A piece of magnesium ribbon is dropped into a beaker of dilute aqueous hydrochloric acid at room temperature. After a while the production of bubbles of gas slows down. Which of the following statements can correctly explain this observation?
1 The particles have less energy.
2 The concentration of hydrochloric acid decreases.
3 The activation energy for the reaction increases.

Answer:

10) C

If it helps, you can write down the reaction equation. This will show you that hydrogen gas will be produced.

Mg + 2HCl → MgCl₂ + H₂

1 – Wrong
Decrease in energy does not decrease the amount of gas produced.

2 – Correct
Since the concentration of one of the reactants decreases, the magnesium would have less hydrochloric acid to react with. This would produce less hydrogen gas bubbles.

3 – Wrong
The activation energy cannot increase all of a sudden mid-reaction.

Since only statement 2 is correct, C is the answer.