Chemical Changes and Reactions PDF

Chemical Changes and Reactions A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products). A chemical bond is the force which holds the atoms of a molecule together, as in a compound. Conditions Ne...

Chemical Changes and Reactions A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products). A chemical bond is the force which holds the atoms of a molecule together, as in a compound. Conditions Necessary for a Chemical Change The following conditions are necessary for a chemical change: Close physical A chemical reaction occurs when two substances are mixed in their solid contact (Mixing) state. Iodine and sulphur react explosively when brought into close contact. Solution A chemical reaction occurs when two substances are mixed in the solution form. Sodium carbonate and tartaric acid vigorously react only in the solution state. Heat Some chemical reactions occur only on heating. CuCO3  Heat CuO + CO2 Light Reactions which occur by the action of light are called photochemical reactions or photolysis. Molecules of the reactants absorb light energy, get activated and then react rapidly. Photosynthesis: 6CO2 + 6H2O  Sunlight C6H12O6 + 6O2 (Glucose) Electricity Chemical reactions such as decomposition of compounds occur only when electricity is passed through the substance. 2H2O  Electric Current 2H2 ↑ + O2 ↑ Pressure Some reactions occur only when substances are subjected to high pressure. N2 + 3H2 2NH3 Catalyst Some chemical reactions need a catalyst to accelerate or decelerate their rates of reaction. Catalysts themselves do not take part in the reaction. A catalyst such as Pt or MnO2 initiates a change in the rate of the reaction without undergoing any change in its chemical composition. 4NH3 + 5O2 Pt 800 C 4NO + 6H2O Positive catalyst: A positive catalyst accelerates a reaction. Negative catalyst: A negative catalyst retards a reaction.  Sound Some chemical reactions proceed only by absorption of sound energy. Sound energy speeds up the reacting molecules, atoms or ions causing a reaction to occur. C2H2  Sound Energy 2C + H2 Characteristics of a Chemical Reaction 1. Evolution of gas In a chemical reaction, a gas may be one of the products. Zn + H2SO4 → ZnSO4 + H2 ↑ 2. Change of colour Some chemical reactions are characterised by a change in the colour of the reactants. Fe + CuSO4 → FeSO4 + Cu 3. Formation of Some chemical reactions are characterised by the formation of a precipitate precipitate. The precipitate is an insoluble solid substance. AgNO3 + NaCl → AgCl + NaNO3 4. Change of state In some reactions, a change of a state is observed. The reaction starts with solid or liquid reactants and ends up with gaseous products and vice versa. NH3(g) + HCl(g) NH4Cl(s) Types of Chemical Changes or Chemical Reactions A chemical reaction is the process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products). 1. Direct A chemical reaction in which two or more substances combine to form a combination or single product. synthesis 1) Combination of two elements: 2Fe(s) + S(s) → FeS(s) Iron Sulphur Iron sulphide 2) Combination of an element and a compound: 2CO(g) + O2(g)  heat 2CO2(g) 3) Combination of two or more compounds: PbO2(s) + SO2(s) → PbSO4(s) Lead dioxide Sulphur dioxide Lead sulphate 2. Decomposition A chemical reaction in which a single compound splits into two or more reaction simple substances. 2HgO(s) → 2Hg(s) + O2↑ Mercuric oxide Mercury Oxygen Decomposition occurs by application of heat or light or by the passage of electric current. Electrolysis of acidulated water: On passing electric current through acidulated water, water produces two volumes of hydrogen gas and one volume of oxygen gas. 2H2O(l)  Electric Current 2H2(g) + O2(g) Thermal decomposition: A decomposition reaction brought about by heat. 2 KClO3  Heat 2KCl + 3O2 In a decomposition reaction: 1) A compound can break up into two or more elements.  (a) 2HgO(s)  2Hg(l)  O2(g) (b) 2H2 O (l)  electric current 2H2(g)  O2(g) 2) A compound can break up to form both elements and compounds. 2 KNO3  Heat 2KNO2 + 3O2 3) A compound can break up to form two or more new compounds. CaCO3(s)  heat 1000  CaO(s)  CO2(g) Thermal decomposition of a metal compound: (i) Metal hydroxide (ii) Metal carbonates  (iii) Metal bicarbonates Metal bicarbonates or metal hydrogen carbonates decompose to give metal carbonate, water vapour and carbon dioxide. NaHCO3  Na2 CO3  H2 O  CO2 sodiumhydrogencarbonate sodiumcarbonte (iv) Metal nitrates 3. Reversible A reaction in which the direction of a chemical change can be easily reaction reversed by changing the conditions under which the reaction is taking place. 3Fe + 4H2O Fe3O4 + 4H2 Thermal dissociation A reaction in which a substance dissociates into two or more simpler substances on the application of heat is called a thermal dissociation reaction. It is a reversible reaction. Heat NH4Cl(s) Cool NH3 + HCl(g) 4. Displacement A reaction in which the more reactive element displaces the less reactive reaction element from its compound. Zinc displaces copper in copper sulphate to form zinc sulphate. 5. Double A reaction in which ions of the reactants exchange places to form two new displacement compounds. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) 6. Double A type of chemical change in which two compounds in a solution react to decomposition form two new compounds by the mutual exchange of radicals. These reactions are of two types—precipitation reaction and neutralisation reaction. Precipitation reaction The insoluble solid formed during a double displacement reaction is called a precipitate. Na2SO4(aq) + BaCl2 → BaSO4(s) + 2NaCl(aq)  Neutralisation reaction The reaction between an acid and a base to form a salt and water is called a neutralisation reaction. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Uses of neutralisation reaction in everyday life: a) Venom of honey bee contains formic acid. When someone is stung by a bee, formic acid enters the skin and produces pain which can be relieved by rubbing the spot with slaked lime or baking soda both of which are bases. b) Acidity is caused by excess secretion of HCl by stomach glands. It can be relieved by taking milk of magnesia or sodium hydrogen carbonate, both of which are bases. On the other hand, deficiency of HCl is covered up by taking any suitable acid in the dilute form. Hydrolysis It is the process in which a salt and water react to form an acidic or a basic solution. 1) Hydrolysis of a salt formed by the reaction of a strong base and a weak acid forms a basic solution which turns red litmus blue. 2) Hydrolysis of a salt formed by the reaction of a strong acid and a weak base forms an acidic solution which turns blue litmus red. Energy Changes in Chemical Reactions Each substance has a fixed amount of stored energy, which is in the form of potential energy. This energy is called its chemical energy. The formation of gas bubbles in a liquid during a reaction is called effervescence. Exothermic change A chemical change which takes place with the release of heat energy is called an exothermic change. C + O2 → CO2 + Heat Endothermic change A chemical change which takes place with the absorption of heat energy is called an endothermic change. C + 2S + Heat → CS2 Photochemical reaction A chemical reaction which proceeds with the absorption of light energy. 6CO2 + 6H2O  Sunlight C6H12O2 Electrochemical reaction A chemical reaction which proceeds with the absorption of electric energy. 2KCl  Electric Current K+ + Cl‒