標題: chemical equilibrium [打印本頁] 作者: 卡迪斯圖達 時間: 09-3-27 10:01 PM
標題: chemical equilibrium
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~作者: skp556 時間: 09-4-2 03:02 PM
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)