Question

In my lab, we conducted an experiment to demonstrate both Hess' Law and the fundamentals of enthalpy via the formation of magnesium oxide. In this experiment, 2 specific sub-reactions were conducted:

1. MgO(s) + 2HCl(aq) > MgCl₂(aq) + H₂O(l)

2. Mg(s) + 2HCl(aq) > MgCl₂(aq) + H₂(g)

In these two reactions, different concentrations of HCl were used: the first reaction called for 100mL of 6M HCl, while the second reaction called for 120mL of 6M HCl.

The lab manual says that this is an important distinction. Why is it important that two different concentrations of hydrochloric acid were used for each reaction?

Answer 1

Hi i am sorry you have used same concentration as your statement "the first reaction called for 100mL of 6M HCl, while the second reaction called for 120mL of 6M HCl."

But no problem, let's see the products

1. MgO(s) + 2HCl(aq) > MgCl₂(aq) + H₂O(l)

2. Mg(s) + 2HCl(aq) > MgCl₂(aq) + H₂(g)

in 2nd reaction we are using most active metal magnesium directly and this reaction is highly exothermic (heat energy released during the course of the reaction) and one of the product is highly flammable hydrogen gas. If we use concentrated HCl for this reaction H2 catches fire quickly and this is a hazardous reaction.

In 1st reaction magnesium oxide is not much reactive compare to pure magnesium metal and also products are non-hazardous, hence we have to use concentrated HCl in case of MgO and diluted HCl in case of Mg(s). This is the correct reason for using different concentrated HCl solutions.

Hope this helped you!

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