Chemistry 30

Consider the following reaction that occurs between hydrochloric acid, HCl, and zinc metal:

HCl_(aq) + Zn_(s) → H_{2 (g)} + ZnCl_{2 (aq)}

Will this reaction occur fastest using a 6 M solution of HCl or a 0.5 M solution of HCl? Explain.

Solution:

The reaction will occur fastest with 6 M HCl, because it is more concentrated than the 0.5 M solution. In the more concentrated solution there are more moles of HCl present - with a higher concentration of reacting particles, collisions will occur more frequently, leading to a faster rate of reaction.

Again consider the reaction between hydrochloric acid and zinc. How will increasing the temperature affect the rate of the reaction? Explain.

Solution:

Increasing the temperature will most likely increase the rate of the reaction, for two reasons:

1. Particles will move around faster at the higher temperature and thus will collide more frequently, resulting in a faster rate of reaction.
2. Particles will collide with more force. Thus, more particles will likely have sufficient energy (E_a) to reach the activated complex and thus have a successful collision.

Solution:

Reaction B would be faster than Reaction A because it has a lower threshold energy (activation energy). Thus, more particles have at least the minimum amount of energy required for a successful reaction.

Reaction A would benefit most by an increase in temperature. Reaction B already have the majority of particles above the threshold energy; having more particles above the threshold would not make a significant difference in the rate.