chemical equilibrium

卡迪斯圖達

An experimental procedure for determining the equ. constant of the following reaction is outlined below:
Ag+ (aq) +Fe2+(aq)  ---->/<----- Ag(s) + Fe3+(aq)
Mix 100cm^3 of 0.100M AgNO3 and 100cm^3 of 0.120M FeSO4(NG4)2SO4 ina  fry bottle
Stand for one hour until equilibrium is attained
Filter the reaction mixture
25cm^3 of the filtrate is titrated with 0.050M KNCS and requires 6.10cm^3 of KNCS for complete reaction
Calculate Kc of the reaction

help me~

skp556

First of all ,  please make sure that you know the purpose of using KCNS for titration . In the titration , KCNS first reacts with Ag+ to form AgNCS ,i.e. Ag+ + NCS- ->AgNCS . At the end point , the solution turns red because of the exhaustion of Ag+ ion and subsequent formation of [Fe(CNS)]2+ . The purpose of the titration
is to find out the amount of Ag+ ion present in the filtrate and hence that in the reaction mixture .

In the calculation part : Ag+ + NCS- -> AgNCS
no.of moles of NCS- titrated = 0.05 x 6.1x10^-3 = 3.05x 10^-4 mol
no. of moles of Ag+ ion present in the filtrate is also 3.05x10^-4 mol because of the same mole ratio .

Hence , the amount of Ag+ ion present in the reaction mixture is :
3.05 x 10 ^-4 mol  X [ (100+100) / 25]  ← this is to find the original no. of moles of Ag+ by using the ratio
=  2.44 X 10 ^-3 mol

Initial       :        0.01 mol         0.012 mol                0 mol       0 mol
                            Ag+       +    Fe2+       rever. ->  Ag      +  Fe3+  ,where x is the no.of moles of Fe3+
Equilibrium      : 2.44X10^-3mol  0.012-x mol       x mol             x mol

no. of moles of Ag+ reacted = no. of moles of Fe3+  formed = x
=0.01 -2.44 X10^-3 =7.56 X 10 ^-3 mol

As a result , we can work out the respective concentrations of the above chemical species .
Reactants:
[Ag+] = 2.44X 10 ^ -3 mol /200X 10^-3  = 0.0122 M
[Fe2+] = (0.012 - 7.56 X10^-3 ) /200X 10^-3 = 0.0222 M
Products :
[Fe3+] = 7.56 X 10 ^-3 /200X 10^-3  =  0.0378 M
** Please be reminded that Ag(s) is not needed to be found in order to calculate Kc .
                [Fe3+]                         ( 0.0378M)
Kc  = ---------------------- = -------------------------  =  139.57 mol ^-2dm^6    => 139.6 mol^-2dm^6//
           [Ag+][Fe2+]           (0.0122M)(0.0222M)

[ 本帖最後由 skp556 於 2009-4-2 03:07 PM 編輯 ]