The heat of combustion of which of the following reactions would be difficult to measure directly in a calorimeter?

Question

a. MgO(s) + 2 HCl(aq) -----  MgCl2(aq) + H2O(l) 

 b.Mg(s) + ½ O2(g) ----------- MgO(s) 

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

Answer 1

There are different kinds of calorimeters, and they vary wildly in price. The bomb calorimeters used in research labs will set you back at least $10,000. Unless you're in an upper level university course, you're not going to be using one of those.

No, what you're going to get is water in a cheap Styrofoam cup, and an alcohol thermometer. If you're lucky, you might get two Styrofoam cups for added insulation! This kind of calorimeter works best if the reaction happens inside the water within the Styrofoam cup(s). If something is being dissolved by an acid, as is the case with the first and third reactions shown, this calorimeter will work (so long as you don't poke a hole in the bottom of the cup and spill acidic water all over the table).

Now let's consider the second reaction, which basically amounts to lighting a ribbon of magnesium on fire. To measure this with your calorimeter, you'd either have to a) hold the burning magnesium right underneath the Styrofoam, likely catching it on fire and ruining your measurement, b) drop the burning magnesium into the cup, where it will explode upon contact with water (and ruin your measurement), or c) use a bomb calorimeter that costs thousands of dollars and requires access to pressurized oxygen to operate.

I think you can see now why the second reaction is harder to measure than the other two, and would likely get you banned from the chemistry laboratory if you tried. This is why unauthorized experiments are strictly forbidden in the lab!