Chemical Reactions and Equations

IMPORTANT NOTES

1. The changes which take place in substances can be broadly classified as physical changes and chemical changes.

2. During a physical change, only the state of the substance changes, but not its chemical composition. Furthermore, a physical change is a temporary change, which can be reversed by changing the physical experimental conditions. At the same time no net energy is absorbed or given out.

3. During a chemical change, the state as well as the chemical composition of a substance changes. Furthermore, a chemical change is a permanent change, which cannot be reversed by changing the physical experimental conditions. At the same time, either energy is absorbed or given out.

4. Whenever, a chemical change occurs, the process which takes place, is called a chemical reaction. 

5. During a chemical reaction one or more of the following changes occur :

(i) Energy (generally in the form of heat or light) is either absorbed or given out. 

(ii) New substances are formed. 

(iii) The state of the reacting substances changes. 

(iv) There may be a change in colour of the reacting substances. 

(v) The temperature of the reacting substances changes. 

(vi) A gas may be evolved during the chemical reaction. 

(vii) The substances formed during a chemical reaction do not change back to the original substance, when the physical experimental conditions are altered.

6. A chemical equation which represents a chemical reaction briefly in words is called a word equation.

7. The substance/substances which take part in a chemical reaction are called reactants.

8. The new substance/substances formed as a result of chemical reactions, are called products. 

9. A plus sign is put in between reactants or products, if their number is two or more. 

10. An arrow ( -->) is put between reactants and products. This sign is read as “to yield” or “to form”. 

11. The direction of the arrow points in the direction in which the reaction proceeds. 

12. A chemical equation is a statement that describes a chemical reaction in terms of symbols and formulae. 

13. A chemical equation expressed in symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to number of atoms of the same elements towards the side of the products, is called a skeletal equation or an unbalanced equation. 

14. An unbalanced equation is unacceptable or is no equation, because, it goes against the law of conservation of mass. 

15. A chemical equation in which the number of atoms of each element are same on the side of reactants and products, is called a balanced chemical equation. 

16. A balanced chemical equation tells us : 

(i) about the substances taking part in a chemical reaction and the products formed. 

(ii) about the symbols and formulae of the reactants and products. 

(iii) about the number of atoms or molecules of the reactants and products involved in the chemical reaction. 

(iv) about the weights of the reactants and products involved in the chemical reaction. 

17. A balanced chemical equation does not tell us : 

(i) the physical state of the reactants and products.

(ii) whether the reaction will come to completion or not. 

(iii) about the speed of the chemical reaction. 

(iv) about the physical conditions which bring about the chemical reaction. 

(v) about changes, such as precipitation, change in colour, evolution of heat, light, etc., during the chemical reaction. 

18. When one or more substances (elements or compounds) undergo a chemical change, with the absorption or release of energy (generally, heat energy) so as to form one or more products, then the change which takes place, is called a chemical reaction. 

19. Chemical composition reaction : When two elements or compounds react chemically to form a single new compound, the chemical reaction which takes place is called a chemical composition reaction or a chemical combination reaction.

 A + B -->AB

20. Chemical decomposition reaction : When a chemical compound decomposes on heating or absorbing some other kind of energy, so as to form two or more new substances (elements or compounds), then the chemical reaction which takes place is called a chemical decomposition reaction or a chemical decombination reaction. 

AB --> A + B 

Chemical decomposition reaction can be further classified into three types.

(a) Thermal decomposition reaction : When a chemical compound decomposes on heating so as to form one or more substances (elements or compounds), then the chemical reaction is called a thermal decomposition reaction.

(b) Photo-decomposition reaction : When a chemical compound decomposes on absorbing light energy, so as to form two or more different substances, then the reaction which takes place is called a photo-decomposition reaction.

(c) Electrochemical reaction : When a chemical compound in an aqueous or fused state decomposes into two different substances on the passage of electric current then the reaction is called an electrochemical reaction. 

21. Chemical displacement reaction : When a more active element displaces a less active element from its aqueous ionic solution, the reaction which takes place is called a chemical displacement reaction. 

22. Metals arranged in tabular form in the order of their decreasing chemical activity is called metal activity series. 

K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > [H] > Cu > Hg > Ag > Au. 

The above list constitutes the metal activity series in which potassium is the most active metal and gold is the least active metal. 

23. Chemical double displacement reaction : A chemical reaction in which two ionic compounds in their aqueous solutions, react by exchanging their ions/radicals, to form two new compounds is called a chemical double displacement reaction. Chemical double displacement reaction can be further classified into two kinds.

(a) Precipitation reaction : When aqueous solutions of two ionic compounds react by exchanging their ions/radicals, to form two or more compounds, such that one of the products formed is an insoluble salt, and hence, forms a precipitate, the double displacement reaction is said to be a precipitation reaction. 

(b) Neutralisation reaction : When an aqueous solution of an acid reacts with a base (alkali) by exchanging their ions/ radicals to form salt and water as the only products, the reaction which takes place is called a neutralisation reaction. 

24. Exothermic reactions : A chemical reaction which proceeds with the release (evolution) of heat energy, is called an exothermic reaction. 

25. Endothermic reaction : A chemical reaction which proceeds with the absorption of heat energy, is called an endothermic reaction. 

26. Oxidation reaction : When a substance gains oxygen or loses hydrogen, the reaction taking place is called an oxidation reaction. 

27. Reduction reaction : When a substance gains hydrogen or loses oxygen, the reaction taking place is called as reduction reaction. 

28. Corrosion of metals : Formation of layers of undesired compounds, such as metallic oxides or hydroxides on the surface of metals is called a corrosion of metals. 

29. Rusting : Slow conversion of iron into hydrated ferric oxide, in the presence of moisture and air is called rusting. 

30. Rust : The flaky, non sticky brown powder formed on the surface of iron, when iron is exposed to moist air, is called rust. 

31. Conditions for rusting : Iron should be exposed to : (i) water, (ii) air, at the same time. 

32. Rancidity : The oxidation of food materials, so that they become stale and start smelling is called rancidity.