Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

Zn reacts with concentration \(\mathrm{NaOH}\) solution to produce (a) \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{H}_{2}\) (b) \(\mathrm{ZnO}_{2}^{2-}\) only (c) \(\mathrm{ZnO}_{4}^{2-}\) and \(\mathrm{H}_{2}\) (d) \(\mathrm{ZnO}_{2}^{2-}\) and \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

(a) \( \text{ZnO}_2^{2-} \) and \( \text{H}_2 \) are produced.

Step 1: Understand the Reaction

When zinc (Zn) reacts with aqueous sodium hydroxide (NaOH), it can form a zincate ion and hydrogen gas as products. In this reaction behavior, Zn acts as an amphoteric metal.

Step 2: Identify the Products

Zinc, when reacting with concentrated NaOH, generally forms the complex ion known as zincate, \[ \text{Zn(OH)}_4^{2-} \]. However, here, Zn forms \( \text{ZnO}_2^{2-} \) and hydrogen gas \( \text{H}_2 \) is also produced.

Step 3: Match Chemical Products to Options

Among the options provided: (a) \( \text{ZnO}_2^{2-} \) and \( \text{H}_2 \)(b) \( \text{ZnO}_2^{2-} \) only(c) \( \text{ZnO}_4^{2-} \) and \( \text{H}_2 \)(d) \( \text{ZnO}_2^{2-} \) and \( \text{O}_2 \), the reaction aligns with option (a), since both \( \text{ZnO}_2^{2-} \) and \( \text{H}_2 \) are the products.

Key Concepts

Zinc

Zinc is a fascinating element that you'll often encounter in chemistry. It's a metal found in the transition metals section of the periodic table, with the symbol Zn and atomic number 30. Zinc exhibits unique properties because it behaves as an amphoteric metal. This means that zinc can react with both acids and bases to form different products.

This dual behavior is quite interesting. In acidic conditions, zinc can dissolve to form zinc ions (4Zn^{2+ }4) and hydrogen gas. However, under basic or alkaline conditions, like with sodium hydroxide, it forms complex ions like zincate. Because of these reactions, zinc is a key player in many industrial processes and chemical reactions.

Sodium Hydroxide

Sodium hydroxide, often referred to as lye or caustic soda, is a strong base with the chemical formula 4 NaOH 4. In its solid form, it's white and highly soluble in water. When dissolved, it forms a solution that's capable of conducting electricity and is very corrosive.

• Sodium hydroxide is commonly used in cleaning products.
• It's used in the manufacture of things like paper and textiles.
• In chemistry, it's a popular reagent due to its effectiveness in neutralizing acids.

Sodium hydroxide's reaction with zinc being a perfect showcase of its role in forming complex ions.

Chemical Reactions

A chemical reaction involves the transformation of reactants into products through a reorganization of atoms and molecules. When thinking about the reaction between zinc and sodium hydroxide, we're looking at a type of chemical process where zinc acts as a reactant with sodium hydroxide. This is an example of a redox reaction, where one substance is oxidized (loses electrons) and the other is reduced (gains electrons).

In this scenario, zinc loses electrons to form hydrogen gas and the zincate ion, showcasing how chemical reactions can create entirely new substances. Understanding the specific products from this reaction gives insight into how metals like zinc behave in different chemical environments.

Zincate Ion

The zincate ion is a complex ion, which comes into play when zinc reacts with sodium hydroxide. It is typically represented as 4 Zn(OH)_4^{2-} 4 or 4 ZnO_2^{2-} 4, depending on the conditions of the reaction.

• The formation of a zincate ion is a testament to zinc's amphoteric characteristics.
• In basic solutions, zinc turns into the zincate ion, pairing with hydroxyl groups.
• This transformation allows zinc to dissolve in alkaline solutions.

Understanding this concept is essential for comprehending how metals can form complex species and participate in various chemical applications. The presence of the zincate ion underscores the versatility of zinc in reaction pathways.