[jubba]

Question 3 (45 minutes)
Data: E0
(Ag+/Ag) = 0.800 V log Kstab ([Ag(CN)2]–) = 21.1
E0
(SCE) = 0.285 V F = 96485 C mol–1
pKsp (AgCN) = 15.8 R = 8.315 J K–1 mol–1
pKsp (AgCl) = 9.75 = 8.206 x 10–2 L atm J K–1 mol–1
An electrochemical cell is prepared by placing a silver electrode in 25.00 mL of a neutral
solution containing potassium chloride and potassium cyanide, then joining a standard
calomel electrode with a potassium nitrate salt bridge. The KCl/KCN solution is then
potentiometrically titrated with a standard 0.1000 M silver nitrate solution at 25 °C. The
potentiometric curve obtained (cell potential against burette reading) is shown below.

Throughout this problem, you may assume that protonation of the cyanide ion is negligible.
(a) The endpoints of the reactions taking place during the titration are marked with A, B and C.
Write a balanced ionic equation for each reaction.
(b) What volume of the titrant is required to reach point B?
(c) Calculate the concentrations of KCl and of KCN in the sample solution.
(d) Calculate the emf readings at points A and C in volts.
(e) What is the molar ratio Cl–/CN– in the solution at point C?
(f) What is the molar ratio Cl–/CN– in the precipitate at point C?

can someone give me a hint for part (a)/(b)

[Borek]

What reactions are taking place in the solution? List them.

Now, there is a combination of stoichiometry and given constants required to determine the order of the reactions.

If there are competing reactions - which one will take place first? The one leaving higher concentration of Ag⁺ or the one leaving lower concentration?

[jubba]

Are the reactions

Ag⁺ + Cl^- ---> AgCl

Ag⁺ + 2CN- ---->[ Ag(CN)₂ ]^-

and Ag⁺ + CN^- ---> AgCN

all of them reduce Ag⁺ concentrations so how do we know which one happens first.

edit: UBBC corrected

[Borek]

Reactions are OK.

Now you have to look at complex formation constant and solubility products to find out the order.

Think about it this way. Imagine there is other solution containing I^- and Cl^-. pK_so AgI is about 18, pK_so AgCl is about 10. If you add some Ag⁺ to the solution both AgI and AgCl may form - but if AgCl is forming, concentration of Ag⁺ remaining in the solution is high enough for AgI to precipitate. Thus AgI will precipitate first, as concentration of Ag⁺ left in equilibrium with the AgI will be way too low for AgCl precipitation. In other words - even if there is AgCl precipitate present in the solution containing I^- it will slowly convert to AgI, as concentration of Ag⁺ being in equilibrium with solid AgCl will be high enough for AgI formation - thus equilibrium will be shifted in the direction of AgI. Once all I^- are used for precipitation, excess AgCl will stay in the solution.

As for complex creation - try to imagine what may happen when you add first drops of Ag⁺?

[plu]

Are the reactions

Ag⁺ + Cl^- ---> AgCl

Ag⁺ + 2CN- ---->[ Ag(CN)₂ ]^-

and Ag⁺ + CN^- ---> AgCN

all of them reduce Ag⁺ concentrations so how do we know which one happens first.

edit: UBBC corrected

Technically speaking, the concentration of Ag⁺ in your titration is actually increasing, as can be seen from the increasing emf values.  You start with a Ag⁺ concentration of 0 (there are no silver ions in the solution).  Then, you titrate some in.  The increase in concentration is very slow though of course due to the series of precipitation/complexation reactions constantly taking place in the solution.  Your endpoints are thus indicated by the clear 'jumps' in emf values at points A, B, and C, indicating the complete (or near complete) precipitation/complexation of one of the anion species.  To figure out which anion is being precipitated/complexed at each of the three points, take a look at the pK_sp values you are given.  The lower the pK_sp of a substance, the more it will dissociate into solution.  Conversely, the higher the pK_sp of a substance, the less it will dissociate into solution.  Thus, whichever one of your two pK_sp values is higher is the pK_sp of the compound that would precipitate out of solution first  

[jubba]

basically i thought the complexion would happen first then the precipitation of AgCn and then AgCl based on the equilibrium constants for each reaction.

but how do you work out the value at B based.

I've said at be AgCn stops precipitating is this right?

If so how do you work out  the titre at B

[Borek]

basically i thought the complexion would happen first then the precipitation of AgCn and then AgCl based on the equilibrium constants for each reaction.

Correct.

but how do you work out the value at B based.

I've said at be AgCn stops precipitating is this right?

If so how do you work out  the titre at B

Sorry, I don't understand

To calculate when AgCN precipitation starts and when does it ends, use stoichiometry of all reactions. Use reaction equations to find out which compounds (Ag(CN)₂^-, AgCN, AgCl) are present in the start-A, A-B, B-C and C- ranges.

[jubba]

thanks for your help

are these the right answers

b)4.94
c)[CN-] .01976
[CL-] 0.02024

d)at A 0.31V
C 0.52V

e)10^-6.05

f)1:1 approx

[Borek]

b)4.94

Twice A. OK

c)[CN-] .01976
[CL-] 0.02024

Ratio is OK, haven't checked if the numbers are exact.

d)at A 0.31V
C 0.52V

More or less OK, have you remembered about dilution?

e)10^-6.05

OK

f)1:1 approx

Identical to ratio of starting concentrations.

[jubba]

Thanks heaps