Single Replacement Reactions — Trading Partners

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In a single replacement (single displacement) reaction, a free element replaces another element within a compound. The general form is A + BC -> AC + B, where element A displaces element B. Whether a reaction occurs depends on the relative reactivity of the elements — the activity series of metals (or halogens) predicts which replacements are thermodynamically favorable. This concept is central to electrochemistry and corrosion science.

Reaction Mechanism

The activity series ranks elements by their tendency to lose electrons (be oxidized). A more reactive metal displaces a less reactive one from solution. Zinc (higher in the activity series) displaces copper from copper sulfate solution: Zn + CuSO4 -> ZnSO4 + Cu. The zinc atoms lose electrons and go into solution as Zn2+ ions, while Cu2+ ions gain those electrons and deposit as solid copper. This electron transfer makes single replacement reactions a subset of redox reactions.

Everyday Examples

Dropping a steel nail into copper sulfate solution turns the nail copper-colored as iron displaces copper. Galvanized steel uses zinc coatings that preferentially corrode instead of the underlying iron — a sacrificial single replacement. The thermite reaction (2Al + Fe2O3 -> Al2O3 + 2Fe) is a dramatic single replacement used in welding railroad tracks, reaching temperatures above 2,500 degrees C.

Importância industrial

Hydrometallurgy uses single replacement to extract metals from ores. Copper cementation recovers copper from mine drainage by passing it over scrap iron. The Kroll process extracts titanium by reducing titanium tetrachloride with magnesium. Single replacement principles underpin all galvanic cells (batteries) — the difference in reactivity between two metals creates the voltage that powers our devices.

Safety Note

Some single replacement reactions are violent — alkali metals react vigorously with water, and the thermite reaction produces molten iron. Always check the activity series before attempting displacement reactions and use appropriate shielding for exothermic reactions.

Fluorine Displacing Chlorine from Sodium Chloride

F₂ + 2NaCl → 2NaF + Cl₂

Fluorine, the most reactive element, displaces chlorine from sodium chloride to form sodium fluoride and chlorine gas. Fluorine is so …

Exotérmico · ΔH = -348,0 kJ

Aluminum Reacting with Iron(III) Oxide (Thermite)

2Al + Fe₂O₃ → Al₂O₃ + 2Fe

The thermite reaction involves aluminum reducing iron(III) oxide to produce aluminum oxide and molten iron. This spectacularly exothermic reaction generates …

Exotérmico · ΔH = -851,5 kJ

Chlorine Displacing Iodine from Potassium Iodide

Cl₂ + 2KI → 2KCl + I₂

Chlorine displaces iodine from potassium iodide solution because chlorine is more electronegative and a stronger oxidizer than iodine. The solution …

Exotérmico · ΔH = -170,0 kJ

Magnesium Reacting with Steam

Mg + H₂O → MgO + H₂

Magnesium reacts with steam to produce magnesium oxide and hydrogen gas. The reaction is vigorous and highly exothermic. Hot magnesium …

Exotérmico · ΔH = -361,0 kJ

Lithium Reacting with Water

2Li + 2H₂O → 2LiOH + H₂

Lithium reacts steadily with water to produce lithium hydroxide and hydrogen gas. While still vigorous, lithium is the least reactive …

Exotérmico · ΔH = -446,0 kJ

Aluminum Reacting with Hydrochloric Acid

2Al + 6HCl → 2AlCl₃ + 3H₂

Aluminum reacts with hydrochloric acid to produce aluminum chloride and hydrogen gas. The reaction requires the initial dissolution of the …

Exotérmico · ΔH = -1049,0 kJ

Aluminum Reducing Chromium Oxide (Aluminothermic)

2Al + Cr₂O₃ → Al₂O₃ + 2Cr

Aluminum reduces chromium(III) oxide in an aluminothermic reaction similar to the iron thermite reaction. The reaction produces aluminum oxide and …

Exotérmico · ΔH = -536,0 kJ

Iron Reacting with Hydrochloric Acid

Fe + 2HCl → FeCl₂ + H₂

Iron dissolves in hydrochloric acid to produce iron(II) chloride and hydrogen gas. The reaction proceeds at a moderate rate, producing …

Exotérmico · ΔH = -89,1 kJ

Iron Displacing Copper from Copper Sulfate

Fe + CuSO₄ → FeSO₄ + Cu

Iron metal replaces copper in copper sulfate solution because iron is higher in the reactivity series than copper. An iron …

Exotérmico · ΔH = -152,3 kJ

Zinc Displacing Copper from Copper Sulfate

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc metal displaces copper from copper sulfate solution because zinc is more reactive than copper in the activity series. The …

Exotérmico · ΔH = -218,7 kJ

Magnesium Reacting with Hydrochloric Acid

Mg + 2HCl → MgCl₂ + H₂

Magnesium reacts vigorously with hydrochloric acid to produce magnesium chloride and hydrogen gas. The reaction is highly exothermic and proceeds …

Exotérmico · ΔH = -462,0 kJ

Zinc Reacting with Hydrochloric Acid

Zn + 2HCl → ZnCl₂ + H₂

Zinc granules react with hydrochloric acid to produce zinc chloride and hydrogen gas. This is a moderately vigorous reaction commonly …

Exotérmico · ΔH = -152,4 kJ

Potassium Reacting with Water

2K + 2H₂O → 2KOH + H₂

Potassium reacts even more violently with water than sodium, instantly producing potassium hydroxide and hydrogen gas. The hydrogen ignites immediately …

Exotérmico · ΔH = -392,2 kJ