Predict the type of reaction for Mg reacting with HCl and provide the balanced equation.

• A. Double displacement; MgCl2 + H2 -> Mg + 2 HCl
• B. Single displacement; Mg + 2 HCl -> MgCl2 + H2
• C. Synthesis; Mg + HCl -> MgH2 + Cl2
• D. Combustion; Mg + HCl -> MgCl + H2

Answer: (B)

When a metal reacts with an acid, you get a single-displacement reaction: the metal kicks out hydrogen from the acid, forming a salt and hydrogen gas. Magnesium is more reactive than hydrogen, so it donates electrons to the hydrogen ions in the acid, becoming Mg2+ and leaving behind chloride ions to pair with the magnesium in the salt MgCl2. The hydrogen ions combine to form H2 gas. The balanced equation shows one magnesium atom reacting with two hydrochloric acid molecules to yield magnesium chloride and hydrogen gas: Mg + 2 HCl → MgCl2 + H2. The 2 HCl is needed so there are two hydrogens to form one molecule of H2, and the chloride ions balance with the magnesium to make MgCl2. Other reaction types don’t fit this situation because the outcome is salt plus hydrogen gas, not a metathesis between two salts (double displacement), not a synthesis forming a single new compound from elements, and not combustion.