Question

Lead and tin are extracted from their chief ores by: (a) Electrolysis and self reduction (b) Carbon reduction and self reduction (c) Self reduction and carbon reduction (d) Self reduction and electrolysis.

Short answer

(c) Self reduction and carbon reduction

Step-by-step solution

Understanding Ore Extraction

Lead and tin are extracted from their ores using specific processes. Lead is typically extracted using a method called 'self reduction', which does not involve carbon or electrolysis. Tin, on the other hand, is usually extracted using 'carbon reduction', where tin oxide is reduced using carbon. This understanding guides us in finding the correct option.

Analyzing the Options

We need to match the processes used for lead and tin to the options given. For lead, we know 'self reduction' is used, and for tin, 'carbon reduction' is the process used. Therefore, the correct answer should combine 'self reduction' and 'carbon reduction'.

Selecting the Correct Answer

On examining the options: (a) Electrolysis and self reduction (b) Carbon reduction and self reduction (c) Self reduction and carbon reduction (d) Self reduction and electrolysis The correct match based on the processes is option (c), which states 'Self reduction and carbon reduction'.

Key concepts

Self Reduction

Self reduction is a fascinating method used in metallurgy where an element in an ore is reduced without the need for an external reducing agent. This process occurs when the ore itself contains enough of the reducing agent elements to allow for the liberation of the metal. For instance, lead sulfide, which is the primary ore for lead, undergoes a process in which the lead is reduced during the roasting stage, due to the presence of both lead sulfide (PbS) and lead oxide (PbO). These compounds interact with each other, and lead is released when these reactions facilitate the reduction of the lead metal.
Not all ores can self-reduce. The ore must naturally contain compounds that can react to release the pure metal. This method is often economically advantageous as it minimizes the need for additional materials. However, it can be limited by the composition of the ore and the metal's chemistry.

Carbon Reduction

Carbon reduction is a widespread method for extracting metals. In this approach, carbon, often in the form of coke, acts as the reducing agent in the smelting process. When the metal oxide from an ore is heated with carbon, the oxygen is removed from the metal oxide, freeing the metal. A classic example is the extraction of tin from its ore, tin oxide (SnO₂).
During this process, tin oxide is mixed with carbon and heated to high temperatures. The carbon reacts with the oxygen in tin oxide to form carbon dioxide, leaving behind pure tin. This chemical process is economically viable for metals found below carbon in the reactivity series because the energy required to reduce them is relatively low. Carbon reduction is significant in the metal industry due to its simplicity and cost-effectiveness.

Lead Extraction

The extraction of lead from its main ore, galena (PbS), is primarily achieved through self reduction. Galena is first roasted in the presence of air to convert part of it into lead oxide (PbO) and sulfur dioxide (SO₂). The roasting process involves heating the ore to a high temperature in an oxygen-rich environment.
Once a portion of the lead sulfide is converted to lead oxide, the lead oxide and remaining lead sulfide react to produce lead metal and sulfur dioxide. This reaction happens as:
\[ 2PbS + 3O_2 → 2PbO + 2SO_2 \]
\[ 2PbO + PbS → 3Pb + SO_2 \]
Lead extraction is thus sensible in utilizing the compounds already present within the ore to obtain the metal without the need for external reducing agents, illustrating the elegance of self reduction.

Tin Extraction

Tin extraction is predominantly achieved through the process of carbon reduction. The main ore of tin is cassiterite (SnO₂). During extraction, cassiterite is crushed and then washed to remove impurities. The concentrate is then heated in a furnace alongside carbon, usually in the form of coke.
As the temperature rises, the tin oxide in the ore reacts with the carbon to produce tin and carbon dioxide. The chemical equation for this reaction is:
\[ SnO_2 + 2C → Sn + 2CO_2 \]
This method is straightforward and beneficial for metals like tin that require less energy to be reduced. Carbon reduction is a practical choice for smelting tin due to its efficiency and cost-effectiveness, making it a staple process in metallurgy.