Explain why zn reacts more slowly with dilute hydrochloric acid then with concentrated hydrochloric acid
The Correct Answer and Explanation is:
Correct Answer:
Zinc reacts more slowly with dilute hydrochloric acid than with concentrated hydrochloric acid because the rate of a chemical reaction is directly dependent on the concentration of the reactants. Concentrated acid has a higher number of reactant particles per unit volume, leading to more frequent and effective collisions with the zinc surface.
Explanation
The reaction between zinc metal and hydrochloric acid is a single displacement reaction where zinc displaces hydrogen to form zinc chloride and hydrogen gas. The balanced chemical equation for this reaction is:
Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)The speed, or rate, at which this reaction occurs is governed by Collision Theory. This theory states that for a reaction to take place, the reactant particles (in this case, zinc atoms and hydrogen ions,
H+H+, from the acid) must collide with each other with sufficient energy and in the correct orientation.
The Role of Concentration:
The concentration of an acid refers to the number of acid molecules (or ions) dissolved in a specific volume of water.
- Concentrated Hydrochloric Acid: This solution contains a very high number of hydrogen ions (
H+H+) packed into a given volume. Because of this high density of ions, there is a much greater probability ofH+H+ions colliding with the surface of the zinc metal at any moment. This increase in the frequency of collisions leads to a higher number of effective collisions—those that have enough energy to result in a reaction. Consequently, the rate of hydrogen gas production is high, and the zinc dissolves quickly. This is observed as vigorous bubbling (effervescence). - Dilute Hydrochloric Acid: This solution has been mixed with more water, so it contains a far lower number of hydrogen ions (
H+H+) per unit volume. With fewer reactant ions available in the same space, the frequency of collisions between theH+H+ions and the zinc atoms is significantly reduced. As a result, there are fewer effective collisions per unit of time, and the reaction proceeds at a much slower pace. The observed effervescence will be gentle and less rapid.
In summary, the higher the concentration of hydrochloric acid, the greater the number of reacting particles, which increases the frequency of effective collisions and, therefore, accelerates the rate of the reaction.
