Question

Consider this reaction: Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g). Which change could decrease the rate of production of ZnCl2?

Answer 1

Consider this reaction: Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g). Which change could decrease the rate of production of ZnCl2?
The answer is: increasing the [HCl] 

This is supported by the "Le Chatelier's Principle" that states, "If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change."
Increasing the temperature of a system in dynamic equilibrium favours the endothermic reaction. The system counteracts the change you have made by absorbing the extra heat.Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction. The system counteracts the change you have made by producing more heat.

Answer 2

Answer:

The changes that could decrease the rate of production of ZnCl₂ are:

• If the H₂ concentration increases.
• If the pressure in the products side increases (or the volume decreases).

Explanation:

In the reaction:

Zn(s) + 2 HCl(aq) ↔ ZnCl₂(aq) + H₂(g)

Zn(s) and HCl(aq) are the reactants and ZnCl(aq) and H₂(g) are the products. The double arrow means that this reaction can proceeds from left to right but also from left to right. This will depend on the conditions that affects the reaction.

Moreover, all the chemical reactions tend to reach an equilibrium state. If somethig happens when the reaction reaches that equilibrium and changes it, the reaction acts trying to reach its equilibruim state again counteracting what make it changed. That's what the Le Chatelier's Principle explains.

So, in the equilibrium state ZnCl and H₂ have been produced, but if the H₂ concentration increases, the reaction will counteract that by increasing the amount of reactants, trying to reach its first equilibrium state, and this will stop the production of ZnCl₂.

Another factor that could decrease the rate of production of ZnCl₂ is the rise of pressure (or volume decrease, which are related) because the H₂ it is a gas and the changes of pressure affects it. In this case, if the pressure rises, the reaction will counteracting that by moving from right to left, increasing the concentration of reactants and the production of ZnCl₂ will stop.

Remember that pressure affects an equilibrium state only when there are gaseous compounds.