By Cara Batema
Updated Aug 30, 2022
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Single replacement, or single displacement, reactions involve one element displacing another within a compound. The substituting element is typically more reactive, resulting in a new element and a new compound as products. This process exemplifies fundamental principles of chemical reactivity and thermodynamics.
When a copper wire is immersed in an aqueous silver nitrate solution, silver metal precipitates while copper ions enter the solution as copper(II) nitrate. Copper replaces silver in the compound, illustrating how a more reactive metal displaces a less reactive one. Likewise, zinc placed in copper(II) nitrate produces metallic copper and dissolves as zinc(II) nitrate.
Many metals readily displace hydrogen from acids. For instance, zinc reacts with hydrochloric acid to form zinc chloride and liberate hydrogen gas. Magnesium follows the same pattern, yielding magnesium chloride and hydrogen. Potassium reacts with sulfuric acid to produce potassium sulfate and hydrogen gas, underscoring the general rule that active metals can liberate hydrogen from acids.
The classic thermite reaction between iron(III) oxide and aluminum is a highly exothermic single replacement reaction. Aluminum oxidizes to aluminum oxide while reducing iron(III) oxide to molten iron. The reaction’s heat output is sufficient to melt the iron product, making it a powerful demonstration of redox chemistry.
Single replacement reactions are not limited to metals. Halogens readily displace less reactive halogens in salts. Chlorine displaces bromine in sodium bromide, producing sodium chloride and elemental bromine. Bromine likewise displaces iodine in potassium or calcium iodide solutions, forming potassium or calcium bromide and iodine.
