Chapter No. 5 CHEMICAL REACTIONS

CHEMICAL REACTIONS: We have already learnt about elements and compounds which are the examples of pure matter. The substances such as hydrogen (H), oxygen (O) etc. are the elements while 2 2 water (HO), carbon dioxide (CO), etc., are the compounds. It is our daily observation 2 2 that water can be changed into ice. It can also be changed into steam. During both these changes, chemical composition of water (HO) and its chemical properties are not 2 changed. It means that liquid water, ice and steam are the three physically different forms of the same substance, i.e., water (HO). On the other hand, when we pass electricity 2 through acidified water (HO), it changes into hydrogen (H) and oxygen (O) which are 2 2 2 entirely different substances with different chemical compositions and chemical properties. Such a change in a substance during which entirely new substances with different chemical compositions and properties are formed is called a chemical change. A chemical change is always brought about by a chemical reaction. In this chapter, we will learn further about chemical reactions.

Chemical Reactions We deal with a large number of chemical reactions in our daily life. During these reactions, atoms present in different substances rearrange themselves form new substances. Burning of coal and natural gas (methane) in air are well known examples of chemical reactions. Chemically coal is carbon (c). It exists in solid state and is black in colour. Its burning in air is in fact a chemical reaction of carbon with oxygen of the air to form carbon dioxide (CO ). Carbon dioxide is a colourless gas. Its chemical 2 composition and chemical properties are entirely different from those of carbon and oxygen. Substances which take part in a chemical reaction are called reactants and those which are formed as a result of the reaction are called products. When methane burns in air, carbon dioxide and water are formed. During the rearrangement of atoms in burning of methane (natural gas), carbon atom of methane gets attached with two oxygen atoms to give carbon dioxide while hydrogen atoms attach themselves with oxygen atom to give water vapours.

Applications of Chemical Reactions Burning, respiration and photosynthesis, etc., are the examples of chemical reactions which take place everywhere in our environment. Fuel (natural gas or petrol, etc.) on its burning in vehicle engine produces different gases. The gases so produced develop pressure to move the piston in the engine and to run the vehicle. Heat produced during burning of fuel in our kitchens is used to cook food. Similarly, heat produced during burning of fuel in industries is used to produce steam from water. During photosynthesis in plants, carbon dioxide (CO) and water (HO) react to produce 2 2 glucose (CH O). This reaction takes place in the presence of sunlight and green 6 12 6 pigment chlorophyll.
During respiration, the oxygen of air reacts with food (glucose) to produce, carbon dioxide and water in the cells of living organisms. The energy produced during this reaction is used to perform all the body functions in living organisms.

CH O (s)        +    6O (g)           6CO (g)         +            6HO       + Energy

Glucose                  Oxygen       Carbon dioxide                 Water

Conversion of milk into yogurt and formation of baking products involve the chemical changes which are brought about by microorganisms. Such chemical changes or reactions are called fermentation reactions.

Chemical Equations and their Balancing A chemical equation is the representation of a chemical reaction in terms of symbols, formulae and signs, etc. In a chemical equation the reactants and products are separated by an arrow. Symbols and formulae of the reactants are written on the left hand side of the arrow whereas the products are written on the right hand side of the arrow. The arrow is directed towards the products. Physical states of reactants and products are also expressed along with their formulae or symbols by (s), (g) and (aq) which stand for solid, gas and aqueous states respectively. For example; the chemical equation representing the reaction of sulphur with oxygen to from sulphur dioxide is written as follows.
The chemical equation written above shows that sulphur in its solid state reacts with oxygen gas. The product of the reaction, i.e., sulphur dioxide is also a gas. The signs (s) and (g) indicate the physical states of the reactants and the products. Similarly, the chemical equation given below indicates that zinc in its solid state reacts with aqueous solution of sulphuric acid and produces aqueous solution of zinc sulphate and hydrogen gas.
5.2.1 Balancing the Chemical Equation The chemical equation in which the number of atoms of each element on both sides of the equation, i.e., reactant side and product side are equal is called a balanced chemical equation. For example, the chemical equation shown below is a balanced chemical equation.
The chemical equation in which the number of atoms of each element on both sides of the equation, i.e., reactant side and product side is not equal is called an unbalanced chemical equation. For example, the chemical equation given below is an unbalanced chemical equation.
Zn(s) + HSO (aq) ZnSO (aq) + H (g) 2 4 4 2 Zinc Zinc sulphate Sulphuric acid Hydrogen
S (s) + O (g) SO (g) 2 2 Sulphur Oxygen Sulphur dioxide
Hcl (aq) + NaOH (aq) NaCl (aq) + HO 2
H (g) + Cl (g) HCl (g)

Unbalanced equations can be balanced by different methods. The trial and error method is commonly used. According to this method, trial and error process of adjusting coefficients before symbols or formulae is continued till the number of atoms of each element on both sides of the equation becomes equal. The working rules for balancing a chemical equation are as follows: (i) Write the unbalanced equation and count the number of atoms of each element on both sides of the arrow. (ii) Work with one element at a time. (iii) Multiply the symbol or formula with suitable integers (2, 3, 4, 5, etc.) on that side of the equation where the number of atoms of a particular element is less and try to balance this element on both sides of the equation. Start multiplying with relatively small numbers. (iv) Repeat the process for all the elements one by one. (v) Balance the diatomic molecules like H, N, O, Cl, etc. at the end. 2 2 2 2 Some examples for balancing the equation are given below: Example 1 Consider the following equation:
Step I Count the number of atoms of each element on both sides of the arrow. Reactants Products Balanced/Unbalanced 2 N atoms 1 N atom N is unbalanced 2 H atoms 3 H atoms H is unbalanced
Step II Add appropriate coefficient to balance N:
Reactants Products Balanced/Unbalanced 2 N atoms 2 N atoms N is balanced 2 H atoms 6 H atoms H is unbalanced.

Step III Now try to balance H atoms.
Reactants Products Balanced/Unbalanced 2 N atoms 2 N atoms N is balanced 6 H atoms 6 H atoms H is balanced Thus the equation is balanced. Example 2
Step I Count the number of atoms of each element or compound on both sides of the arrow: Reactants Products Balanced/Unbalanced 1 C atom 1 C atom C is balanced 4 H atoms 2 H atoms H is unbalanced 2 O atoms 3O atoms O is unbalanced Step II
Add appropriate coefficients: Reactants Products Balanced/Unbalanced 1 C atom 1 C atom C is balanced 4 H atoms 4 H atoms H is balanced 4 O atoms 4O atoms O is balanced Thus the equation is balanced. Example 3
Step I Count the number of atoms of each element or compound on both sides of the arrow. Reactants Products Balanced/Unbalanced 1 Ca atom 1 Ca atom Ca is balanced 2 Cl atoms 1 Cl atom Cl is unbalanced 2 Na atoms 1 Na atom Na is unbalanced 1 C atom 1 C atom C is balanced 3 O atoms 3 O atoms O is balanced.

Step II Add appropriate coefficients to balance Na and Cl.
Reactants Products Balanced/Unbalanced 1 Ca atom 1 Ca atom Ca is balanced 2 Cl atoms 2 Cl atoms Cl is balanced 2 Na atoms 2 Na atoms Na is balanced 1 C atom 1 C atom C is balanced 3 O atoms 3 O atoms O is balanced Thus the equation is balanced.
Balance the following equations. Fe(s) + Cl (g) 2 FeCl (s) 3
Al(s) + O (g) 2 AlO (s) 2 3
NaCO (aq) 2 3 + HClO (aq) 3 NaClO (aq) 3 + CO (g) 2 + HO 2
CO (g) 2 + HO 2 CH O (aq) 6 12 6 O (g)

Law of Conservation of Mass (Matter) Law of conservation of mass was put forward by a French Chemist Lavoisier in 1785. This law states that during a chemical reaction, mass is neither created nor destroyed but it changes from one form to another. In other words during a chemical reaction, total mass of the products is equal to the total mass of the reactants.

Apparatus / Material Required Conical flask, weight balance, sodium chloride solution, silver nitrate solution Procedure § Take a small amount of sodium chloride solution in a conical flask and silver nitrate solution in a small test tube. § Place the test tube (containing silver nitrate solution) in the flask along its wall in such a way that two solutions do not mix with each other.

Through the activity 5.3 we observe the formation of white precipitate of silver chloride (AgCl) as a product of the reaction between sodium chloride (NaCl) and silver nitrate (AgNO) solutions. The balanced chemical equation for the reaction is as follows: 3
We also notice that during a chemical reaction total mass of the products is equal to the total mass of reactants. This verifies the law of conservation of mass.

Types of Chemical Reactions Thousands of chemical reactions are taking place all the time in the world. They are classified into different types. Here we will discuss only two types, i.e., addition reactions and decomposition reactions. 5.4.1 Addition Reactions The chemical combination of two or more substances to form one compound is called addition reaction. The following are the examples of addition reactions:
5.4.2 Decomposition Reactions A chemical reaction during which a compound splits up into two or more simple substances is called a decomposition reaction. Usually heat is required to bring about decomposition of compounds. The following are some examples of decomposition reactions.
2KClO (s) 3 Potassium chlorate
2KCl (s) Potassium chloride+ 3O (g) 2 Oxygen
CaCO (s) 3 Calcium carbonate
CaO (s) Calcium oxide+ CO (g) 2 Carbon dioxide

Energy Changes in Chemical Reactions In order to know about the nature of chemical change in various reactions we need to know about the change in energy of substances. The energy of a substance is a particular amount of energy due to which the structure of the substance remains stable. A substance undergoes a chemical change or chemical reaction when its energy is changed. The change in energy of a substance takes place by absorbing or releasing heat or light. On the basis of the change in energy, chemical reactions can be classified into two types, i.e., exothermic and endothermic reactions. 5.5.1 Exothermic Reactions Exo means outside and therm means heat. Exothermic reactions are those reactions during which heat is given out. Burning is a common example of exothermic reaction. Fossil fuel (coal, natural gas, etc.) burns in the air to release heat.
Fireworks are also the examples of exothermic reactions (Figure 5.4).
Figure 5.4: Fireworks
C (s) + O (g) 2 CO (g) 2 + Heat
CH (g) 4 + 2O (g) 2 CO (g) 2 + 2HO 2 + Heat
Fe (s) + S (s) FeS (s) + Heat

During activity 5.5, sides of the beaker become warm. The heat which makes the mixture in the beaker and sides of the beaker warm is released by exothermic reaction between calcium oxide and water.
5.5.2 Endothermic Reactions Endo means inside. The reactions during which heat is absorbed are called endothermic reactions. Thermal decomposition of calcium carbonate to produce carbon dioxide on commercial scale is an endothermic reaction.
CaO (s) Calcium oxide+ HO (l) 2 WaterCa(OH) (aq) 2 Calcium hydroxide+ Heat
CaCO (s) 3 Calcium carbonate+ Heat CaO (s) Calcium oxide+ CO (g) 2 Carbon dioxide

During activity 5.6, you will feel that the sides of the beaker become cold. This is because an endothermic reaction takes place by absorbing heat from the surrounding walls of the beaker.
Formation of nitric oxide from nitrogen and oxygen and hydrogen iodide from hydrogen and iodine are also the examples of endothermic reactions.
5.5.3 Importance of Exothermic Reactions in Daily Life Exothermic reactions have great importance in our daily life. They are extensively used to fulfill our needs of heat energy for various purposes. The heat released during burning of fuel at our homes is used for cooking food and to warm our rooms. The heat released during burning of petrol or diesel in the vehicle engine increases pressure of the products (gases) to push and move the piston in the cylinder. The force of the piston turns the wheels and makes the vehicle move. Heat produced by the burning of fuel in thermal power stations is used in generating electricity. Heat produced during digestion of food in our body keeps us warm and alive. Ignition of dynamite and gunpowder are also highly exothermic reactions and are termed as explosions. Such explosions are used for blasting in mines.

The process during which a substance changes into entirely new substance with different chemical composition and properties is called chemical reaction. § During chemical reactions, atoms present in different substances are rearranged to form new substances. § The substances which take part in a chemical reaction are called reactants and those which are formed as a result of the reaction are called products. § The representation of a chemical reaction in terms of symbols, formulae and signs used for indicating physical states of the substances is called chemical equation. § The use of co-efficients to balance the number of different types of atoms in a chemical equation is called the balancing of chemical equation. § Law of conservation of mass states that during a chemical reaction, the total mass of the reactants is equal to the total mass of the products. § Addition reactions involve the chemical combination of two or more substances to form one compound. § A chemical reaction which involves the splitting up of one compound into two or more simple substances is called decomposition reaction. § The chemical reactions during which heat is evolved are called exothermic reactions. § The chemical reactions during which heat is absorbed are called endothermic reactions. § Heat evolved during exothermal reaction is used to cook food, drive vehicles and generate electricity.

 

 

 

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