In the extraction of iron, the waste gas from the furnace is a mixture of

Analysis

This question focuses on the industrial extraction of iron using a blast furnace. We need to identify the gaseous by-products that exit the furnace after the reduction of iron ore (usually hematite, $Fe_2O_3$).

Concept Explanation

In a blast furnace, several chemical reactions occur as iron ore, coke (carbon), and limestone are heated:

1. Combustion: Coke burns in the blast of hot air to produce carbon dioxide ($CO_2$) and heat.
2. Reduction: As $CO_2$ rises, it reacts with more coke to form carbon monoxide ($CO$), which is the primary reducing agent for the iron ore.
3. Exhaust: The air blast contains a large amount of nitrogen ($N_2$) because air is approximately 78% nitrogen. Since nitrogen is inert, it does not participate in the chemical reactions and passes through the furnace unchanged.

Consequently, the waste gases leaving the top of the furnace consist of the nitrogen that entered with the air, the carbon dioxide produced from combustion, and the excess carbon monoxide that did not react with the ore.

Step-by-Step Solution

1. Identify the input: The blast furnace uses hot air, which is primarily $N_2$ and $O_2$. The $N_2$ passes through the furnace as an inert gas.
2. Identify the chemical reactions: • $C + O_2 \rightarrow CO_2$ • $CO_2 + C \rightarrow 2CO$
3. Determine the output: The gases exiting the furnace include the unreacted nitrogen ($N_2$), the carbon dioxide ($CO_2$) produced, and the carbon monoxide ($CO$) that was used as a reducing agent but remained in excess.
4. Evaluate the options: • Option A and D include $PH_3$ (phosphine), which is not a product of iron extraction. • Option B mentions $C$ (carbon), which is a solid, not a gas. • Option C correctly lists the three main gaseous components: $N_2$, $CO$, and $CO_2$.

Conclusion

The mixture of waste gases from the blast furnace is composed of nitrogen, carbon monoxide, and carbon dioxide.

Exam Tip: Always remember that in any industrial process involving air, nitrogen is almost always present in the waste stream because it is chemically unreactive under these conditions. This simple logic helps eliminate many distractors in chemistry exams!