RATE OF REACTION-CHEMICAL EQUILIBRIUM-II : BRIEF ANSWERS

ANSWER IN BRIEF

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1. What is a chemical reaction ? Give example.
   
   ANS:A chemical reaction is a process by which the form of a substance changes chemically, i.e. a substance is converted to another completely different substance.
   Example :
   
   
   
   1. Rusting of iron in humid atmosphere whereby iron metal is converted into iron oxide.
      
   2. When barium chloride reacts with dilute sulphuric acid precipitates of barium sulphate are formed.
      
   
   
2. Explain slow and fast reactions giving examples.
   
   ANS:
   SLOW REACTION : A reaction which takes longer time to complete is called a slow reaction. Rusting of iron, digestion of food and formation of petroleum in earth's crust are example of slow reactions.
   
   FAST REACTION : A reaction which takes very short duration for completion is called a fast reaction. Heating of magnesium in open air, preparation of ghee and preparation of ammonia by Haber's process are examples of fast reaction.
   
   
3. Give the definitions of forward and reverse reactions.
   
   ANS:The reaction in which the reactants react to form products is called FORWARD REACTION.
   
   With the same set of conditions (of forward reaction) if the products combine to form original reactants, then that reaction is called REVERSE REACTION.
   
   
4. What is rate of a reaction ? State its unit.
   
   ANS:The rate of a reaction is defined as the decrease in the concentration of reactants per unit time or the increase in the concentration of products per unit time.
   

   [Change in concentration of
   reactant or product]
   Rate of Reaction = ----------------------------------
   Time taken for change

   
   Its unit is molar/second (M-S^-1) or molar/minute.
   
   
5. Explain physical change giving example.
   
   ANS:During a reaction(process) if the state of the substance changes but there is no change in its chemical constitution, the change is called 'physical change'.
   
   For example: Heating of ice(solid) gives water(liquid) and further heating of water gives water vapour(gas).
   
   
6. Explain chemical change giving example.
   
   ANS: During a reaction(process) if the bonds existing in reactant molecules break and new bonds are formed in the product molecules(with or without physical change), the change is called 'chemical change'.
   
   For example : When a piece of zinc(Zn) metal is placed in aqueous solution of hydrochloric acid(HCl), the bonds of molecule of HCl break and chloride ion combines with zinc ion to form zinc chloride(ZnCl₂) whereas hydrogen ions combine to form hydrogen gas molecules.
   
   
7. Explain : Molarity.
   
   ANS:The number of gram moles of a substance present in one litre of a solution is known as the MOLARITY of that substance. Thus,
   

   [Number of gram moles of
   the substance]
   Molarity   =    ------------------------------
   volume of the solution
   

   
   A solution with 1 molarity contains 1 gram mole of a substance dissolved in 1 litre of solution. Such a solution is called 1 molar solution and its concentration is indicated as 1 M.
   1 molar = 1 mole/litre
   
   
8. Explain in brief the effect of a catalyst on a chemical reaction.
   
   ANS:A catalyst helps to increase the rate of a chemical reaction. For example, in the manufacture of ammonia by Haber's process, iron is used as a catalyst to increase the rate of the reaction.
   

   
   [Fe]
   N_2(g) + 3H_2(g)     ↔      2NH_3(g) + Heat
   [K₂O]
   

   
   
9. Mention the factors affecting the rate of a reaction.
   
   ANS:The factors affecting the rate of a reaction are :
   
   
   
   1. State of the substance/ surface area
      
   2. Concentration of the reactants
      
   3. Temperature
      
   4. Catalyst
      
   
   
10. Explain Robert Boyle's concept of acid and base.
    
    ANS:According to Robert Boyle :
    
    
    
    • Acids are substances which are sour in taste, which turn the blue litmus paper red and liberate hydrogen gas on reaction with metals like Zn or Mg.
      
    • Bases are tasteless and they turn red litmus paper to blue.
      
    • When an acid reacts with a base, a salt and water are produced.
      
    
    
11. Explain Arrhenius' concept (theory) of acid and base.
    
    ANS:
    
    
    
    • According to Swante Arrhenius, acids are substances that produce hydrogen ion (H⁺) whereas bases are substances that produce hydroxyl ion (OH^-) on ionisation.
      
    • For example :
      

      HCl → H⁺ + Cl^-
      NaOH → Na⁺ + OH^-

      
      Thus HCl is an acid whereas NaOH is a base.
      
    • Accordingly, the production of H⁺ ion for an acid and OH^- ion for a base is necessary.
      
    • The theory fails in explaining the basic nature of ammonia (NH₃) as it does not ionise and give OH^-.
      
    • The theory fails because it does not give importance to the solvent. H⁺ ion exists only in a solution and it cannot exist freely.
      
    
    
12. Mention the criteria for chemical equilibrium.
    
    ANS:Following are the criteria for chemical equilibrium :
    
    
    
    • The reaction should occur in a closed system.
      
    • The reaction should be reversible.
      
    • The factors affecting the reaction such as temperature, pressure, concentration of reactants and products should not change.
      
    • The rate of the forward reaction must be equal to that of the reverse reaction.
      
    
    
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    ANS: