Let's consider single displacement reaction, where one element displaces another in a compound. This reaction can be summarized by the following general equation:
A(s) + BC(aq) => B(s) + AC(aq)
Figure 1: A is a free metal (s), BC is an ionic compound dissolved in water (aq), B is a free mental (s), and AC an ionic compound dissolved in water (aq).
In this type of reaction, there is a competition between the two elements (A and B). During this single displacement, redox reaction one element loses electron (or electrons) to the other element.
We can simplify the reaction by writing net ionic equation, that will show only the electron exchange between elements:
A(s) + B+ (aq) => A+(aq) + B(s)
Figure 2: A(s) is a free metal that is donating an electron and dissolving into the solution-forming cation A+(aq), a cation B+ (aq) is accepting an electron and converting to the free metal B(s).
Above reaction occurs when free metal A(s) is more reactive then ion B+ (aq) in the solution.
However, in cases where free metal A(s) is less reactive then ion B+ (aq) the reaction does not occur.
There are four signs to look for, when analyzing a chemical reaction: