Class 10 l Science l Chemistry l Lesson 1: Chemical equations and reactions

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1. Chemical Equations and Reactions Class 10 Chemistry

2. Chemical Reaction A chemical reaction is a change in which one or more substances or reactants react to form a new substance with entirely different properties. Reacting species are known as reactants and the new species formed as result of the reaction are called products . Example: 2 Na (s) + 2 H 2 O 2 NaOH (aq) + H 2 (g) Reactants products

3. Identification of Chemical Reaction A chemical reaction can be identified by either of the following observations: Change in state Change in colour Evolution of gas Change in temperature Formation of precipitate

4. Chemical equations A chemical equation is the symbolic representation of a chemical reaction. A chemical equation can be written in 3 forms: Word equation(ex: Hydrogen + Oxygen Water ) Paragraph (ex: when hydrogen is combined with oxygen, water is formed) Symbolic representation (ex: 2H 2 + O 2 2H 2 O )

5. Balanced chemical equation A balanced chemical equation is that in which the total number of atoms of each element are equal on both sides of the equation.

6. Making a chemical equation more informative The physical states of the reactants and products can be represented by using the symbols, (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solutions. The word (aq) aqueous is written if the reactant or product is present as a solution in water Example: 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Precipitate can also be represented by using an arrow pointing downwards ( ) instead of using symbol (s). Gaseous state of an evolved gas can be represented by using an arrow pointing upward direction ( ) instead of using symbol (g).

7. Types of chemical reactions Combination reaction Decomposition reaction Exothermic and endothermic reactions Displacement reaction Oxidation and reduction

8. Combination reaction A reaction in which two or more reactants combine to form a single product, is called combination reaction. Example: Calcium oxide reacts vigorously with water to form calcium hydroxide. The reaction is highly exothermic, as a lot of heat is produced during the reaction. CaO(s) + H 2 O Ca(OH) 2 (aq) + Heat General equation: A + B -------> AB

9. Decomposition reaction A reaction in which a single reactant breaks down to form two or more products, is known as decomposition reaction. Thermal decomposition Those reactions use the energy in the form of heat for decomposition of the reactant. Ex: CaCO 3 (s ) CaO(s) + CO 2 (g) Heat General equation: AB -------> A + B

10. 2) Electrolysis These reactions involve the use of electrical energy for the decomposition of the reactant molecules. Example: 2 H 2 O 2 H 2 + O 2 3) Photolysis These reactions involve the use of light energy for the purpose of decomposition. Example : 2NaCl 2Na + Cl 2 Electric current

11. Exothermic and endothermic reactions The reactions which are accompanied by the evolution of heat are called exothermic reactions or the reactions in which heat is released along with the formation of products are called exothermic reactions. Example : CH 4 + 2O 2 CO 2 + 2H 2 O + Heat The reactions which occur by the absorption of heat/energy are called endothermic reactions. Example: 6CO 2 + 12H 2, O C 6 H 12 O 6 +6 O 2 +6 H 2 O

12. Displacement reaction When a more reactive element displaces less reactive element from its compound, it is called displacement reaction. (i) Single displacement reaction : It is a type of reaction where an element reacts with a compound and takes the place of another element in that compound is called single displacement reaction. Example: Zn + CuSO 4 ZnSO 4 + Cu (ii) Double displacement reaction: the reaction in which two different ions or group of atoms in different ions or group of atoms in the reactant molecules are displaced by each other is called double displacement reaction. Also called as precipitate reaction. Example: Na 2 SO 4 + BaCl 2 BaSO 4 + 2 NaCl

13. Neutralization reaction The reaction in which acid react with base to form salt and water are called neutralisation reaction Example: NaOH + H 2 SO 4 Na 2 SO 4 + H 2 O

14. Oxidation and reduction reactions Oxidation Reduction Addition of oxygen 2Cu + O 2 → 2CuO Reduction of oxygen 2KClO 3 → 2KCl +3O 2 Removal of hydrogen 2H 2 S + O 2 → 2S + 2H 2 O Addition of hydrogen 2Na + H 2 → 2NaH Removal of electrons (e - ) Zn → Zn 2+ + 2e - Addition of electrons (e - ) Zn 2+ + 2e - → Zn

15. Oxidising agent The substance which can bring about oxidation of other substances is called an oxidising agent. Reduction agent The substance which can bring about reduction of other substances is called a reducing agent Redox reactions Those reactions in which oxidation and reduction take place simultaneously are called redox reactions. Example: CuO + H 2 → Cu + H 2 O

16. Corrosion The phenomenon due to which open surface of the metals are slowly eaten away by the reaction of air, water and chemicals present in the atmosphere, is called corrosion. The process of corrosion of iron is called rusting. Painting , galvanising , electroplating are some of the methods to prevent corrosion.

17. Rancidity It is the process of slow oxidation of oil and fat present in the food materials resulting in the change of smell and taste. Methods to prevent rancidity: Keeping food in airtight containers Refrigerating food at low temperatures Packing food in packets containing nitrogen Avoid keeping cooked in direct sunlight By adding antioxidants (Butylated Hydroxy Anisole; Butylated Hydroxytoluene )

18. I hope you all understood