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Chemical Equilibrium and Acids - Bases

Questions - Answers

1. Define pH. What is the pH of 10−8 M HCl?

A: Negative logarithm of activity of Hydrogen ion is called pH.

2. Define equilibrium constant. Write KP, KC and relation between them for the reaction 

A: The ratio of product of molar concentrations of products to the product of molar concentrations of reactants at equilibrium.

3. What is meant by ionic product of water? What is the value of KW at 25°C? Give its units.

A: The product of molar concentrations of H+ ions and OH ions in pure water or aqueous solution.

KW = 1.0 × 10−14 moles2/ lit2

Units = moles2/ lit2
 

4. What is a conjugate acid − base pair. Give the corresponding conjugate acid and base for

a) H2O b) HCO3 and c) HSO4

A: A pair of acid & base differed by a proton is conjugate acid − base pair. Conjugate acid is formed when a proton is gained by the base. Conjugate base is formed when a proton is donated by the acid.

For H2O: Conjugate acid is H3O+, conjugate base is OH

For HCO3: Conjugate acid is H2CO3, conjugate base is CO3−2.

For HSO4: Conjugate acid is H2SO4, conjugate base is SO4−2.
 

5. Explain Lewis acid - base theory with suitable examples.

A: Acid: Any substance that accepts electron pair to form a coordinate covalent bond.

Types of Lewis acids: All cations (CO+3, Fe+3)

* Elements with electron sextet (O, S).

* Molecules with vacant d-orbitals (SF4, SiF4).

* Molecules with incomplete octet (AlCl3, BCl3).

Base: Any substance that donates electron pair to form a coordinate covalent bond.

Types of Lewis bases: All anions (F, Cl).

* Molecules with lone pairs 

* Molecules with multiple bonds (C2H2, C2H4).
 

6. Discuss the industrial synthesis of sulphur trioxide.

A: 2 SO2 (g) + O2 (g) ⇌ 2 SO(g) +Heat

Effect of pressure: According to Lechatlier's principle by increase of P, equilibrium shifts towards right. But at high P, the towers used in the process are corroded due to acidic nature of SO3. So optimum pressure 1.5 to 1.7 atmosphere is used.

Effect of Temperature: As the reaction is exothermic, T must be decreased. But at low T, the rate of the reaction is slow. So optimum T (673 K) is used to get more SO3.

Effect of concentration: Either by addition of SO2 or O2 or both or by the removal of SO3 from the system, equilibrium shifts towards forward direction to give more amount of SO3. V2O5 is used as catalyst.
 

7. Define solubility product.

The solubility of Ag2CrO4 is 1.3 × 10−4 moles/ lit. What is the solubility product?

A: The product of molar concentrations of cations and anions of salt present in a saturated solution.

Ag2CrO4  ⇌ 2 Ag+ + CrO4−2
KSP = (2 S)2 (S) = 4 S3 = 4(1.3 × 10−4)3
          = 8.79 × 10−12

8. What is a bronsted base? Give 1 example.

A: Proton acceptor is called bronsted base.

e.g.: H2O, OH, SO4−2, CO3−2
 

9. What is common ion effect?

A: The suppression of ionisation of a weak electrolyte, when a solution having common ion (of strong electrolyte) added is called common ion effect.

e.g.: Ionisation of CH3COOH is suppressed if CH3COONa is added.
 

10. Calculate the pH of 0.1825% HCl aqueous solution.
A:

                                       = 5 × 10−2
pH = −log[H+] = −log (5 × 10−2)
= −(0.6990 − 2) = − (−1.3010)
= 1.3010

 

11. Calculate the pH for 0.01 M Ca(OH)2 solution.
A: N = [OH] = 10−2 × 2
pOH = −log [OH] = −log [10−2 × 2]
= − [0.3010 − 2] = − [−1.6990]
= 1.6990
pH = 14 − pOH = 14 − 1.6990
      = 12.3010

7(a). Chemical Equilibrium


4 Marks questions:

1. Give important characteristics of chemical equilibrium.
A: Characteristics of Chemical equilibrium:
* Chemical equilibrium can be attained from any direction.
* Chemical equilibrium is dynamic in nature.
* At equilibrium, the rate of forward reaction is equal to the rate of backward reaction.
* At equilibrium both the forward as well as backward reactions take place simultaneously, continuously.
* Addition of a catalyst does not alter the equilibrium but speeds up the attainment of equilibrium.
* At equilibrium, colour, density, concentration, pressure remains unchanged.
* Change in pressure and temperature may change the position of the equilibrium.

2. State law of mass action. Derive Kc for the equilibrium.
A:  xA + yB    ⇌  mC + nD            
Law of mass action: It was stated by Guldberg and Waage. According to this law "The rate of a chemical reaction at a given temperature is directly proportional to the product of active mass of the reacting substances".
           In a hypothetical reaction xA + yB  ⇌   mC + nD
In forward reaction:
                  rf  ∝  [A]x [B]y                               rf = rate of forward reaction
                  rf = Kf [A]x [B]y                          kf = rate constant for forward reaction
In backward reaction:
               rb ∝  [C]m [D]n                     rb = rate of backward reaction
               rb = Kb [C]m [D]n                 kb = rate constant for backward reaction
At equilibrium state rf = rb


            
              Kc = concentration equilibrium constant.

3. Give the relation between Kc, Kp. Write this relation for the following.
a) N2 (g) + 3 H2 (g)   ⇌ 2 NH3 (g)
b) 2 NOCl (g)   ⇌ 2 NO (g) + Cl2 (g)
c) H2 (g) + I2    ⇌ 2 HI (g)
A: Kp = Kc (RT)Δn
a) Kp = Kc (RT)−2       ∴  Kp > Kc
b) Kp = Kc (RT)1        ∴   Kp< Kc
c) Kp = Kc (RT)ο        ∴    Kp = Kc

4. Why is it useful to compare Q and K?
What is the situation when (a) Q = K (b) Q < K (c) Q > K?
A: By comparing Q (Qc or Qp) with K (Kc or Kp), one can predict the direction of the reaction.
When a) Q = K, reaction at equilibrium.
b) Q < K, forward reaction takes place.          

c) Q > K, reverse reaction takes place.

5. State Lechatlier's Principle. What is the effect of pressure, temperature during the synthesis of SO3?
A: If a system at equilibrium is disturbed either by change in concentration, temerature or pressure, then the equilibrium shifts towards the direction in which that effect is nullified or minimised.
Effect of pressure: According to Lechatlier's principle, high pressure favours forward reaction, but the towers used in this process are corroded due to acidic nature of SO3. So optimum pressure 1.5 to 1.7 atmosphere is used.
Effect of temperature: As the reaction is exothermic, T must be decreased. But at low T, the rate of the reaction is slow. So optimum temperature 673 K is used to get more SO3.

2 Marks Questions:

1. What is homogeneous equilibrium? Give one example.
A: The equilibrium in which the reactants and products are in the same phase.
e.g.: N2 (s) + 3 H2 (g)   ⇌ 2 NH3 (g)

2. What is heterogeneous equilibrium? Give one example.
A: The equilibrium in which the reactants and products are in different phases.
e.g.: CaCO3(s)  ⇌  CaO (s) + CO2 (g)

3. At certain temperature, Kc for the reaction SO2 (g) + NO2 (g)  ⇌  SO3 (g) + NO (g) is 16. If initially 1 mole of all the four gases are taken in 1 litre vessel. What are equilibrium concentration of NO & NO2?
A: SO2 (g) +      NO2 (g)  ⇌  SO3 (g) +     NO (g)
    1 Mole           1 Mole             1 Mole           1 Mole        (initial)
     1 − x              1 − x               1 + x              1 + x           (at equilibrium)


   

Equilibrium concentrations of
NO = 1 + x = 1 + 0.6 = 1.6 Moles/lit
NO2 = 1 − x = 1 − 0.6 = 0.4 Moles/lit

4. For the reaction PCl5(g) ----> PCl3 (g) + Cl2 (g), 0.4 moles of PCl5, 0.2 moles of PCl3 and 0.6 moles of Cl2 are taken in a 1 litre flask. If Kc = 0.2, predict the direction in which reaction proceeds.
A:   Q = [PCl3][Cl2] / [PCl5] = 0.2 × 0.6 / 0.4 = 0.3 
            Kc = 0.2
            Q> Kc

  ∴ The reaction proceeds in backward direction.

8 Marks questions:
1. State and explain Lechatlier's principle and apply to the equilibrium.
                       N2 (g)    +   3 H2 (g)   ⇌ 2 NH3 (g)
A: Lechatlier's principle: If a system at equilibrium is disturbed either by change in concentration, temperature or pressure, then the equilibrium shifts towards the direction in which that effect is nullified or minimised.
Effect of pressure: If pressure is increased, the equilibrium shifts in the direction in which volume is decreased (decrease of no. of moles). If pressure is decreased, the equilibrium shifts in the direction in which volume is increased (increase of no. of moles).
Effect of temperature: If the temperature is increased, endothermic reaction is favoured. If the temperature is decreased, exothermic reaction is favoured.
Effect of concentration:
       * Addition of reactants favour forward reaction.
       * Removal of products favour forward reaction.
       * Addition of products favour backward reaction.
       * Removal of reactants favour backward reaction.

Synthesis of NH3 by Haber's process:
N2 (g) + 3 H2 (g)       2 NH3 (g)              ∆H = -92 K.J.
                         
Effect of temperature:
   Forward reaction is exothermic. Temperature must be decreased to favour forward reaction. But practically at low temperature, the rate of the reaction is slow. Hence an optimum temperature 725 to 775 K is maintained in presence of finely powdered iron catalyst and Mo promoter to get more yield of NH3.
Effect of pressure: 3 moles of H2, one mole of N2 reacts to give 2 moles of NH3. By increasing pressure, equilibrium shifts towards lower volume (less no. of moles) side to give more amount of NH3. A high pressure of 200 atm is maintained.
Effect of concentration: Either by addition of N2, H2 or both, or by the removal of NH3 from the system, forward reaction is favoured to give more yield of NH3.

Optimum conditions:
           Temperature: Optimum temperature (725 - 775 K)
           Pressure: High pressure (200 atm)
           Catalyst: Finely powdered Iron
           Promoter: Mo
           Concentration: Addition of N2, H2 in 1 : 3 ratio or removal of NH3

 

2. State and explain Lechatlier's principle and apply to the equilibrium.
                 2 SO(g) + O2 (g)   ⇌ 2 SO3 (g)
A: Lechatlier's principle: If a system at equilibrium is disturbed either by change in concentration, temperature or pressure, then the equilibrium shifts towards the direction in which that effect is nullified or minimised.
Effect of pressure: If pressure is increased, the equilibrium shifts in the direction in which volume is decreased (decrease of no. of moles). If pressure is decreased, the equilibrium shifts in the direction in which volume is increased (increase of no. of moles).
Effect of temperature: If the temperature is increased, endothermic reaction is favoured. If the temperature is decreased, exothermic reaction is favoured.

Effect of concentration:
       * Addition of reactants favour forward reaction.
       * Removal of products favour forward reaction.
       * Addition of products favour backward reaction.
       * Removal of reactants favour backward reaction.
Synthesis of SO3 by contact process:
              2 SO2 (g) + O2 (g)       2 SO3 (g)              ∆H = -189 K.J.
Effect of temperature: Forward reaction is exothermic. Temperature must be decreased to favour forward reaction. But practically at low temperature, the rate of the reaction is slow. Hence an optimum temperature 673 K is maintained in presence of V2O5 catalyst to get more yield of SO3.
Effect of pressure: 2 moles of SO2, 1 mole of O2 reacts to give 2 moles of SO3. By increasing pressure, equilibrium shifts towards lower no. of moles (low volume) side to give more amount of SO3.  But at high pressure, the towers used in this process may be corroded due to acidic nature of SO3. To avoid this problem, optimum pressure 1.5 to 1.7 atmospheres is used.

Effect of concentration: Either by addition of SO2 or O2 or both, or by the removal of SO3 from the system, equilibrium shifts towards forward direction to give more amount of SO3
Optimum conditions:
     Pressure: Optimum pressure (1.5 to 1.7 atmosphere)
     Temperature: Optimum temperature (673 K)
     Catalyst: V2O5
     Concentration: Addition of SO2, O2 or removal of SO3    

7(b). Acids & Bases

4 Marks Questions.

1. Explain Bronsted-Lowry theory of acids & bases.
A: Acid: A Substance that donates proton to another substance eg. HCl, HNO3.
     Base: A Substance that accepts proton from another substance.
            e.g.: NH3, Cl-, HSO4-

Conjugate acid - base pair: The acid-base pair which differs by a proton.
                             NH3 + H2O ⇌ NH4+ + OH-
                            H2O & OH-; NH4+ & NH3 are conjugate acid-base pairs.

Neutralisation: The transfer of a proton from acid to base.
      e.g.: One proton of H2O is transferred to NH3 to form NH4+

2. Explain Lewis theory of acids & bases     
A:  Acid: Any substance that accepts electron pair to form a coordinate covalent bond.
        eg: BF3, BCl3
Types of Lewis acids:
      * All cations : CO+3, Fe+3
      * Elements with electron sextet: O, S
      * Molecules with multiple bonds: CO2, SO2
      * Molecules with available d-orbitals: SF4, SiF4
      * Molecules with incomplete octet: AlCl3, BF3
Base: Any substance that donates electron pair to form a coordinate covalent bond
            eg: H2O, NH3.
Types of Lewis bases:
      * All anions: F-, Cl-
      * Molecules with lone pairs: ROH, NH3
      * Molecules with multiple bonds: C2H2, C2H4

    All bronstead bases are also Lewis bases. For example NH3 accepts a proton to form NH4+ So it is bronstead base, NH3 donate one electron pair so it is Lewis base. But all Lewis acids need not be Bronstead acids as they need not contain protons (eg: Fe+3, Cu+2, SO2)
 

3. What is pH? Find the pH of 0.05 M Ba (OH)2 aqueous solution.
A: Negative value of logarithm (to the base 10) of [H+] ion concentration (moles/lit) in a solution.
                    pH  =  -log10 [H+]
Normality =  N  =  [OH-]  =  2 × 0.05  =  0.1  =  10-1
        ⇒ pOH = -log10 [OH-)  =  -log[10-1]  =  - (-1) = 1
         ∴  pH = 14 - pOH = 14 - 1 = 13.

 

4. What are buffer solutions? Classify them with suitable examples.
A: Buffer solution: The solution whose pH remains constant upon the addition of few ml of strong acid or strong base.
     Acid buffer solution: The solution which is formed by mixing weak acid and its salt with a strong base                            

e.g.: CH3COOH  +  CH3COONa

Base buffer solution: The solution which is formed by mixing weak base and its salt with a strong acid.
                            e.g.: NH4OH  +  NH4Cl

 

5. What is 'common ion effect'? Give its significance.
A: Common ion Effect: The suppression of ionization of a weak electrolyte, when a solution (having common ion with weak electrolyte) of strong electrolyte is added to the solution of weak electrolyte.
Applications of Common ion effect:
      * In precipitation purification of NaCl by passing HCl gas
      * In controlling H+ ion concentration in buffer solution
      * In Analytical Chemistry
      * Precipitation of cations of groups in qualitative analysis.
      * In grallimetric analysis.

 

6. What is "Solubility product? Give its applications.
A: Solubility Product: The product of molar concentrations of cations and anions of salt present in a saturated solution. Solubility product of a Salt Ax By can be explained as follows.
                                 AxBy  ⇌ xA+ + yB-
     Solubility Product (KSP) = [Ay+]x [Bx- ]y

Applications of Solubility Product:
      * In the precipitation of III group hydroxides, sulphides of II and IV group cations.
      * In the manufacture of sodium bicarbonate.

 

7. (a) The solubility product of Salt A B is 10-10 moles2/lit2. What is Solubility of
the Salt?
     (b) The Solubility of Salt A2 B is 2 × 10-3 moles/lit. What is its solubility product?

8. What is conjugate acid base pair? Give the corresponding conjugate acid and base for
a) HSO4 and b) NH3
A: An acid and base differed by a proton is called conjugate acid - base pair.
e.g.: H2O + NH3    ⇌ NH4+ + OH
        Acid1    Base2        Acid2       Base1
a) Conjugate acid of HSO4 is H2SO4
    Conjugate base of HSO4 is SO4−2
b) Conjugate acid of NH3 is NH4+
    Conjugate base of NH3 is NH2

9: Calculate pH of 0.1 M acetic acid having Ka = 1.8 × 10−5.
A: CH3COOH        CH3COO  + H+
              c(1−α)                c.α              c.α
Ka = c.α2

pH = −log [H+]
      = −log [1.34 × 10−2]
      = − [log 1.34 + log 10−2]
      = −[0.1271 − 2]
      = − (−1.87)
      = + 1.87

2 Marks Questions

1. All Bronsted bases are Lewis bases. Explain.
A: NH3 is proton acceptor, so it is Bronsted base.
          NH3 + H+    NH4+
          NH3 is an electron pair donor, so it is Lewis base.


   

2. All Lewis acids are not Bronsted acids. Why?
A: BF3 is electron pair acceptor, so it is Lewis acid.


              
As BF3 can't donate a proton (proton donor is Bronsted acid), it is not a Bronsted acid.

3. What are Arrhenius acids and bases?
A: Acid: A substance that donates H+ in its aqueous solution.
e.g.: HCl (aq)   ⇌ H+ (aq) + Cl (aq)
Base: A substance that donates OH in its aqueous solution.
e.g.: NaOH (aq)  ⇌ Na+ (aq) + OH (aq)

4. Define ionic product of water. Give its value at room temperature.
A: The product of the concentration of H+ and OH ions in water or aqueous solution.
               Kw = 1.0 × 10−14 mole2/lit2 (at 25°C)

       

5. Calculate [H+] for a solution whose pH is 4.75.
A: pH = 4.75
pH = −log10 [H+]
[H+] = 10−pH = 10−4.75
                          = 100.25 − 5 = 100.25 × 10−5
                          = 1.778 × 10−5 M

6. What is the pH of 10−8 M HCl?
A: [H+] = [H+]HCl + [H+] H2O
       = 10−8 + 10−7 = 10−7 [10−1 + 1]
       = 1.1 × 10−7
pH = −log [1.1 × 10−7] = −[log 1.1 − 7 log 10]
                                         = − [0.0414 − 7]
                                         = − [−6.958]
                                         = + 6.96

7. Calculate the pH of 10−8 M NaOH.
A: [OH] = [OH]NaOH + [OH]H2O
                       = 10−8 + 10−7 = 10−7 [10−1 + 1]
                       = 1.1 × 10−7
pOH = −log [1.1 × 10−7] = − [log 1.1 − 7 log 10]
                                            = − [0.0414 − 7] = − (−6.958)
                                            = + 6.96
pH + pOH = 14
∴  pH = 14 − pOH = 14 − 6.96 = 7.04

8. 100 ml of pH = 4 solution is mixed with 100 ml of pH = 6 solution. What is the pH of resultant solution?

pH = −log (5 × 10−5) = −[log 5 − 5 log 10]
                                      = − [0.6990 − 5] = −(−4.3010)
                                      = + 4.3

Posted Date : 29-10-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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