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imp question for chemical reaction and equation

                                   Chemical Reaction and Equation


Q1. What do you mean by chemical reaction?

Ans  A chemical reaction is a process which transforms one or more substances into new substances. During chemical reaction new substances with new properties are formed.

                 CaO + H2O —–  Ca(OH)2


Q2. How can we indicate that a certain chemical reaction takes place?

Ans  The chemical reactions are indicated by the following observations:

a.        The chemical reaction takes place by the change of state.

b.       The chemical reaction occurs by change of colour.

c.        It occurs with the evolution of gas.

d.       It occurs with evolution or absorption of heat energy.


Q3. Define chemical equation?

Ans Achemical equation is a statement used in chemistry to represent a chemical reaction.

                   CaO + H2O —–  Ca(OH)2


Q4. Define reactants?

Ans The substance which take part in the chemical reaction are called reactant.

          CuSO4 + Fe——- Cu + FeSO4


Q5. Define product?

Ans  The substances which are formed as a result of chemical reaction are called products.

         CuSO4 + Fe——- Cu + FeSO4


Q6. What do you mean by symbol equation?

Ans A shorthand representation of a chemical change in terms of symbols and formulae of substance involved in it.

        CuSO4                  +     Fe         ——-         Cu    +   FeSO4

    Copper Sulphate              Iron                                Copper     Iron Sulphate


Q7. Write the steps to write chemical equation.


a.        The symbols and formulae of reacting substances are written on the left hand side with + sign between them.

b.       The symbols and formulae of various products are written in the right hand side with + sign between them.

c.        An arrow sign or sign of equality is put between the reactants and products.


Q8. What do you mean by balanced chemical equation?

Ans  A chemical equation which has an equal number of atoms of each element in the reactants and products are called balanced chemical equation.


Q9. What do you mean by unbalanced chemical equation?

Ans An equation in which the number of atoms of each kind are not equal in the reactants and products is called unbalanced chemical equation or skeletal equation.


Q10. What is hit and trial method of balancing equation?

Ans  In this method we make trials to balance the equation by using the smallest whole number coefficients.


Q11. How hit and trial method works in balancing the chemical equation?


i.       Write the skeleton equation with correct formulae of reactants and products.

ii.      Count the number of atoms on both side of the equation and make them equal by multiplying the formulae.

iii.    To equate the number of atoms on both sides

iv.    Then pick the second element to balance the partly balanced equation.

v.     Atoms of elementary gases are lastly balanced.

vi.    In inonic equation the charges must also be balanced in addition to the balancing the atoms.


Q12. Define Endothermic reaction?

Ans The chemical reaction which occur with the evolution of heat along the formation of products are called exothermic reaction.      Reactants  ———  Products  +  Heat

                C + O2    _———–  CO2  +  Heat


Q13. Define endothermic reation?

Ans The chemical reaction which occurs with the absorption of heat are called endothermic reaction.

                  Reactant  + Heat  ———  Product

           N2 + O2 + Heat ——  2NO


Q14. Define Combination reaction

Ans  The reactions in which two or more substances combine to form a single substance are called combination reactions.

           2Mg + O2  ——— 2MgO


Q15. Define Decombination reaction?

Ans  The reaction in which a compound breaks up into two or more simpler substances are known as decomposition reaction.

          2FeSO4  ————  Fe2O3 + SO2 + SO3


Q16. What is displacement reaction?

Ans  The chemical reaction in which one element takes the place of the another element in a compound are called displacement reaction.

            Zn  +  H2SO4                       ZnSO4  +   H2

Q17. What is double displacement reaction?

Ans  The reaction in which two compounds react to form two other compounds by mutual exchange of atoms or group of atoms are called double displacement reaction.

           Ag NO3  +  NaCl                     AgCl  +   NaNO3


Q18. What is reactivity series?

Ans   The arrangement of metals in the decreasing order of their reactivities is called reactivity series.


Q19. Write the decreasing order of reactivity series.

Ans                                                                                                                                        (Most Reactive)

 Potassium                                                             K                                                     

                 Sodium                                                                 Na

                Barium                                                                    Ba

                Clacium                                                                  Ca

                Magnesium                                                           Mg

                Aluminium                                                             Al

                Zinc                                                                        Zn

                Iron                                                                         Fe

                Cobalt                                                                    Co

                Nickel                                                                     Ni

                Tin                                                                          Sn

                Lead                                                                       Pb

                Hydrogen                                                              H

                Copper                                                                   Cu


                Silver                                                                      Ag

                Gold                                                                        Au

                Platinum                                                                 Pt


Q20. What is redox reaction?

Ans  When oxidation and reduction occur side by side such reactions are called redox reaction or oxidation reduction reaction.


                   CuO   +     H2                         Cu    +     H2O


Q21. What is oxidized reaction?

Ans   If the substance gain oxygen during a reaction it is said to be oxidized.


Q22. What is reduced reaction?

Ans   If a substance loses oxygen during a reaction it is said to be reduced.


Q23. What is oxidation reaction?

Ans   It is a reaction which involves the addition of oxygen and removal of hydrogen.

                4Na   +  O2                       2Na2O


Q24. What is reduction reaction?

Ans   It is a reaction which involves the addition of hydrogen or removal of oxygen.

                 H2  +  S                             H2S


Q25. What is oxidizing agent?

Ans  It is a substance which gives oxygen or removes hydrogen and causes the oxidation of the other substance.


Q26. What is reducing agent?

Ans  It is a substance which gives hydrogen or removes oxygen and causes reduction of the other substance.


                                CuO   +     H2                         Cu    +     H2O


Q27. What do the notations (s), (l), (g) stands for in a chemical reaction?

Ans   The notations (s) stands for solid, (l) stands for liquid, (g) stands for gas.


Q28. What is the necessity of balancing chemical equation?

Ans  According to law of conservation of mass, matter can neither be created nor be destroyed in a chemical reaction. This         means that the total mass of thee reactants and products must be same.


Q29.  Why should magnesium ribbon be cleaned before burning in air?

Ans Magnesium ribbon is cleaned before burning so that coating of impurities formed on its surface is removed and it becomes pure magnesium


Q30.  A solution of a substance X is used for white washing. Name the substance X and write its formula. Write the reaction of this substance with water.

Ans  Calcium oxide or quick lime. CaO.

         CaO + H2O —–  Ca(OH)2


Q31. Why is double the amount of gas collected in one of the test tubes in electrolysis of water than the amount collected in the other? Name the gas.

Ans  Water decomposes to give hydrogen and oxygen. 2H2O ====== 2H2 + O2

         For 2 vol. of  H2O, 2 vol. of H2 and one vol. of O2 are produced. Therefore the amount of hydrogen gas collected in the test tube is double of the amount of oxygen collected in the other test tube.


Q32. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Ans  Iron displaces copper ions from the copper sulphate solution. Therefore the colour of the solution fades.

         CuSO4 + Fe——- Cu + FeSO4


 Q33. What is balanced chemical equation? Why should chemical equation be balanced?

Ans  A chemical equation which has equal number of atoms of each element in the reactants and products is called balanced chemical equation. The chemical equation must be balanced because according to the law of conservation of mass. Total mass of the reactants before the chemical reaction must be equal to the total mass of the products after the chemical reaction.


Q34.  Why respiration is considered an exothermic reaction? Explain.

Ans  During respiration we need energy. We get this energy from food. During digestion food is broken down into simpler substances like glucose. This glucose combines with oxygen in the cells of our body producing energy. This reaction is called respiration. Thus during respiration energy is released and hence it is considered an endothermic reaction.

        C6H12O6 + 6O2 ———  6CO2 + 6H2O + ENERGY

Q35. Why are decomposition reaction called the opposite reactions? Write equation for these reactions.

Ans The decomposition reaction are those in which a compound breaks up into two or more simpler substances.

         CaCO3  ————  CaO + CO2

        2H2O ———-  2H2 + O2

        Combination reaction are those reaction in which two or more substances combine to form single substances.

         2H2 + O2  ———–   2H2O

        NH3 + HCl ———- NH4Cl


Q36. In the refining of silver, the recovery of silver nitrate solution involved displacement by copper metal. Write the reaction.

Ans  2AgNO3 + Cu ——– 2Ag + Cu


Q37.What do you mean by precipitation reaction? Explain with example.

Ans The reaction in which precipitates are produced are called precipitation reaction.

        2AgNO3 + BaCl ——- 2AgCl + Ba(NO3)2


Q38. Explain the following in terms of gain or loss of oxygen with example?

Ans  Oxidation: It is a reaction which involves addition or gain of oxygen.

          4Na + O2 —–   2Na2O

           2Cu + O2 —— 2CuO

        Reduction: It is a reaction which involves loss or removal of oxygen.

           CuO  + H2   —-  Cu  + H2O

          ZnO +  C ——  Zn + CO


Q39. A shiny brown coloured element X on heating in air becomes black in colour. Name the element X and the black coloured compound formed.

Ans  The element X is copper, CuO. And black coloured coating is of copper oxide.


Q40. Why do you apply paint on iron articles?

Ans  To protect iron article from rusting or corrosion.


Q41. Oil and fat containing food items are flushed with nitrogen. Why?

Ans  Oil and fats containing articles are flushed with nitrogen to prevent them from getting oxidized. This will protect them from becoming rancid.


Q42. Explain the term:


1.        Corrosion: It is a process of deterioration of a metal as a result of its reaction with air or water present in the environment. This process is called corrosion. Almost all the metals except noble metals such as gold, platinum.

2.        Rancidity: When fats and oils are exposed to air they get oxidized and become rancid and their smell and colour changed. This phenomenon is called rancidity.


Q43. How do we prevent Rancidity?

Ans  To prevent rancidity of food materials containing fats and oils., certain substances are added which prevent oxidation and hence rancidity. Like some food materials are stored in air tight containers or the bags containing the food materials (like chips) are flushed with inert gas such as nitrogen to prevent them from getting oxidized.



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