RATE OF REACTION-CHEMICAL EQUILIBRIUM-III : LONG ANSWERS

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1. Explain : Reversible and Irreversible Processes/Reactions.
   
   ANS:
   
   
   
   • A chemical reaction is the one in which reactants react to form products.
   
   
   • If in the same set of conditions of reaction, the products also recombine to form original reactants, then the reaction is said to be reversible reaction.
   
   
   • If in the same set of conditions of reaction, the products do not recombine to form the original reactants, then such reaction is called irreversible reaction.
   
   
   • Example of a REVERSIBLE REACTION :The preparation of ammonia by Haber's process is a reversible reaction because after the formation of ammonia from nitrogen and hydrogen, at some stage, ammonia (the product) starts breaking down into nitrogen and hydrogen (the reactants).
     

     
     700K
     200atm
     
     N_2(g) + 3H_2(g)     ↔      2NH_3(g) + Heat
     iron oxide
     

     
     
     
   
   
   • Example of an IRREVERSIBLE REACTION :Methane burns in air to form carbon dioxide and water but at no stage carbon dioxide and water can react back to form methane.
     

     
     CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(l)

   
   
   • Reversible reactions occur in closed system only.
   
   


2. Explain : Physical Equilibrium with example.
   
   ANS:
   
   
   
   • In equilibrium state when there is a change only in the physical state of reactants and products without any change in their chemical form, the state of equilibrium is called PHYSICAL EQUILIBRIUM.
   
   
   • Let us take an example of evaporation of water (see figure).
     
     
     
     
   
   
   • When water is taken in a closed vessel, at constant temperature, it is slowly converted to water vapour. The concentration of water vapour increases with time. This results in the increase in vapour pressure within the container. Consequently some vapour is converted back to liquid water.
   
   
   • Initially, the rate of evaporation is greater than the rate of liquefaction (condensation) as the vapour pressure is low. but after some time the rate of liquefaction becomes equal to the rate of evaporation. In other words, the number of molecules of liquid water converted to vapour is equal to the number of molecules of vapour converted back to liquid. Thus an equilibrium is established.
   
   
   • The vapour pressure at that point is maximum and is constant (if temperature has not changed).
   
   
   • Since liquid water is converted to water vapour and its chemical structure remains unchanged, this equilibrium is called physical equilibrium.
     

     
     Evaporation
     H₂O_(l)         ↔         H₂O_(g)
     water    Condensation    vapour

   
   
   
   


3. Explain : Chemical Equilibrium with example.
   
   ANS:If during equilibrium state the reactants and products undergo a chemical change with or without a change in their physical state, the state of equilibrium is called CHEMICAL EQUILIBRIUM.
   
   For example, if decomposition of solid calcium carbonate (CaCO₃) by heating it is carried out in a closed vessel, calcium oxide (CaO) and carbon dioxide (CO₂) gas are produced. The products then recombine to form CaCO₃. At one stage the rate of decomposition of CaCO₃ equals that of reformation of CaCO₃ and the reaction appears to have stopped. This is called chemical equilibrium.
   
   CaCO_3(s)     ↔     CaO_(s) + CO_2(g)
   


4. Explain : Rate of Reaction with the help of a graph and Derive the expression for average rate of reaction.
   
   ANS:
   
   
   
   • During any chemical reaction, initially the concentration of reactants is high.
   
   
   • As the reaction proceeds, the concentration of the reactants decreases whereas that of the products increases.
   
   
   • At one stage, the rate of forward reaction equals the rate of reverse reaction and chemical equilibrium is established.
   
   
   • Experimental observations show that, if during a chemical reaction, neither the products are removed nor the reactants are added, then the rate of reaction is greater in the beginning. It then decreases gradually.
   
   
   • The rate of a recation is therefore mentioned as 'Average Rate of Reaction'.
   
   

   
   Decrease in concentration
   of reactants
   Average rate of reaction = ---------------------------
   Time taken for change.
   
   Increase in concentration
   of products
   =--------------------------------
   Time taken for change

   
   
   
   • The unit of rate of reaction is molar/second or molar/ minute.
   
   
   
   
   MATHEMATICAL EXPRESSION :
   Using R for reactant and P for product, we can denote a simple chemical reaction as follows :
   

   
   R   →    P

   
   
   Here 1 mole reactant denoted by [R] gives 1 mole product denoted by [P].
   

   (NOTE: [R] is molar concentration of reactant
   and [P] is molar concentration of product)

   
   
   The decrease in the concentration of reactant [R] and the increase in the concentration of product [P] are equal. Only their magnitudes change, i.e. the average rate of reaction is shown negative in case of reactants since their concentration decreases and the average rate of reaction is shown positive in case of products since their concentration increases.
   

   Suppose [R]₁ = concentration of reactant at time t₁
   [P]₁ = concentration of product at time t₁
   [R]₂ = concentration of reactant at time t₂
   [P]₂ = concentration of product at time t₂
   

   
   Then during time interval Δt = t₂ - t₁,
         the change in concentration of reactant Δ[R]
                           =[R]₂ - [R]₁
      and the change in concentration of product Δ[P]
                           =[P]₂ - [P]₁
   
   And the average rate of reaction, r_av
   

        Change in concentration
   = ---------------------------
        Change in time
   
           Δ[R]
   = - --------
           Δt
   
           Δ[P]
   = + --------
           Δt
   

   
   Thus, the unit of the rate of reaction is molar/second (Ms^-1) or molar/minute.
   


5. Giving the example of the preparation of ammonia, describe reversible reaction and chemical equilibrium. Draw the graph of velocity of reaction v/s time.
   
   ANS:
   
   
   
   • The preparation of ammonia is a reversible reaction.
     
     N_2(g) + 3H_2(g)    ↔    2NH_3(g)
   
   
   • The reaction between nitrogen and hydrogen under suitable conditions can be written as
     
                       forward
     N_2(g) + 3H_2(g)    →    2NH_3(g)
     
     Thus, the forward reaction yields ammonia.
   
   
   • The decomposition of ammonia can be written as :
     
               reverse
     2NH_3(g)    →     N_2(g) + 3H_2(g)
     
     Thus, the reverse reaction yields original reactants, i.e. nitrogen and hydrogen.
   
   
   • We can write both the forward and reverse reactions collectively as
     
     N_2(g) + 3H_2(g)    ↔    2NH_3(g)
   
   
   • Such reactions are carried out in a closed vessel. They never attain completion. When the rate of forward reaction equals the rate of reverse reaction, equilibrium state is established. The following graph represents equilibrium state.
   
   
   • From the graph, it is clear that, at equilibrium,
     Rate of forward reaction = Rate of reverse reaction
   
   


6. Derive the expression for the Equilibrium Constant for the reaction :
   
   A + B → C + D.
   
   ANS:Consider a reversible reaction :
   

   
   V_f
   A + B    ↔    C + D
   V_r
   

   
   

   Where
   V_f = speed of forward reaction
   V_r = speed of reverse reaction

   
   According to the Law of Mass Action,
   
       V_f α [A] [B]
   ∴ V_f = K_f [A] [B]
   

   Where K_f = Velocity Constant
   for forward reaction

   
   Similarly,
   
       V_r α [C] [D]
   ∴ V_r = K_r [C] [D]
   

   Where K_r = Velocity Constant
   for reverse reaction

   
   Now, at equilibrium state, the rate of forward reaction is equal to the rate of reverse reaction.
   ∴ V_f = V_r
   ∴ K_f [A] [B] = K_r [C] [D]
   

   
      K _f     [C] [D]
   ∴ --- = ---------
       K_r     [A] [B]
   
               [C] [D]
   ∴ K_e = ---------
               [A] [B]
   

   
   

   K_f
   where K_e = ----
   K_r
   

   
   K_e is called EQUILIBRIUM CONSTANT.
   
   Thus the equilibrium constant is the ratio of the product of concentrations of products to the product of concentrations of reactants.
   
   If the concentration is denoted in mol/litre, then K_e is written as K_c and if the concentration is denoted in pressures [in case of gaseous substances], then K_e is written as K_p.
   


7. With the help of an experiment explain the concept of Dynamic Equilibrium.
   
   ANS:When the rate of forward reaction equals the rate of reverse reaction chemical equilibrium is established during a reversible reaction. Apparently the process seems to have stopped. Although no change is apparent at equilibrium the reaction has, in fact, not stopped. This can be established experimentally.
   
   As shown in figure 1, flask A contains ammonia and flask B contains heavy hydrogen gas(deuterium) initially. the valve connecting both the flasks is closed. When ammonia in flask A is decomposed, nitrogen and hydrogen are obtained. Thus at some stage undecomposed ammonia, hydrogen and nitrogen are present in flask A. The reaction is reversible so after some time the decomposition reaction appears to have stopped and equilibrium establishes. If the equilibrium is static, then the reaction has really stopped.
   
   To confirm whether the reaction has stopped or not, as shown in figure 2, the valve is opened to allow the gases in both flasks to diffuse on their own. If the reaction has stopped, we cannot expect any change now, but after some time we observe that both the flasks contain all four gases in addition to ammonia containing deuterium, i.e., N₁²H₃. This can happen only if the reaction is still taking place. however, the pressures in both the flasks do not change showing that equilibrium still persists.
   
   This shows the DYNAMIC NATURE of chemical equilibrium, i.e. the equilibrium is not a static process but a dynamic process in which the reactants are being consumed at the same rate as they are reformed from the products.
   


8. Describe various factors affecting the rate of a chemical reaction.
   
   ANS:The major factors affecting the rate of a chemical reaction are:
   
   
   
   1. State of a substance/Area of surface for contact
   
   
   2. Temperature
   
   
   3. Concentration of reactants
   
   
   4. Catalyst
   
   
   
   
   State of substance/Area of surface : [Generally the reactions of solids are slower than those of fluids as solids cannot mix so easily as fluids do.]
   For a solid reactant, the rate is faster if the reactant is in powder form rather than solid block (lump) form because the area of surface for contact is greater in powder form.
   For example : The reaction of an acid with iron powder is faster than its reaction with solid iron piece.
   
   Temperature : Generally, the rate of a reaction increases with the increase in temperature because at high temperature the collision rate of molecules is higher. This results in increase in rate of reaction.
   For example : Sugar dissolves faster in hot water than in cold water.
   
   Concentration of Reactants : According to the Law of Mass Action, the rate of a reaction is directly proportional to the concentration of reactants. Thus, higher the concentration, faster is the rate.
   
   Catalyst :Generally, catalysts increase the rate of reaction.
   For example : In the manufacture of ammonia by Haber's process, iron is used as catalyst to increase the rate of the reaction.
   [NOTE : In the new version of Text Book, the EFFECT OF CATALYST has been deleted. Students need not explain it in answer. The EFFECT OF CONCENTRATION has not been described separately in the book but as we describe it somewhere else in other form it has been included here.]
   


9. Describe various concepts/theories about Acid and Base.
   
   ANS:There were several theories presented to describe acid and base.
   
   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 bitter and they turn red litmus paper to blue.
   
   
   • When an acid reacts with a base, a salt and awter are produced.
   
   
   
   
   According to 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.
   
   
   
   
   Bronsted-Lowry (Proton-Transfer Theory) : This theory tried to remove limitations of Arrhenius' concept as it gave importance to the solvent and H⁺ ion. However, this theory also had its limitations.
   
   Lewis' Theory : This theory is more relevant. Lewis defined acid and base based on the transfer of electron pair. It is general and applies to all substances defined as acid and base.
   


10. Write a detailed explanatory note on p^H.
    
    ANS:
    
    
    
    • In 1909, Sorensen gave the concept of p^H.
    
    
    • p^H is defined as the negative logarithm of hydrogen ion concentration (in mole/litre) to the base 10.
    
    
    • H⁺ ion cannot exist freely.
    
    
    • It combines with H₂O molecule to form H₃O⁺ ion. Therefore p^H is defined as
      
      p^H = -log₁₀ [H₃O⁺]
    
    
    
    • In neutral water hydrogen ion concentration is equal to the hydroxyl ion concentration. This is clear from the following reaction.
      
      H₂O_(l) ↔ H⁺_(aq) + OH^-_(aq).
      
          ∴[H⁺] = [OH^-]
      
          ∴p^H = p^OH
    
    
    
    • It has been found experimentally that at 25°C, the concentration of H⁺ and OH^- ions in neutral water is 1.0 x 10^-7 mol/litre. Thus,
      
      [H⁺] = [OH^-] = 1.0 x 10^-7 mol/litre
      
      Now,
      p^H = -log₁₀ [H⁺]
           = -log₁₀(1.0 x 10^-7)
           = 7
      
      Similarly, p^OH = -log₁₀ [OH^-] = 7
      
      ∴ p^H + p^OH =14
    
    
    
    • In an acidic solution the concentration of H₃O⁺ ion is more and the p^H value of the solution will be less than 7.
    
    
    • In a basic solution the concentration of H₃O⁺ ion is less and the p^H value of the solution will be greater than 7.
    
    
    • In a neutral solution the concentration of H₃O⁺ ion is equal to the concentration of OH^- ion and the p^H value of the solution will be 7.
    
    
    • For accurate p^H measurement, p^H meter is used. For approximate measurement, p^H paper and p^H indicators are used.
    
    
    • The p^H values of strong acids and strong bases can be calculated from their concentrations as they are completely ionized and their concentration equals H⁺ or OH^- ion concentrations.
    
    
    • To calculate p^H values of weak acids and weak bases, their ionization constants(K_a or K_b) must be known along with their concentrations as they do not ionize completely.
    
    


11. Explain giving examples : Energy changes during chemical reaction.
    
    ANS:
    
    
    
    • Generally all chemical reactions are accompanied by energy changes. Heat is either evolved or absorbed during a chemical reaction.
    
    
    • A chemical reaction in which heat is released is called EXOTHERMIC REACTION.
      For example : During combustion of a candle(or any fuel) heat is released.
      
    • Similarly, dissolution of concentrated sulphuric acid or lime in water produces heat.
    
    
    • Neutralisation reaction between a strong acid and a strong base is also exothermic.
    
    
    • A chemical reaction in which heat is absorbed is called ENDOTHERMIC REACTION.
      For example : Dissolution of ammonium chloride (NH₄Cl), sodium chloride (NaCl) or sodium nitrate in water absorbs heat and the test tube feels cool.
    
    
    • Due to this reason, pads soaked in aqueous solution of sodium chloride or ammonium chloride are placed on forehead to absorb heat(from a person's body) during fever.
    
    


12. 
    

    ANS: