Chapter 3 · Question 13

Explain the thermit reaction with a balanced chemical equation and state one important application. Why cannot metals at the top of the activity series (like Na, Mg, Al) be reduced from their oxides using carbon?

Q13

Explain the thermit reaction with a balanced chemical equation and state one important application. Why cannot metals at the top of the activity series (like Na, Mg, Al) be reduced from their oxides using carbon?

Answer Revealed

Direct Answer:

The thermit reaction is a highly exothermic displacement reaction in which aluminium powder reduces iron(III) oxide to molten iron:

\text{Fe}_2\text{O}_3\text{(s)} + 2\text{Al(s)} \rightarrow 2\text{Fe(l)} + \text{Al}_2\text{O}_3\text{(s)} + \text{Heat}

. The enormous heat generated melts the iron, which is then used to join railway tracks or cracked machine parts. Metals at the top of the activity series (K, Na, Ca, Mg, Al) have a higher affinity for oxygen than carbon has. Therefore, carbon cannot pull oxygen away from their oxides — the reduction does not occur. Instead, these metals are obtained by electrolytic reduction of their molten compounds (e.g., molten chlorides).

Simple Explanation

The thermit reaction is like a controlled firework: mix aluminium powder with iron oxide and light it — a violent reaction produces so much heat that the iron melts and flows as a liquid. This liquid iron is used to weld railway tracks together. Now, why can't we use carbon (coke) to extract sodium or aluminium? Because these metals love oxygen more than carbon does. Carbon is like a weak magnet for oxygen compared to them — it simply cannot pull the oxygen off. So we use electricity instead — passing current through their molten salts forces the metal to deposit at the cathode.

Exam-Ready Structure

The thermit reaction and electrolytic extraction of highly reactive metals (Sections 3.4.4–3.4.5) represent the two extremes of the reactivity spectrum in extractive metallurgy. 1. The thermit reaction: (a) It is a displacement reaction between a highly reactive metal (aluminium) and the oxide of a less reactive metal (iron). Since Al is above Fe in the reactivity series, it displaces iron from its oxide. (b) Equation:

\text{Fe}_2\text{O}_3\text{(s)} + 2\text{Al(s)} \rightarrow 2\text{Fe(l)} + \text{Al}_2\text{O}_3\text{(s)} + \text{Heat}

. The reaction is highly exothermic — the heat released is so intense that the iron produced is in the molten state. (c) Application: Joining railway tracks or repairing cracked machine parts (as shown in Fig. 3.11). The molten iron flows into the gap between the rails and solidifies, creating a strong weld. A similar reaction with MnO₂ produces molten manganese:

3\text{MnO}_2 + 4\text{Al} \rightarrow 3\text{Mn} + 2\text{Al}_2\text{O}_3 + \text{Heat}

. 2. Why carbon cannot reduce highly reactive metal oxides: (a) Metals at the top of the activity series (K, Na, Ca, Mg, Al) have a very high affinity for oxygen — higher than carbon's affinity for oxygen. (b) Thermodynamically, carbon is not a strong enough reducing agent to pull oxygen away from these metal oxides. The reaction

\text{Metal oxide} + \text{C} \rightarrow \text{Metal} + \text{CO}

is not feasible for highly reactive metals. 3. Electrolytic reduction as the alternative: These metals are extracted by electrolysis of their molten compounds (usually chlorides): (a) Sodium is obtained by electrolysis of molten NaCl. At the cathode:

\text{Na}^+ + \text{e}^- \rightarrow \text{Na}

. At the anode:

2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-

. (b) Aluminium is obtained by electrolytic reduction of molten aluminium oxide. 4. The NCERT flow chart (Fig. 3.10) systematises this: high-reactivity metals → electrolysis of molten ore → pure metal. Medium-reactivity metals → carbonate/sulphide ore → calcination/roasting → oxide → reduction with carbon → purification. Low-reactivity metals → sulphide ore → roasting → metal → refining.

Key Points

Thermit reaction: $\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3$ + Heat (highly exothermic)
Application: joining railway tracks/cracked machine parts with molten iron
Top-activity metals (K-Al) have higher oxygen affinity than carbon → carbon cannot reduce them
Electrolytic reduction of molten compounds is used instead (e.g., NaCl electrolysis for Na)
Cathode: metal deposited ( $\text{Na}^+ + \text{e}^- \rightarrow \text{Na}$ ); Anode: Cl₂ liberated ( $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$ )

Explain the thermit reaction with a balanced chemical equation and state one important application. Why cannot metals at the top of the activity series (like Na, Mg, Al) be reduced from their oxides using carbon?