The reaction Mg + 2HCl → MgCl2 + H2 is an example of a a) single displacement reaction.
In this reaction, magnesium (Mg) displaces hydrogen (H) from hydrochloric acid (HCl). This type of reaction is characterized by one element being replaced by another in a compound, which fits the definition of a single displacement reaction. The magnesium combines with the chloride ions from the HCl to form magnesium chloride (MgCl2), while the hydrogen gas (H2) is released. In contrast, double displacement reactions involve the exchange of ions between two compounds, decomposition reactions break a single compound into its components, and synthesis reactions combine multiple reactants into a single product.