Zinc (Zn) - Chemical Properties & Reactions
Chemical Properties Overview
Zinc (Zn) is a d-block element with atomic number 30. It exhibits a variable oxidation state but predominantly exists in the +2 oxidation state due to its stable $d^{10}$ electronic configuration.
- Reactivity Series Position: Zinc is placed above hydrogen in the electrochemical (reactivity) series, indicating its ability to displace hydrogen from dilute acids and steam.
- Electronegativity: Its electronegativity on the Pauling scale is approximately 1.65, suggesting a tendency to form covalent bonds with more electronegative elements, though it primarily forms ionic compounds in its +2 state.
- General Reactivity: Zinc is a moderately reactive metal. It is amphoteric, reacting with both acids and strong bases to produce hydrogen gas. It readily loses its two valence electrons to form $Zn^{2+}$ ions.
Action of Air and Oxygen
Zinc reacts with air/oxygen upon heating, forming zinc oxide. At room temperature, a thin, protective layer of zinc oxide forms on its surface, preventing further corrosion (passivation).
- Reaction with Air/Oxygen (upon heating): $2Zn(s) + O_2(g) \xrightarrow{\text{heat}} 2ZnO(s)$ (Zinc oxide is yellow when hot and turns white on cooling.)
Action of Water and Steam
Zinc’s reaction with water depends on the temperature.
- Reaction with Cold Water: Zinc does not react with cold or boiling water.
- Reaction with Steam: When heated to red-hot, zinc reacts with steam to form zinc oxide and hydrogen gas. $Zn(s) + H_2O(g) \xrightarrow{\text{heat}} ZnO(s) + H_2(g)$
Action of Acids and Bases
Zinc, being an amphoteric metal, reacts with both acids and strong bases.
Action of Acids
Zinc reacts with dilute acids to produce hydrogen gas. With oxidizing acids, the products vary depending on the concentration of the acid.
- With Dilute Hydrochloric Acid (HCl): $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$
- With Dilute Sulfuric Acid (H_2SO_4): $Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$
- With Very Dilute Nitric Acid (HNO_3): $4Zn(s) + 10HNO_3(very\ dilute) \rightarrow 4Zn(NO_3)_2(aq) + NH_4NO_3(aq) + 3H_2O(l)$ (Note: The product of reduction of nitrate varies with concentration. With less dilute HNO3, oxides of nitrogen like NO or N2O might be formed.)
- With Concentrated Sulfuric Acid (H_2SO_4) (Hot): $Zn(s) + 2H_2SO_4(conc.) \rightarrow ZnSO_4(aq) + SO_2(g) + 2H_2O(l)$
- With Concentrated Nitric Acid (HNO_3) (Hot): $Zn(s) + 4HNO_3(conc.) \rightarrow Zn(NO_3)_2(aq) + 2NO_2(g) + 2H_2O(l)$
Action of Bases (Alkalis)
Zinc reacts with strong alkalis (e.g., NaOH, KOH) to form soluble zincate complexes and hydrogen gas.
- With Sodium Hydroxide (NaOH): $Zn(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow Na_2Zn(OH)_4 + H_2(g)$ (Sodium tetrahydroxozincate(II))
Key Laboratory Test/Identification Reactions for $Zn^{2+}$ Ions
Zinc ions ($Zn^{2+}$) are typically identified in qualitative analysis through their characteristic reactions with specific reagents.
- Reaction with Sodium Hydroxide (NaOH) Solution:
- Observation: A white gelatinous precipitate is formed, which is soluble in excess sodium hydroxide solution.
- Equations:
- $Zn^{2+}(aq) + 2OH^-(aq) \rightarrow Zn(OH)_2(s)$ (White precipitate of zinc hydroxide)
- $Zn(OH)_2(s) + 2OH^-(aq) \rightarrow [Zn(OH)_4]^{2-}(aq)$ (Soluble, colourless tetrahydroxozincate(II) complex ion)
- Reaction with Ammonium Hydroxide (NH_4OH) Solution:
- Observation: A white gelatinous precipitate is formed, which is soluble in excess ammonium hydroxide solution.
- Equations:
- $Zn^{2+}(aq) + 2NH_4OH(aq) \rightarrow Zn(OH)_2(s) + 2NH_4^+(aq)$ (White precipitate of zinc hydroxide)
- $Zn(OH)_2(s) + 4NH_3(aq) \rightarrow [Zn(NH_3)_4]^{2+}(aq) + 2OH^-(aq)$ (Soluble, colourless tetraamminezinc(II) complex ion)
- Reaction with Hydrogen Sulfide (H_2S) Gas in Alkaline Medium:
- Observation: A white precipitate of zinc sulfide is formed when H_2S gas is passed through a neutral or ammoniacal (alkaline) solution containing $Zn^{2+}$ ions. No precipitate forms in acidic medium.
- Equation: $Zn^{2+}(aq) + H_2S(g) + 2NH_4OH(aq) \rightarrow ZnS(s) + 2NH_4^+(aq) + 2H_2O(l)$ (White precipitate of zinc sulfide)