Ask A Scientist Chemistry Archive

Question:
Can I use Sodium Nitrate as an example of an endothermic reaction?
 george t tuggle

Answer 1: 
I'm a scientist, so it's my job to be picky.... ;-)
You mean, "Can I use the solvation of sodium nitrate
in water as an example of an endothermic reaction?"
 
As the Magic 8-Ball says, "My sources say yes."
Atkins' Physical Chemistry (4th edition) says that the
limiting enthalpy of solution at STP for NaNO3 is
+20.4 kJ mol^-1....which is even more endothermic
than the enthalpy of solution of NaCl (+3.9 kJ mol^-1).
 
-topper
 
Answer 2:
From personal experience, potassium chloride works real well too.
There's also a mixture of two salts that is so endothermic that it
gets cold enough to freeze water.  I'll see if I can find the recipe.
-jschultz
 
Answer 3:
Joe, maybe you are thinking about the case where you
mix crystals of barium hydroxide octahydrate [Ba(OH)_2 . 8H2O]
with crystals of ammonium nitrate [NH_4 NO_3].
The reaction, which is endothermic, is
 
Ba(OH)_2 . 8H2O (s) + 2NH_4NO_3(s) -> 2NH3(g) +10H2O(l)+Ba(NO_3)_2(aq)
 
with DH = 170 kJ / mole.  This experiment MUST be done with
proper ventilation due to the evolution of ammonia.
See chapter 6 of Ebbing's "General Chemistry" text for a
description of this experiment.
 
-topper
 
Answer 4:
Oh, I forgot to mention....in order to see this reaction freeze
water, you mix the two crystalline substances together in a beaker
or Erlenmeyer flask throughly, and then set the beaker/flask onto
a puddle of water on a wooden board. In a couple of minutes, the flask
and the board are frozen solidly together!
 
-topper
 
Answer 5:
That's the one!  And yes...we usually do this one in a big lecture hall
too.
-jschultz