A student placed 3.0 grams of Mg into some HCl in two different experiments. In each case, it reacted according to the following equation:

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g).

In the first experiment, it took 32 minutes for all of the Mg to react. In the second experiment, it took 54 minutes for all of the Mg to react. Which of the following could account for the change in reaction rate of the second experiment?

a catalyst was added

the Mg was powdered into H2

the H2 was decreased

the temperature was decreased

Answer: -

A catalyst is a substance that speeds up the chemical reaction. In the first experiment it took 32 minutes for all the magnesium to react.

In the next it took 54 minutes for all the magnesium to react. So the reaction was slower the second time.

Thus a catalyst was not used.

Powdering up the Magnesium would mean increasing the surface area. More the surface area faster the reaction. Since the reaction is slowing down,

Magnesium was not powdered

If the temperature was decreased then kinetic energy would decrease of the reactant molecules and the reaction would slow down due to less number of collisions between reactant moleules.

So the temperature was decreased is a correct option.