Which metal releases the most heat when reacting with excess hydrochloric acid?

The correct option is Magnesium.

To understand why magnesium releases the most heat, we can look at the chemical reactivity of the metals listed: iron, nickel, zinc, and magnesium.

When a metal reacts with hydrochloric acid, a single displacement reaction occurs where the metal oxidizes and hydrogen gas is released. The general equation for this reaction with a divalent metal (such as magnesium, zinc, iron, or nickel) is:

$M\left(s\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to M{\mathrm{Cl}}_2\left(\mathrm{aq}\right)+H_2\left(g\right)$

These reactions are exothermic, meaning they release heat energy to the surroundings. The amount of heat released (the enthalpy change of the reaction) is directly related to the reactivity of the metal. A more reactive metal has a greater tendency to lose electrons and form positive ions, resulting in a more energetically favorable reaction that releases more heat.

By referencing the metal reactivity series, we can compare the relative reactivities of the given metals (from most reactive to least reactive):

Magnesium (Mg) > Zinc (Zn) > Iron (Fe) > Nickel (Ni)

Because magnesium is the most reactive metal among the choices, its reaction with hydrochloric acid is the most vigorous and releases the greatest amount of heat energy.