Why the copper is able to replace the silver, but the reverse reaction will not result in a replacement reaction?

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Explanation: A single-replacement reaction is a reaction in which one element replaces a similar element in a compound. The general form of a single-replacement (also called single-displacement) reaction is:

A+BC \rightarrow AC+B

In this general reaction, element Ais a metal and replaces element B, also a metal, in the compound. When the element that is doing the replacing is a nonmetal, it must replace another nonmetal in a compound, and the general equation becomes:

Y+XZ \rightarrow XY+Z

Yis a nonmetal and replaces the nonmetal Zin the compound with X.

Metal Replacement

Magnesium is a more reactive metal than copper. When a strip of magnesium metal is placed in an aqueous solution of copper(II) nitrate, it replaces the copper. The products of the reaction are aqueous magnesium nitrate and solid copper metal.

\text{Mg}(s)+\text{Cu(NO}_3)_2(aq) \rightarrow \text{Mg(NO}_3)_2 (aq) + \text{Cu}(s)

This subcategory of single-replacement reactions is called a metal replacement reaction because it is a metal that is being replaced (copper).

b.have there own function.