waecE-LEARNING
Chemistry Paper 2,Nov/Dec 2012  
Questions:   1 2 3 4 7 8   Main
General Comments
Weakness/Remedies
Strength







































































Question 3

 

(Question 3

                                                                                                                         

(a)        (i)     What is an acid-base indicator?
           (ii)     When is an indicator said to be most suitable for an acid-bas titration?
          (iii)     Consider the acid-base titration reaction represented by the following equation:
                                    NaOH(aq) + CH3COOH(aq)           CH3COONa(aq) + H2O(l)
            I.          Name a suitable indicator for the reaction.
            II.        Explain briefly your answer in 3(a)(ii) I.                                

                                                                                                                         

(b)        (i)         Write the ionic equation for the reaction between zinc and silver trioxonitrate (V)
solution.
            (ii)        Which type of reaction occurs between zinc and silver trioxonitrate (V) solution?
                        Give a reason for your answer.

(c)        (i)         Consider the reaction represented by the following equation:
                        CaCO3(s) + 2HCl(aq)       CaCl2(aq) + CO2(g) + H2O(l).      
                        State three factors that could increase the rate of the reaction.

  1.  Explain briefly the observation that increase in temperature generally   

increases the rate of reaction. 

  1.  (i)        What is solubility?

(ii)      Distinguish between saturated solution and unsaturated solution.

  1. A saturated solution of volume 10 cm3 yielded 0.06 g of its dry salt at 25oC.

           Calculate the solubility of the salt in gdm-3.
This question was fairly attempted by candidates.  In (a), candidates were unable to correctly define an acid-base indicator and were unable to explain the suitability of an indicator for a particular acid-base titration.  Some candidates used colour in acid and base as a bases for defining indicators instead of changes in pH.
In (b), a majority of the candidates could not write ionic equations although a few of them could identify the reaction as displacement/redox.
In (c), most candidates gave general factors instead of being specific e.g. “concentration” instead of “increase in concentration of HCl” “surface area.” instead of surface area of CaCO3”.
In (d), candidates did not give the correct/complete definition of solubility.  In defining solubility, some candidates lost marks for writing “in 1dm3 of solution” instead in 1 dm3 or solvent.

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OBSERVATION

 The expected answers include:
3.
(a)        (i)         An acid-base indicator is a weak acid or weak base (or organic compound) that has
one colour in acid medium and another colour in alkaline medium. 
                       OR
An acid-base indicator is a weak acid or weak base whose colour in the dissociated
form is different from the colour in the undissociated form.
                                                                                   
            (ii)        An indicator is suitable, when it changes its colour (sharply) at the equivalence      
            point of the titration reaction.
(iii)       I.          Phenolphthalein         
            II.        CH3COONa formed in the reaction is hydrolysed in water to give
excess hydroxide ions.           
The resulting solution at equivalence point is alkaline and phenolphalein    changes colour in alkaline medium.
                                                                                                                       
(b)        (i)         Zn(s) +2Ag+(aq) → Zn2+(aq) + 2Ag(s)      

(ii)        Redox reaction because there is a transfer of electrons from zinc to Ag+ .
OR 
Displacement because zinc displaces Ag+ ions from its aqueous solution
                                                                                                           
(c)        (i)         -  temperature;                   
- concentration of HCl    
- surface area of CaCO3  

            (ii)        Increase in temperature results in the particles gaining kinetic energy. 
                        This causes the particles to move faster and collide more frequently.
                        Effective collision is increased hence an increase in rate of reaction.
                       
(d)       (i)         Solubility is the maximum amount of the solute (moles or grams) that will
                        saturate/dissolve in 1.0dm3 of solvent at a particular temperature.   
           
            (ii)        At a particular temperature a saturated solution cannot dissolve any more
                        solute but an unsaturated solution can dissolve more solute.

            (iii)       10 cm3 of saturated solution contains 0.06 g 
                        \1000 cm3 = 1000 x 0.06 g  
                        10
        = 6.0 gdm-3            

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