Exam Style Question - 2

Question: A student added magnesium ribbon to 1.0 mol dm^-3 hydrochloric acid, then repeated the experiment with 2.0 mol dm^-3 acid, keeping everything else the same. Use collision theory to explain why the reaction is faster with the more concentrated acid, and why the rate of each reaction decreases as it proceeds. (4 marks)In 2.0 mol dm^-3 acid there are more H⁺ ions in the same volume, so there are more collisions per unit time between H⁺ ions and the magnesium surface.The proportion of collisions with energy above the activation energy does not change, because the temperature is the same. However, since there are more collisions in total, there are more successful collisions per unit time and the rate increases.As the reaction proceeds, H⁺ ions and magnesium are used up. The concentration of the acid falls, so the collision frequency and therefore the rate decrease, until the limiting reagent runs out and the reaction stops.The same reasoning applies to gases: increasing the pressure puts more molecules into the same volume and increases the frequency of collisions.