Chapter 1 chemistry chemical Equations and Reactions
- 1. TYPES OF CHEMICAL REACTIONS Chemical Reactions and Equations
- 2. COMBINATION REACTION A reaction in which a single product is formed from two or more reactants is known as a combination reaction. CaO(s) +H2O(l) → Ca(OH)2(aq) + Heat
- 3. COMBINATION REACTION C(s) + O2(g) → CO2(g) + Heat 2H2(g) + O2(g) → 2H2O(l) + Heat we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.
- 4. DECOMPOSITION REACTION When a single reactant breaks down to give simpler products. This is a decomposition reaction. Thermal decomposition:- 2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g) CaCO3(s)→ CaO(s) + CO2(g)
- 5. THERMAL DECOMPOSITION:- Take about 2 g lead nitrate powder in a boiling tube. Hold the boiling tube with a pair of tongs and heat it over a flame. You will observe the emission of brown fumes. These fumes are of nitrogen dioxide (NO2). The reaction that takes place is – 2Pb(NO3)2(s) → 2PbO(s) +4NO2(g) + O2(g)
- 6. ELECTROLYSIS /ELECTRICAL DECOMPOSITION When electric current is passed through water it breaks to form Hydrogen gas and oxygen gas. 2H2O 2H2 + O2
- 7. PHOTOLYSIS When Silver chloride is exposed to sunlight it breaks up to form Silver and Chlorine gas. 2AgCl(s) 2Ag(s) + Cl2(g)
- 8. DISPLACEMENT REACTION When a more reactive element displaces a less reactive element from its salt, it is called displacement reaction. Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) Zn(s) + CuSO4(aq)→ZnSO4(aq) + Cu(s) Pb(s) + CuCl2(aq)→ PbCl2(aq) + Cu(s)
- 9. DOUBLE DISPLACEMENT REACTION The reactions in which there is an exchange of ions between the reactants are called double displacement reactions. Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + NaCl(aq) This insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.
- 10. REDUCTION – OXIDATION REACTIONS The reactions in which one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions. 2Cu + O2 → 2CuO CuO +H2→ Cu + H2O ZnO + C → Zn + CO
- 11. EXOTHERMIC- ENDOTHERMIC REACTIONS Reactions in which heat is given out along with the products are called exothermic reactions. C(s) + O2(g) → CO2(g) + Heat 2H2(g) + O2(g) → 2H2O(l) + Heat Reactions in which energy is absorbed are known as endothermic reactions. 2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g) CaCO3(s)→ CaO(s) + CO2(g)
- 12. CORROSION:- When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. Example :- Rusting of Iron, the black coating on silver and the green coating on copper are other examples of corrosion.
- 13. RANCIDITY:- When fats and oils are oxidised, they become rancid and their smell and taste change. . Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised.
- 14. CHEMICAL EQUATIONS A word-equation shows change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products, and shows the direction of the reaction.
- 15. CHEMICAL EQUATION Chemical equations can be made more concise and useful if we use chemical formulae instead of words. A chemical equation represents a chemical reaction.
- 16. BALANCED CHEMICAL EQUATIONS:- The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction.
- 17. Why should a magnesium ribbon be cleaned before burning in air? 2Mg + O2 2MgO MgO + H2O Mg(OH)2
- 18. WRITE EQUATION AND BALANCE IT:- Hydrogen gas combines with nitrogen to form ammonia. 3H2 + N2 2NH3 Hydrogen sulphide gas burns in air to give water and sulpur dioxide. 2H2S + 3O2 2SO2 +2H2O Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.