Discussion Forum

1)

When 100 mL of 1.0 M  HCl was mixed with 100mL   of 1.0 M NaOH in an insulated beaker at constant pressure, a temperature increase of 5.7° C was measured for the beaker and its contents (Expt. I). Because the enthalpy of neutralisation of a strong acid with a strong base is a constant  (-57.0kJmol-1), this experiment could be used to measure the calorimeter constant. In a second experiment  (Expt.2), 100mL  of 2.0 M  acetic acid  (K_a = 2.0 x 10^-5) was mixed with 100 mL of 1.0 M   NaOH  (under identical conditions to  Expt. 1)  where a temperature rise of 5.6° C was measured.

Enthalpy of disscociation (in KJmol^-1) of acetiic acid  obtained from the Expt. 2 is 

Answer:

Option A

Explanation:

Let C JK^-1 be the heat energy capacity of the calorimeter.

                        Maa of solution

             $=200 mL \times 1g mL^{-1}=200g$

 Heat evolved  in Exp.I

    =   $57\times 1000\times 0.1(mol)=5700J$

        $\Rightarrow  5700J=(200\times 4.2+C)\times 5.7$

                 $\Rightarrow  100=200\times 4.2+C$  ......(i)

Let x kJ/mol  is  heat evolved in neutralisation of acetic acid.

 $\Rightarrow x\times 1000\times 0.10=(200\times4.2+C)\times5.6$

             $\Rightarrow \frac{x\times 100}{5.6}=200\times4.2+C$         ...........(ii)

From (i)  and (ii): x=56 kJ/mol

    $\Rightarrow$       Enthalpy  of ionisation of acetic acid 

                                    = -56-(-57)= 1 kJ/MOL