What makes metal + acid displacement reactions exothermic?

Question

In a lab report, we were using Hess's Law to calculate enthalpies using a few reactions we did in class, one of which was magnesium in hydrochloric acid. The question was to explain why the change is exothermic.

What exactly is it about displacement reactions of metals in acids that makes them exothermic?


In this case,

Mg + 2HCl --> MgCl2 + H2 + xkJ

Answer 1

I'll give you my opinion. This is not the word of God here, just thoughts.

I see three things are happening here: First, a redox reaction; Second, you are making and breaking bonds; Third, you are organizing solvent molecules more firmly in products than in reactants.

Redox: Magnesium is losing two electrons and two H+ ions are gaining one electron apiece. I think this is energetically very favored, which is another way to say it is exothermic. I don't have the oxidation potentials in my head or I would tell you how much it is favored by. The bulk of energy released will come from here, I think.

Enthalpy (bond breaking and making): H-Cl is not a very strong bond. The H-H bond is stronger than the H-Cl bond. Heat energy will be released from this source.

Not that it matters heat-wise, but I think this thing is likely entropy favored also, since one of the products is a gas that escapes. The dissolved Mg2+ and Cl- ions from the salt will organize lots of solvent molecules through ion-dipole interactions, but likely the formation of a free gas will overwhelm that and make the reaction entropy favored.

Speaking of which, you're going to have formation of many strong ion-dipole interactions on formation and dissolution of the salt, and those are also exothermic, because they are stronger than the dipole-dipole interactions in the raw HCl. Much heat from here too.

Heat energy out: The combination of breaking two H-Cl bonds and forming an H-H bond, forming many ion-dipole interactions, reducing Mg(O) to Mg(II).

Answer 2

There is no actual reaction forming the salt. The exothermic nature of the reaction comes from the redox reaction, in which electrons are transferred from Mg atoms to H+ ions. Work out the difference between these two in the electrochemical series - that is the origin of the exothermic nature of this reaction.

Answer 3

what makes it exothermic is due to the formation of very stable salt by-products

Answer 4

1c 2c 3a 4b 5b 6c 7b 8d (you could also argue c tho) 9c 10d (not Absolutely positive on this one) 13Methane + Oxgen gas --> Carbon Dioxide + Hydrogen Peroxide 14 2HgO --> 2Hg + O2 do the rest yourself...