Ammonium sulfide ((NH4)2S) is a widely used reagent in qualitative analytical chemistry. To prepare — Physical Chemistry — Kinetics Chemistry Question
Ammonium Sulfide Reagent Analysis
Ammonium sulfide ((NH4)2S) is a widely used reagent in qualitative analytical chemistry. To prepare the reagent, hydrogen sulfide gas is bubbled through a ammonia solution (c = 4 – 5 mol dm–3) and then some water is added. The solution prepared in this way is almost never pure. It can contain either ammonia or ammonium hydrogen sulfide in excess when a lower or higher than stoichiometric amount of gas is absorbed.
* 10.00 cm3 of an ammonium sulfide reagent solution was diluted to 1.000 dm3. 10.00 cm3 of the resulting stock solution was transferred into a distillation flask and ~40 cm3 of water was added. Then, 25.00 cm3 of cadmium nitrate solution with a concentration of 0.1 mol dm–3 was added into the collector flask (where the distilled components would condense). Moreover, 20.00 cm3 of a sulfuric acid solution (c = 0.02498 mol dm–3 ) was added into the distillation flask.
* Approximately one half of the solution in the distillation flask was distilled into the collector flask. (In the collector flask, the formation of a yellow precipitate could be seen.)
* The content of the distillation flask was washed completely into a titration flask. After adding a few drops of methyl red indicator it was titrated with a NaOH solution (c = 0.05002 mol dm–3). The volume of the titrant used to reach the equivalence point was 10.97 cm3.
* Bromine water was added to the solution in the collector flask (the precipitate dissolved), and the excess of bromine was removed by boiling the solution for 15 minutes. Bromine oxidizes all sulfur containing anions into sulfate ions. The hydrogen ions formed in the reactions in the collector flask were neutralized by 14.01 cm3 of a NaOH solution with a concentration of 0.1012 mol dm–3.
Calculate the exact composition of the reagent ammonium sulfide solution.
Model Answer
Sulfuric acid acidifies the solution, so all the sulfide and hydrogen sulfide ions are converted to H2S which is distilled into the cadmium nitrate solution.
The amount of sulfuric acid added is 0.4996 mmol. The amount of NaOH that reacted with the excess sulfuric acid is 0.5487 mmol. So, the amount of the hydrogen ions that reacted is 0.4505 mmol.
Hydrogen ions from sulfuric acid can react in two ways:
S2– + 2 H3O + = H2S + 2 H2O
NH3 + H3O + = NH4+ + H2O (in the case of excess ammonia)
To decide between the two cases let us see what happens in the collector flask:
Cd2+ + H2S + 2 H2O = CdS + 2 H3O +
CdS + 4 Br2 + 12 H2O = Cd2+ + SO4 2– + 8 Br– + 8 H3O+
Thus, 1 mol of H2S causes the formation of 10 moles of H3O + (2 moles from the first and 8 moles from the second reaction).
NaOH reacts with the hydrogen ions formed in the reactions. The amount of NaOH used is 1.418 mmol. This means that there was 1.418 mmol / 10 = 0.1418 mmol of hydrogen sulfide in the collector flask.
Since the hydrogen ion equivalent of 0.1418 mmol hydrogen sulfide (0.2836 mmol) is much less than the hydrogen ions that reacted from the sulfuric acid (0.4505 mmol), we can conclude that some of the sulfuric acid reacted with excess of ammonia.
The amount of ammonia in 10.00 cm3 of the stock solution is 0.1669 mmol.
So, the ammonia concentration of the reagent solution (which is 100 times greater than that of the stock solution) is 1.669 mol dm–3.
Finally, the ammonium sulfide concentration in the reagent solution is 1.418 mol dm–3.