. The details on covalency will be discussed later. In compound like aluminium sulphate there are two types of valencies- electrovalency between 3+ 2- 2- Al and SO4 and covalency within S and O atoms in forming SO4 . Similarly in compound like Na2CO3 the valency of Na and CO3 are 1+ and 2- respectively while in CO32- the valency of C is 4 as it is sharing four of its elelctrons with the three oxygen atoms. Note that each covalent bond is made up of two electrons contributed one each by the the two linking atoms. 2- O Na CO + 2 3 Na + CO3 ; C (covalency of C=4) electrovalency O O Note that both electrovalency and covalency are expressed by a single term called Oxidation Number(ON) or Oxidation State(OS). The only difference between valency and ON is that there is a sign in ON(+ve and -ve) while there no sign in valency. These things will be made more clear in later sections. CHEMICAL FORMULAE Molecular Formula: A molecular formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. It identifies each type of element by its chemical symbol and identifies the number of atoms of such element to be found in each individual molecule of the compound. The number of atoms(if greater than one) is indicated as a subscript. Molecular formula is valid for covalent molecular substances like CO2, NH3, H2SO4, N2O5 etc. which exist in the form of discrete molecules. Empirical Formula: Empirical formula of a compound is a simple expression of the relative number of each type of atom or ratio of the atoms of different elements present in it. Empirical formula is valid for non-molecular substances which remain as network solids. All ionic compounds like NaCl, CaSO4, KNO3 etc. and a few covalent network solids like SiO2 fall into this category of non-molecular substances. In any ionic compound, say for example NaCl, there is no individual NaCl molecule. Na+ and Cl- ions are arranged alternately in a repeated manner in three dimension to form a gigantic network. In such cases molecular formula cannot be known rather their empirical formula which gives a simple ratio of the elements present in the compound can be known. Dr. S. S.Tripathy Logic of Inorganic Reactions 3 Crossover Rule: While writing chemical formula of an ionic compound, valency of basic and acid radicals are crisscrossed. In other words the valency of basic radical becomes the subscript of acid radical and vice versa. The following examples will suffice. 32 Al (SO ) Al (SO4) 2 4 3 2 2 Zn (CO3) ZnCO3 Note that parenthesis is used for a compound radical which contains more than one element such as CO3, SO4 etc. only when there is any subscript associated with it. In the first example, parenthesis is used for SO4 but not in the second example for CO3. For monoatomic radicals containing one element such as Al, Cl, O etc parenthesis is not used even if there is a coefficient [e.g. AlCl3 and not Al(Cl)3] Question arises why does this crisscrossing of valencies are done? This is done to equalise the total positive and negative charge as the molecule is neutral i.e net charge of the molecule should be zero. 2- SO4 3+ Al 2- SO4 3+ Al 3 2 2- SO4 Al (SO4) Al2(SO4)3 The hollows(depressions) in the above picture of the basic radical is the site from which electrons have been lost and the mount(bulging out) portions in the acid radical are the sites at which electron have been accepted. There must be a complete matching between the hollows with the mounts. In aluminium sulphate, there are two Al3+ ions, so the total +ve charge is 2 X (+3) = +6; and 2- that is why there are three SO4 ions, so that the total -ve charge is 3 X (-2) = -6. In zinc 2+ 2- carbonate, one Zn ion has +2 charge and one CO3 ion has -2 charge and thus the charge balance occurs. While writing formula for such compounds in which the valencies of acid and basic radicals are same or are simple multiples of each other, then the valencies are simplified. In zinc carbonate the valency 2 for each radical get cancelled while writing the formula. In cupric ferrocyanide, the valencies get simplified by dividing with a factor 2. Hence to conclude, empirical formula gives the simplest whole number ratio of basic and acid radicals. 4 2 2 2 4 Cu [Fe(CN)6] Cu [Fe(CN) ] Cu [Fe(CN)6] 2 6 IMPORTANT: In ionic compounds the formula we talk of are empirical formula not molecular formula. Dr. S. S.Tripathy 4 Concepts in Chemistry VALENCY TABLES AND FORMULA WRITING TIPS The formula writing is the most vital part in the study of chemical sciences. Unless and until you write formula correctly it is not worthwhile to proceed further. So practise adequately to master over it. Students often neglect this part of chemistry and become permanently weak in the subject. You should not try to cram the valencies of different basic and acid parts(radicals) given in the tables below rather just read them and practise writing formulae of compounds for a large number of times. Check them yourself and know your mistakes. Within a few days of regular practice and self check, these will automatically be stored in your memory. No special effort will be necessary for the purpose. BASIC RADICALS (TABLE-I) Valency=1 Valency=2 Valency=3 Valency=4 Valency=5 sodium(Na) magnesium aluminium(Al) stannic arsenic(As) potassium(K) (Mg) chromic (Sn) antimonic mercurous(Hg)* barium(Ba) (Cr) plumbic (Sb) cuprous(Cu) zinc(Zn) antimonous (Pb) vanadium(V) ammonium(NH4) cupric(Cu) (Sb) platinum aurous(Au) mercuric(Hg) auric(Au) (Pt) silver(Ag) cobaltous arsenous(As) titanium hydrogen(H) (Co) cobaltic(Co) (Ti) lithium(Li) nickel(Ni) bismuth(Bi) rubidium(Rb) lead(Plumbous) ferric(Fe) cesium(Cs) (Pb) thallium(Tl)(ous) ferrous(Fe) manganic antimonyl(SbO) srontium(Sr) (Mn) bismuthyl(BiO) stannous(Sn) scandium(Sc) chromous(Cr) thallium(Tl)(ic) manganous (Mn) berryllium(Be) cadmium(Cd) calcium(Ca) palladium(Pd) platinum(Pt) 2+ Dr.