Zinc metal is located in 4th period in the periodic table and belongs to the d block. Zinc is an amphoteric element and zinc oxide, zinc hydroxide are also amphoteric compounds. Zinc forms only +2 ion and it is a different factor than other 3d metal ions. Most 3d metals form variable oxidation numbers.
In this tutorial, we will learn followings.
Zinc is widely distributed in earth and is about abundant as copper. Zn is found as sphalerite (ZnS) or zinc blend, and Smithsonite (ZnCO3). Zinc is obtained by roasting ad reduction of ZnO with carbon. Zinc is a shiny white metal with a bluish-gray luster.
Following tests are done to identify Zn2+ ion and they are explained in detail in this tutorial.
Heat a small portion of zinc compound on a charcoal block using a blow pipe, an incrustation of zinc oxide (ZnO) is given. This incrustation is red in hot state and turn to white when it becomes cold. After cooling, moistened with a drop of Co(NO3)2 and heat the mixture again. A green mass of cobalt zincate ( CoZnO2)is given.
Addition a hydrogen sulfide gas (H2S) to aqueous Zn2+ in a basic medium will a white precipitate, zinc sulfide (ZnS). Basic medium is prepared by aqueous ammonia solution.
Add dilute NaOH to Zn2+ aqueous solution. A white gelatinous precipitate, zinc hydroxide ( Zn(OH)2 ) is given. When adding excess NaOH, white precipitate dissolves and colourless solution is obtained.
Addition of aqueous ammonia to the Zn2+ aqueous solution will form the Zn(OH)2 white precipitate. With excess aqueous NH3, that precipitate dissolve and form a colourless tetraamminezinc(ii) ion solution.
Both zinc chloride and sodium chloride are colourless solutions. But identification process of zinc ion and sodium ion is a simple thing to do. Add aqueous sodium carbonate solution to aqueous zinc chloride solution and aqueous sodium chloride solution and observe the differences. A white precipitate forms in one solution which is zinc carbonate. Sodium carbonate is a water soluble compound.Learn qualitative analysis of carbonate ion
Yes. You can. First alkaline both solutions by adding ammonium sulfide because ksp of zinc sulfide is higher. Sodium sulfide is very much soluble in water and form a strong alkaline solution due to instability of sulfide ion in the water.
When hydrogen sulfide is sent to the two aqueous solutions, a white precipitate forms in one solution. You know that white precipitate is zinc sulfide.Learn sulfide ion identification tests and qualitative analysis
First thing you should know is zinc oxide is an amphoteric oxide. Then how we know zinc oxide has amphoteric characteristics and magnesium oxide does not have?
Add both solids to two water samples. You may see two white precipitates are deposited at the bottom of the solutions.
Amphoteric oxides will react with both bases and acids. Due to metal oxides are basic, both oxides will react with base and no use of that using an acid for testing.
So add aqueous NaOH to two solutions. One white precipitate will start to dissolve and form colourless solution. Non-disslovong precipitate is related to the magnesium oxide.Lust of all amphoteric elements, oxides and hydroxides
Both zinc carbonate and magnesium carbonate are white solid compounds and form white precipitates in the water (insoluble). Therefore, we cannot identify them from adding to the water.
You have two solid white precipitates in the laboratory. Now you want to identify them. Also there is a information that these two white precipitates may be zinc sulfide or zinc carbonate. How you are going to test these two compounds?
In both compounds, zinc ion is common. So we have to do the test for anions, sulfide and carbonate. Consider the qualitative analysis for sulfide ion and carbonate ion.
Both ZnCl2 and CaCl2 are colourless solutions. Chloride ion is same for both compounds. So we have to do identify Zn2+ ion and Ca2+
H2S gas is treated with ZnCl2 and CaCl2 solutions in the basic medium. A white precipitate forms in a one solution. That white precipitate is ZnS. Then we know, in what solution Zn2+ ions exist. CaS is soluble in water.
Both Zn2+ and Al3+ ions reacts with NaOH to give white precipitates ( Zn(OH)2 and Al(OH)3 ). Zn(OH)2 and Al(OH)3 are amphoteric hydroxides and again dissolve in NaOH solution to give colourless solutions.
So Zn2+ + NaOH cannot be used to identify Zn+2 ions from Al3+.
The 3d sub shell of zinc is completely filled. Important thing is Zn behave as group IIA metal (alkali earth metal) than like a transition metal.
Zn readily reacts with dilute acids and also reacts with oxygen, sulfur, phosphorous and the halogens upon being heated. Only important oxidation number of zinc is +2.
Questions Asked by Students
When dilute HCl is added to ZnCO3, carbon dioxide gas is emitted producing ZnCl2. With NH3, Zn2+ ion forms colourless [Zn(NH3)4]2+ coordination complex cation.
Zn + 2NaOH → Na2ZnO2 + H2
Zn reacts with aqueous NaOH and produce aqueous Na2ZnO2 and hydrogen gas.
Zn + HCl → ZnCl2 + H2
Zn reacts with dilute HCl and produce ZnCl2 and hydrogen gas.
Zinc carbonate is a white precipitate. With little bit of aqueous NaOH, white precipitate remains unchanged. But with excess NaOH, white precipitate dissolve and produce [Zn(OH)4]2-, a coordination complex.
When sodium hydroxide is added to the aqueous zinc chloride solution, a white precipitate ( Zn(OH)2 ) forms.
ZnCl2 + NaOH &8594; Zn(OH)2 + NaCl
With excess NaOH, white precipitate dissolves and produce colourless aqueous solution.
Zn(OH)2+ NaOH &8594; Na2[Zn(OH)4]
zinc carbonate gives zinc oxide + carbon dioxide ?
When zinc carbonate is heated, it gives zinc oxide and carbon dioxide as products.
ZnCO3 &8594; ZnO + CO2
No. Zinc carbonate forms a white insoluble precipitate in the water.
Zinc carbnate has weak basic characteristics. Carbonate ion is not stable in the water. So it produce some hydroxyl ions in the water to make the aqueous solution basic.
All 3d orbitals are filled in Zn2+ ion. So Zn2+ does not show colours in aqueous solutions and solid. In aqueous soutions, Zn2+ compounds are colourless and at solid state, they are white.
W can test Zn and Al ions from aqueous ammonia solution. With ammonia solution, both Zn and Al ions give white precipitates. With excess ammonia solution, one white precipitate dissolve and give colourless solution. That is related to Zn ion.
Qualitative Analysis for anions and cationsEmission of gases Identify ammonium salts identify d block cations by ammonia solution Identify halides Identify carbonate ions identify chromium compounds Testing for thiosulfate (S2O32-) Testing for phosphate ion