Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Hydrogen will not reduce (a) heated cupric oxide (b) heated ferric oxide (c) heated stannic oxide (d) heated aluminium oxide

Step-by-Step Solution

Answer

Hydrogen will not reduce heated aluminium oxide (d).

Step 1: Identify the Oxides

First, identify each of the given oxides and their corresponding metals: - Cupric oxide is the oxide of copper (CuO). - Ferric oxide is the oxide of iron (Fe₂O₃). - Stannic oxide is the oxide of tin (SnO₂). - Aluminium oxide is the oxide of aluminum (Al₂O₃).

Step 2: Understand Reduction by Hydrogen

Hydrogen can reduce metal oxides to their metals when those metals form stable oxides. Generally, hydrogen can reduce oxides of metals that are less reactive than hydrogen, according to the reactivity series.

Step 3: Examine Reactivity of Metals Involved

Look at the reactivity series to determine which metals are more or less reactive than hydrogen. Among the metals given: - Copper and tin are less reactive than hydrogen, so hydrogen can reduce their oxides. - Iron is more reactive than hydrogen, but hydrogen can still reduce iron oxides under certain conditions. - Aluminum is much more reactive, making its oxide stable against reduction by hydrogen.

Step 4: Determine Which Oxide Cannot Be Reduced by Hydrogen

Compare the stability and reactivity of each oxide: - Cupric oxide and stannic oxide can be reduced by hydrogen. - Ferric oxide can also be reduced by hydrogen under high temperatures and suitable conditions. - Aluminium oxide cannot be reduced by hydrogen because aluminum's strong affinity for oxygen and its high reactivity make the oxide very stable.

Key Concepts

Reactivity Series

The reactivity series is a crucial tool in chemistry that ranks elements, particularly metals, based on their ability to displace other elements from compounds. This ranking is based on how readily they lose electrons to form positive ions.
In a typical reactivity series:

• Highly reactive metals like potassium and sodium appear at the top.
• Less reactive metals like silver and gold are found at the bottom.

Understanding the placement in the reactivity series helps predict chemical behavior. For example, elements higher in the series can displace those lower down from their compounds. In the context of metal oxide reduction, a metal lower in the series than hydrogen can potentially be reduced by it.

Reducing Agents

A reducing agent is a substance that donates electrons to another, facilitating a reduction reaction. In this process, the reducing agent itself becomes oxidized by losing electrons.
Common reducing agents include:

• Hydrogen, which is often used to reduce metal oxides.
• Carbon monoxide, another common industrial reducing agent.

In the context of reducing metal oxides, the choice of reducing agent is dependent on the relative reactivity of the metal. The agent should be strong enough to donate electrons and remove oxygen from the oxide, converting it back to its base metal.

Aluminium Oxide Stability

Aluminium oxide ( Al₂O₃ ) is known for its remarkable stability. This stability arises from the strong chemical bonds between aluminium and oxygen atoms. Being highly reactive, aluminium forms a protective oxide layer which is difficult to break.
Key aspects of aluminium oxide's stability include:

• High melting point, making it resistant to both physical and chemical change.
• Corrosion resistance, allowing it to remain unaffected by environmental conditions.

These features make Al₂O₃ immune to reduction by weaker agents like hydrogen. Advanced methods such as electrolysis are needed to extract aluminium from its oxide.

Hydrogen Reduction

Hydrogen reduction involves using hydrogen as a reducing agent to extract metals from their oxides. Hydrogen, when used, reacts with oxygen to form water and leaves behind the pure metal.

• This method is viable for oxides of metals less reactive than hydrogen.
• For example, copper and tin oxides can be reduced using hydrogen.

However, it is ineffective for very stable oxides like Al₂O₃ due to aluminium's strong affinity for oxygen. The decision to use hydrogen depends on the properties of the metal oxide in question, balancing reactivity and stability.