Chemical Reactions And Equations Class 10 Science Notes

Revision Notes for Class 10 Science Chapter 1 – Chemical Reactions and Equations Chemical Change: A change that results in the formation of one or more new compounds. Chemical changes are also known as chemical reactions. Observations in a Chemical Reaction In a chemical reaction or chemical change, the following observations can be made, • Formation of new substances. • Change in mass. • Changes in energy. • Evolution of gas. • Change in temperature. • Permanent change in colour and/or state. • Formation of a precipitate. Examples : A. Cooking of food B. Rusting of iron C. Heating of Lead nitrate D. Souring of milk E. Ripening of fruit. Class X Science www.vedantu.com 1 Chemical Formulae - A substance's chemical formula is a symbolic representation of the number of atoms present in a single molecule of that material. a. One water molecule has the formula H2O . It demonstrates that one molecule of water is made up of two hydrogen atoms and one oxygen atom. b. One molecule of sulphuric acid has the formula H2SO4 . It illustrates that one molecule of sulphuric acid has 2 hydrogen atoms, 1 sulphur atom, and 4 oxygen atoms. Chemical Equations - The chemical equation of the reaction is the representation of a chemical change in terms of symbols and formulae of the reactants and products. Example: KMnO4 + Potassium permanganate HCl Hydrochloric acid → KCl + MnCl2 + H 2O + Cl2 Potassium Manganese Water Chlorine chloride chloride Steps for writing Chemical Equation A. Step I: Writing Skeletal Equation A word-equation depicts the transformation of reactants into products by placing an arrow between them ( → ). The arrow indicates the reaction's direction and might be read as "to yield," "to form," or "to give." The reactants are on the left (LHS) of the arrow, while the products are on the right (RHS). A plus sign (+) links the different reactants and products together. A balanced account of a chemical transaction is a complete chemical equation, which symbolically depicts the reactants, products, and their physical states. Class X Science www.vedantu.com 2 B. Step II: Balancing of Chemical Equation A balanced chemical equation is one in which the number of atoms of each element is equal on both sides of the equation. On the reactant and product sides of the equation, a chemical equation is balanced if the number of atoms of each type involved in the reaction is the same. Equations must be balanced at all times. The amount of hydrogens on both sides should be an even number whenever H2O is present on one side (2 atoms of hydrogen in water). If there are 4 H2O hydrogen atoms, then 4  2 = 8 hydrogen atoms are present. If there are 7 H2O hydrogen atoms, then 7  2 = 14 hydrogen atoms, indicating that all the numbers are even. There must be an even number in front of HCl on the reactant side. (We'll figure out what that number is later.) As a result, there will be an even amount of chlorine atoms (i.e., KCl=1 , MnCl2 = 2 , Cl2 = 2 . i.e., 1 + 2 + 2 = 5 ). However, the quantity of chlorine atoms in the product is unusual. KCl has the only odd number of chlorine atoms. Let's alter it to the simplest even number we can think of, which is 2. KMnO4 + HCl → 2KCl + MnCl2 + H2O + Cl2 Since the number of K atoms in 2 KCl = 2 , we have to put 2 in front of KMnO4 for balancing K atom.  2KMnO4 + HCl → 2KCl + MnCl2 + H2O + Cl2 In 2 KMnO4 , there are 2 K, 2 Mn, and 8 O. So, add these numbers in front of K, Mn and O, (K is already done).  2KMnO4 + HCl → 2KCl + 2MnCl2 + 8H2O + Cl2 If there are 8 H2O on the product side, there should be 16 H (8 x 2) on the reactants side as well.  2KMnO4 + 16HCl → 2KCl + 2MnCl2 + 8H2O + Cl2 Class X Science www.vedantu.com 3 Now the only unbalanced one is Chlorine. On the left hand side, there are 16 Cl. On the right hand side, firstly, there are 2 Cl in 2 KCl + 4 Cl in 2 MnCl2 , making total of 6(2 + 4). So, 10 more Cl atoms are to be accounted for. So, place 5 in front of Cl2 to make it 10 (5 x 2).  2KMnO4 + 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2 The Hit and Trial approach is a technique for balancing chemical equations. Short Technique for Balancing a Chemical Equation Let us take the same chemical reaction and try to balance it using a short technique which will save you time and effort in the exam. Let the number of molecules on each side be, aKMnO4+ bHCl → cKCl + dMnCl2 + eH2O + fCl2 Now comparing LHS and RHS, K: a = c Mn: a = d O: 4a = e H: b = 2e Cl: b = c + 2d + 2f Let us take a = 2, then c=d=a=2 b = 2e = 8a = 16 So, e = 8 So, 16 = 2 + 2(2) + 2(f) Class X Science www.vedantu.com 4 f=5 Thus, the required equation is: 2KMnO4+ 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2. From a chemical equation we can conclude: A. Various reactants and products names B. Reactant and product formulae C. The number of moles of the reactants and products in relation to each other D. Masses of reactants and products relative to each other E. Volumes of gaseous reactants and products in relation to each other In short – the Steps Involved in Balancing a Chemical Equation can be Listed As: Step 1: Write the skeleton equation correctly. Step 2: Start with the compound that has the maximum atoms or maximum types of atoms and the atoms present in it are balanced first. Step 3: Start by balancing elements that only appear once on each side of the arrow. Then, on each side, balance elements that appear more than once. Step 4: The Elementary substances are balanced last. Step 5: To make all the coefficients whole numbers, the entire equation is multiplied by a suitable number if necessary. Class X Science www.vedantu.com 5 Types of Reactions A. Combination or Synthesis Reactions: The reactions in which two or more chemicals combine to generate a single new compound. Types of Combination reactions: 1. Combination of two Elements to Form a Compound Burning of hydrogen in air or oxygen to produce water. 2H 2(g) → O2(g) + 2H 2 O( l ) Hydrogen Oxygen Water 2. Combination Reactions involving an Element and a Compound Burning of carbon monoxide in oxygen to form carbon dioxide. 2CO(g) + O2(g) → 2CO2(g) 3. Combination Reactions Involving Two Compounds Combination of ammonia and hydrogen chloride to produce ammonium chloride. NH3(g) + Ammonia Colourless HCl(g) → Hydrogen chloride Colourless NH 4Cl(s) Ammonium chloride White B. Decomposition Reactions: Decomposition reactions, on the other hand, are the polar opposite of combination reactions. A decomposition reaction occurs when a complex is broken down into two or more simple components using heat or electricity. Class X Science www.vedantu.com 6 Thermal decomposition occurs when a substance decomposes owing to heat, whereas electrolytic decomposition occurs when a substance decomposes due to electricity. Electrolysis is the process of decomposing a substance by putting an electric current through it. Photolysis: The decomposition of a compound with light is called photolysis. 1. Mercuric oxide, when heated, undergoes thermal decomposition, to give mercury and oxygen. Thermal decomposition 2HgO(s) ⎯⎯⎯⎯⎯⎯⎯ → Mercuric oxide Red 2Hg (l) + Mercury Silvery liquid O 2(g)  Oxygen 2. Similarly, heating blue copper nitrate crystals causes thermal breakdown, resulting in black copper oxide, reddish brown nitrogen dioxide fumes, and colourless oxygen gas. Thermal decomposition 2Cu(NO3 )2(s) ⎯⎯⎯⎯⎯⎯⎯ → 2CuO(s) + 4NO2(g) + O2(g) Copper nitrate Copper Nitrogen Blue oxide dioxide Black Oxygen Reddishbrown 3. When water is acidified with a little amount of sulphuric acid and a direct current is run through it, electrolytic breakdown occurs, resulting in hydrogen and oxygen. Electrolytic Decomposition 2H 2O(l) ⎯⎯⎯⎯⎯⎯⎯⎯ → Water 2H 2(g) + O2(g) Hydrogen Oxygen 4. When molten lead bromide is exposed to an electric charge, it decomposes into lead and bromine. Class X Science www.vedantu.com 7 Electrolytic decomposition PbBr2 ⎯⎯⎯⎯⎯⎯⎯ ⎯ → Lead bromide (molten) Pb(s) + Lead Br2(l) Bromine Activity Series: The metal activity series is a list of metals arranged in decreasing chemical activity order. C. A Substitution or Displacement Reaction occurs when the atoms of one element replace the atoms of another element in a compound's molecules. Elements in the Activity Series that are higher in the hierarchy displace elements that are lower in the hierarchy. Electropositive elements that are more abundant displace electronegative elements that are less abundant. Conversely, electronegative elements with a greater charge density will displace electronegative elements with a lower charge density. For e.g., Fe(s) + 2CuSO 4(aq) → Iron Copper Sulphate Blue Iron (II) Sulphate Green Copper Reddish Brown + H 2SO 4(aq) → MgSO 4(aq) + H 2O(g) Mg (s) Magnesium 2KI(aq) 4FeSO 4(aq) + Cu (s)  + Potassium Iodide Sulphuric acid Magnesium Sulphate Water Cl2(g) → 2KCl(aq) + I 2(s) Chlorine Potassium Chloride Iodine The iodine that has been freed dissolves in the chloroform, turning it purple. Class X Science www.vedantu.com 8 D. Double Displacement Reactions / Metathesis Reactions: The mutual exchange of ions in which two chemicals react to generate two distinct molecules. A + B0 +C+ D0 → A + D0 +C+ B0 Na + Cl0(aq) + Ag + NO30 aq → Na + NO30 aq + Ag +Cl0(s)  Sodium chloride Silver nitrate Sodium nitrate Silver chloride White 2H + Cl0(aq) + Pb 2+ (NO3 ) 20 aq → 2 H + NO30 aq + Pb 2+Cl20( s )  hydrochloric Lead nitrate Nitric acid Lead chloride acid White Zn 2+SO4{{^2-}_(aq)}+ Ba 2+Cl2{{^0}_(aq)} → Zn 2+Cl 20 aq + Ba 2+SO4 2− ( s )  Zinc sulphate Barium chloride Zinc chloride Barium Sulphate White Types of Double Displacement Reactions: 1. Precipitation In all of the following processes, a white material is generated that is insoluble in water. A precipitate is the insoluble substance that forms. A precipitation reaction is a reaction that results in the formation of a precipitate. 2. Neutralization The reactants are a base and an acid, and the products are salt and water. Neutralization is a form of double displacement reaction in which the reactants are a base and an acid, and the products are salt and water. The positive charge of the acid's hydrogen ion and the negative Class X Science www.vedantu.com 9 charge of the base's hydroxyl ions or oxide ions lose their electrical charge and form covalent water molecules. Na + OH 0 + H + Cl0 → Na + Cl0 + H 2O Base Acid Salt Water Zn 2+O2- + 2H+ NO30 → Zn 2+ (NO3 ) 20 + H 2O A neutralization reaction is basically a reaction between H+ and OHions i.e., H + (aq) + OH0(aq) → H 2O(l) E. Oxidation - Reduction Reactions/Redox Reactions Classical Concept of Oxidation and Reduction "Oxidation is a reaction in which oxygen is added or hydrogen is removed from a substance." I. Addition of Oxygen - Magnesium oxide is formed when oxygen reacts with magnesium. 2Mg (s) + O 2 → Magnesium Oxygen 2MgO(s) Magnesium oxide II. Removal of Hydrogen - To liberate free iodine, hydrogen is removed from hydro iodic acid. 2HI(g) + 2H 2(g) → Hydroiodic Hydrogen acid I 2(g) Iodine III. Removal of Oxygen - Copper metal is formed by removing oxygen from copper oxide. Class X Science www.vedantu.com 10 → CuO(s) + CO(g) Cu (s) Copper oxide Carbon Black monoxide + CO2( g ) Copper Reddish Brown Carbondidoxide IV. Addition of Hydrogen - When hydrogen reacts with chlorine, hydrogen chloride gas is formed. H 2(g) + Cl 2(g) → Hydrogen Chlorine 2HCl(g) Hydrogen chloride • A reduction reaction is one in which oxygen is removed from a material and hydrogen is added. • Redox reactions occur when oxidation and reduction reactions occur at the same time. Example of Redox Reaction: Mg (s) + H 2SO 4(aq) → MgSO 4(aq) + Magnesium Sulphuric acid H 2( g ) Magnesium Sulphate Hydrogen Example of Non-redox Reaction NaCl(aq) + AgNO3(aq) → AgCl(s)  Sodium Chloride Silver Nitrate Silver chloride + NaNO3(aq) Sodium Nitrate Redox reactions are those in which oxidation and reduction occur at the same time. Class X Science www.vedantu.com 11 Oxidizing and Reducing Agents A. Oxidizing Agent: A substance that brings about oxidation. B. Reducing Agents: A substance that brings about reduction. Another Definition of Oxidation and Reduction 1. The addition of an electronegative element or radical, or the removal of an electropositive element or radical, is referred to as oxidation. 2. Addition of an electropositive element or radical, or removal of an electronegative element or radical, is known as reduction. Electronic Concept of Oxidation and Reduction 1. Oxidation: The process in which there is a loss of electrons. 2. Reduction: The process in which there is a gain of electrons. The Effects of Oxidation Reactions on Everyday Life Corrosion Many metals are chemically active elements that are easily affected by moisture, oxygen, acids, and other things. When iron products are new, they are shiny, but after some time, they become coated with a reddish-brown powder. The rusting of iron is the common name for this phenomenon. The difficulty with iron (and many other metals) is that it oxidises, and the resulting oxide does not attach firmly to the metal's surface, leading it to flake off readily. This eventually leads to structural weakening and metal breakdown. Class X Science www.vedantu.com 12 As a result, when metal is attacked by substances in its environment, it is said to corrode, and the process is known as corrosion. When copper vessels or artefacts are exposed to air and water, what happens? They gradually tarnish as a thin layer of green oxide forms on them. Silver, too, quickly develops a thin black oxide covering when exposed to damp air. In wet weather, even the heaviest metal lead tarnishes. The black coating on silver and the green coating on copper are instances of corrosion in which the oxides generated create a strong connection to the metal's surface, blocking further oxygen exposure and thereby reducing corrosion. Rancidity Have you ever tasted or smelled fat/oil-containing food materials that have been left out for an extended period of time? Rancidity refers to an undesirable change in a food's flavour and odour. The oxidation process causes fats and fatty foods to deteriorate, which is the most common cause of rancidity. When an oxygen atom replaces a hydrogen atom in a fatty acid molecule, the molecule becomes destabilised. Factors that accelerate fat oxidation include salt, light, water, bacteria, moulds trace metals (iron, zinc, etc.). Antioxidants are chemicals that prevent fat from oxidising or becoming rancid (such as BHT, BHA, vitamin E, and vitamin C, and spices such as sage and rosemary). To avoid spoilage, they are added to foods containing fats and oils. Keeping food in airtight containers or airtight wrapping also helps to slow down oxidation. Some high-fat meals such as potato chips are wrapped in materials that shield them from light and oxygen and the containers are flooded with nitrogen to further exclude oxygen. Vacuum packaging is sometimes employed in processed goods to completely eliminate the presence of oxygen. Class X Science www.vedantu.com 13