Homework Help: HCl + Mg

What do you think is going to happen, even if you cannot justify it yet?

The reaction between Magnesium metal and Hydrochloric acid is,
Mg (s) + 2 HCl (aq) --> H2 (g) + MgCl2 (aq)

When does this reaction occur? Can the reactants just be located anywhere and this reaction will proceed, or do they need to be combined for something to happen?
How do they ‘combine’ (on a molecular level) when you drop the Mg into the HCl solution?

 

Yes, the more concentrated HCl solution should react with the Magnesium at a higher rate.

In addition to the questions I asked above,
Think how a concentrated solution of HCl differs from a dilute solution of HCl in terms of the number of H+ ions available to react with the Mg.

 

Hydrochloric acid is a strong electrolyte; it will split up completely into H+ and Cl- ions in solution.

The Chloride ions (Cl-) are merely spectator ions in this reaction, it is the H+ ions which are reacting with the Magnesium metal.

The ionic form of the reaction is:
Mg (s) + 2 H+ (aq) + 2 Cl- (aq) --> H2 (g) + Mg+2 (aq) + 2 Cl- (aq)
Canceling out the Chloride ions (since they are spectators) gives us this net ionic reaction,
Mg (s) + 2 H+ (aq) --> H2 (g) + Mg+2 (aq)

 

I tried this reaction at school becasue we were testing out the new pH probes that we got. We had HCl up to 6M and we used varying amounts, however we did not see a distinct change. But thats not to say that it should show a linear change, becasue it should. Chances are we just got some bad acid.

 

There are some good reasons that you would not see large differences. First, the reaction is between a solid and a liquid. The reaction happens at the interface. The reaction rate is therefore dependent on surface area of the magnesium ribbon which is steadily growing smaller. The reaction generates bubbles on the surface that grow in size and effectively shield a significant area of the magnesium from the solution before detatching. Finally, unless the reaction is very strongly stirred, you are relying on the fairly weak currents generated by the upwelling bubbles to deliver fresh HCl solution to the surface of the Mg ribbon.

 

Would the HCl be considered a catalyst in this, because the more Hydrochloric acid you add, the quicker the reaction, and catalyst's are meant to speed up reactions. So to repeat, would the Hydrochloric acid be a catalyst in mixing HCl + Mg?

 

A Catalyst is a substance that initiates or accelerates a chemical reaction without itself being affected. (according to http://wordnetweb.princeton.edu/perl/webwn?s=catalyst)

 

from what i can tell, yes it does, as the HCl is not changed at the end of the reaction and it seems to accelerate the reaction, so it would be fair on my part to conclude that YES, HCl is a catalyst when mixed with Mg.

 

MG+2HCl -> MGCl2 + H2...
Ohhh.. Thanks Borek. Because it IS changed by the end, it has broken up. Therefore, it is NOT a catalyst, as it does not match the definition of a catalyst.. Unless my chemical equation is wrong (which is completely possible), in which case, I would need the correct equation in order to determine if the HCl is a catalyst or not.

 

Thanks Borek :D