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Copy of Qualitative Analysis of Cations Lab Report
Transcript of Copy of Qualitative Analysis of Cations Lab Report
When hydrochloric acid is added, the chloride ion only precipitates three ions:
Most chloride salts are soluble with chloride:
Lead chloride is the most soluble of the three precipitates, so it is the only one that will dissolve in hot water.
black thick liquid
Silver chloride makes a soluble complex ion with the excess ammonia added to the solution.
Silver precipitates again with chloride when chloride replaces ammonia in the equation, turning the complex ion into a precipitate.
Mercury(I) chloride goes through redox. Mercury(I) is reduced by ammonia to form liquid elemental mercury and oxidized by ammonia to form a white precipitate with mercury(II).
Most ions are insoluble with sulfates, including lead(II), so a white precipitate forms.
Aluminum and zinc will form soluble complex ions with the excess hydroxide.
Iron, bismuth, and copper all precipitate because they are insoluble with hydroxide.
Manganese(II) and cobalt(II) precipitate as an insoluble hydroxide, but quickly oxidize to Manganese(IV) and cobalt(III) with hydrogen peroxide, still staying as precipitates.
The positive hydrogen atoms attatch to the negative hydroxides to form water, leaving the copper, iron, and bismuth ions in solution, dissolving the precipitate.
Magnesium(IV) oxide and cobalt(III) hydroxide do not react with the acid. They only dissolve once there is a reducing agent, so it can oxidize.
As hydrogen peroxide is added, manganese(IV) and cobalt(III) will oxidize and produce oxygen gas. The hydroxides and hydrogen atoms will come together to form water. Manganese(IV) and cobalt(III) will change from brown and black solids to ions in solution.
Manganese will oxidize again as the bismuthate ions act as the oxidizing agent to form purple permanganate ion.
Cobalt comes out of solution when potassium nitrite is added to it. It forms a yellow precipitate and a yellow toxic gas forms, nitrogen oxide and red-brown nitrogen dioxide.
Copper reacts with excess ammonia, forming a complex ion. It turns from a light blue to a dark blue liquid.
Bismuth reacts with ammonia forming an insoluble hydroxide and iron reacts with hydroxide and forms an insoluble hydroxide.
blood red liquid
Both bismuth and iron are soluble in sulfuric acid, so the percipitate dissolves. The hydroxides and hydrogen atoms join to form water, leaving iron and bismuth ions in solution.
Iron becomes a deep red complex ion when excess thiocyanate ion is added.
Bismuth percipitates out when chloride ions are added because it forms bismuth oxychloride, a white solid.
Tin(II) chloride is the reducing agent added, causing bisuth to oxidize and turn into a black metallic solid.
Aluminum and zinc become ions in solution again after adding ammonia and nitric acid. Zinc then reacts with the excess ammonia and forms a soluble complex ion, while aluminum forms an insoluble hydroxide that is a transluscent, gelatinous precipitate.
transluscent, gelatinous precipitate
(if completely pure)
Acid is added in order for the aluminum ion to go back into solution. Then, ammonia is added and aluminum hydroxide precipitates again. Aluminon, a red dye, is added to help see the precipitate better.
Adding acid will separate the zinc ion from the complex ion, allowing it to float in solution. When potassium hexacyanoferrate(II) is added, it forms a precipitate with zinc.
Hierarchy is needed in order to confirm only one ion
at the end of the branch. It’s hard to distinguish which ion is in a clear solution, or which ion formed a white precipitate. In addition, many ions react the same with acid or base. However, certain types of acids or bases cause some ions to react slightly differently. Therefore, scientists take advantage of those unique reactions to separate the ions. The most unique reactions are done towards the beginning of the lab. For example, chloride that only precipitates with three ions is done first in order to quickly separate some ions. The goal is to isolate one ion, so the ions slowly go through small to smaller groups as more reactions are done to it until it finally stands alone. Solubility rules and color change are especially taken advantage of during this lab in order to distinguish one ion from another, leading us to confirming the existence of all ten cations in the end.
It is imperative to wash the precipitates before proceeding to the next step. If you don’t, then the lab won’t go as you planned. You’ll have other unwanted ions that will cause your reactions to change. In some instances, you will get precipitates of the wrong color or precipitates when you wanted a clear solution. In order for things to go as planned, only the ions specified should be in the reaction.
Copper precipitates when hexacyanoferrate(II) is added. It replaces the ammonia and changes it from a complex ion, to a solid.