Title 03-Chemical Reactions.pdf ✅

CHEMICAL REACTIONS 3 CHAPTER CONTENTS ➢ INTRODUCTION ➢ CHEMICAL EQUATIONS ➢ COVALENT BOND ➢ BONDING IN METALS ➢ INTRODUCTION In the previous class, you have read about physical an chemical changes. Chemical changes result from chemical reaction taking placed between substances. In this chapter we shall deal with the chemical reactions and their representation in the form of chemical equations. The processes in which a substance or substances undergo change to produce new substances with new properties are known as chemical reactions. for example, when calcium carbonate is heated, calcium oxide (lime) and carbon dioxide are formed. The breaking up of calcium carbonate into calcium oxide and carbon dioxide is, thus, a chemical reaction because calcium carbonate changes into new substances, calcium oxide and carbon dioxide.  Reactant : The substance which takes part in a chemical reaction is called reactant. For example, in the breaking up of calcium carbonate into calcium oxide and carbon dioxide, calcium carbonate is the reactant. Similarly, sodium and water are the reactants when they react.  Product : A product is a new substance formed in a chemical reaction. For example, hydrogen and sodium hydroxide are the products of the reaction between sodium and water. hydrogen 2 water sodiumhy droxide 2 sodium Na + H O → NaOH + H Similarly, in the breaking up of calcium carbonate, calcium oxide and carbon dioxide are the products. carbon dioxide 2 calciumoxide calciumcarbonate CaCO3 → CaO + CO You know, atoms in a molecule are held together by a force of attraction called bond. The molecules do not participate directly in a chemical reaction. First they break down into atoms and these atoms then take part in the reaction. New bonds are formed between the atoms to form the products. That is, there take place rearrangement or regroupings of atoms in various ways to give products. For example, when ammonium cyanate is heated, different bonds in ammonium cyanate molecules are broken and new bonds are formed to produce urea. ammonium cyanate H4N — N == C == O → urea H2N — C — NH2 || O Here, we see that the molecular formulae of both ammonium cyanate and urea are the same, but their properties are quite different and they are two different compounds. Such compounds are known as isomers of each other and the reactions that produce such isomers are called isomerization reactions.  Valency :
The number of electrons shared by an atom is called its valency. It is also called the combining capacity of an atom, e.g., Cl atom can share one valence electron, its valency is 1, Oxygen can share two valence electrons, its valency is 2. Nitrogen can share 3 valence electrons, its valency is 3, Carbon can share 4 valency electrons, therefore its valency is 4 and so on. It means if carbon combines with Chlorine, Carbon will share four valence electrons with four chlorine atoms, therefore the molecular formula of the covalent compound will be Element C Cl Valency 4 1 i.e., CCl4 is formula Some more examples are : Element H O H Cl H S N H P Cl N O Valency 1 2 1 1 1 2 3 1 3 1 5 2 H2O HCl H2S NH3 PCl3 N2O5 Some Common Monoatomic Ions +1 Charge Formula +2Charge Formula +3 Charge Formula Name of ion Name of ion Name of ion Copper ion (Cuprous ion) Cu+ Barium ion Cobalt ion Ba2+ Co2+ Aluminium ion Auric ion Al3+ Au3+ Potassium ion K + Strontium ion Sr2+ Chromium (III) ion Cr3+ Silver ion Ag+ Iron (II) ion (Ferrous ion) Fe2+ Iron (III) ion (Ferric ion) Fe3+ Sodium ion Na+ *Copper (II) ion Cu2+ Scandium ion Sc3+ Lithium ion Li+ *Lead (II) ion Pb2+ Arsenic ion As3+ Cadmium ion Cd2+ Bismuth ion Bi3+ Magnesium ion Mg2+ Antimony ion Sb3+ Aurous Au+ Manganese (II) ion Mn2+ *Mercury (I) ion 2+ Hg2 Zinc ion Zn2+ – 1 Charge Formula – 2Charge Formula – 3 Charge Formula Name of ion Name of ion Name of ion Bromide ion Br– Oxide ion O 2– Nitride ion N 3– Chloride ion Cl– Sulphide ion S 2– Phosphide ion P 3– Fluoride ion F – Boride ion B 3– Iodide ion I – • These elements show more than one valency. So a Roman numeral shows their valency in a bracket. Some Common Polyatomic Ions – 1 Charge Formula – 2Charge Formula – 3 Charge Formula Name of ion Name of ion Name of ion Hydrogen carbonate or bicarbonate ion − HCO3 Carbonate ion Manganate ion 2− CO3 2− MnO4 Phosphate ion Arsenate ion 3− PO4 3− AsO4 Hydrogen sulphate or (bisulphate ion) − HSO4 Thiosulphate ion Silicate ion 2− S2O3 2− SiO3 Arsenite ion 3− AsO3 Hydroxide ion OH– Sulphate ion 2− SO4 Phosphite ion 3− PO3
Nitrate ion − NO3 Sulphite ion 2− SO3 Chlorate ion − ClO3 Chromate ion 2− CrO4 Borate ion 3− BO3 Nitrite ion − NO2 Dichromate ion 2− Cr2O7 Ferricyanide ion [Fe(CN)6] 3– Permanganate ion − MnO4 Hydrogen phosphate ion 2− HPO4 Acetate ion CH3COO– Oxalate ion 2− C2O4 Cyanide ion CN– Hypophosphite ion − H2PO2 – 4 Charge Meta aluminate ion − AlO2 Carbide ion C 4– +1 Charge Ferrocyanide ion [Fe(CN)6] 4– Ammonium ion + NH4  Tests of Chemical Reaction : A chemical reaction must satisfy the following : (i) There must be either evolution or absorption of heat, i.e., a chemical reaction must be accompanied with change in temperature. (ii) The reaction must occur between fixed quantities of the reactants. (iii) There must not be either gain or loss of matter, i.e., a chemical reaction should follow the law of conservation of mass. (iv) The products obtained as a result of chemical reaction must have properties different from those of the reactants. ➢ CHEMICAL EQUATIONS All chemical reactions are represented by chemical equations. A chemical equation is a shorthand representation of a chemical reaction using the symbols and formulae of substance involved in the chemical reaction. The symbols and formulae of the substances (elements or compounds) are arranged to show the reactants and products of a chemical reaction. Examples : 1. When potassium nitrate is heated, it gives potassium nitrite and oxygen. This reaction may be represented in the form of a chemical equation as follows. oxygen 2 potassiumnitrite 2 potassiumnitrate KNO3 → KNO + O 2. Zinc and dilute sulphuric acid react to form zinc sulphate and hydrogen. This reaction is represented by a chemical equation as Zn + H2SO4 → ZnSO4 + H2  Rules for writing chemical equation : Certain rules have to be followed while writing a chemical equation. 1. The reactants taking part in the reaction are written in terms of their symbols or molecular formulae on the left-hand side of the equation. 2. A plus (+) sign is added between the formulae of the reactants. 3. The products of reaction are written in terms of their symbols or molecular formulae on the right-hand side of the equation. 4. A plus (+) sign is added between the formulae of the products. 5. In between the reactants and the products an arrow sign (→) is inserted to show which way the reaction is occurring. A + B ⎯→ C + D In this chemical equation, A and B are the reactants, and C and D are the products. The arrow indicates that the reaction proceeds towards the formation of C and D.  Balancing of Chemical Equation : Observe the following two chemical equations : Zn + H2SO4 ⎯→ ZnSO4 + H2 .....(i) Na + H2O ⎯→ NaOH + H2 .....(ii)
In equation (i), the number of atoms of Zn, H, S and O are equal on both sides, i.e., the equation is balanced  Balanced Equations : The equations in which atoms of various elements on the reactants' and the products side are equal. Equation (ii) is not balanced because the number of hydrogen atoms in not equal on both sides. It is called a skeleton chemical equation.  Reason of Balancing Equations : The number of atoms of elements on both sides of a chemical equation should be equal in accordance with the law of conservation of mass.  Balancing : The process of making atoms of various elements equal in an equation on either side is called balancing.  Steps in Balancing of Chemical Equations : A number of steps are involved in balancing a chemical equation, e.g., Na + H2O → NaOH + H2 Step-1 : Examine the number of atoms of different elements present in unbalanced equations. Number of atoms in reactants Number of atoms in products Na H O 1 2 1 1 3 1 Step-2 : Pick an element to balance the equation. In the above equation Na and O are balanced, Hydrogen is not. Step : To balance Hydrogen on both sides we need to multiply H2O by 2 which makes Hydrogen atoms equal to 4 on the reactants' side. To make Hydrogen 4 on the products' side, multiply NaOH by 2. Now oxygen has become 2 on both side. But Sodium atoms has become two on the products' side. Multiply Na by 2 on the reactants side so that they become equal on both side. The steps are as follows : (i) Na + 2 H2O → NaOH + H2 (ii) Na + 2 H2O → 2NaOH + H2 (iii) 2 Na + 2 H2O → 2NaOH + H2 The equation is now balanced. Example : Fe + H2O ⎯→ Fe3O4 + H2 Step-1 : Element Number of atoms in reactants Number of atoms in products Fe H O 1 2 1 3 2 4 Step-2 :Pick up the compound which has the maximum number of atoms whether a reactant or a product, and in that compound select the elements which has the highest number of atoms, e.g., we select Fe3O4 in the above equation : To balance oxygen atoms, In reactants In products Initial To balance 1 (in H2O) 1 × 4 4 (in Fe3O4) 4 × 1 To equalise the number of atoms, we put the coefficient on the left side of the formula. A coefficient is a small whole number, like coefficients used in algebraic equations. You must keep in mind that we can put coefficients but we cannot change the subscripts in the formula, i.e., to balance Oxygen atoms, we can put the coefficient 4 as 4 H2O and not H2O4 or (H2O)4. Now the partly balanced equation becomes as follows : Fe(s) + 4 H2O(g) ⎯→ Fe3O4(s) + H2(g) (Partly balanced) Step-3 : Pick up the second element to balance this partly balanced equation. Let us try to balance hydrogen atoms. In partly balanced equation. Atoms of Hydrogen. In reactants In products Initial To balance 8 (in 8 H2O) 8 × 1 2 (in H4) 2 × 4 To equalise the number of Hydrogen atoms, we use 4 as the coefficient of H2 in the products. Fe(s) + 4 H2O(g) ⎯→ Fe3O4(s) + 4 H2 Step-4 : Pick up third element to be balanced. The element which is left to be balanced is Fe. In reactants In products Initial To balance 1 (in Fe) 1 × 3 3 (in Fe3O4) 3 × 1