Reactions involving energy changes: ENDOTHERMIC reactions

Spontaneous endothermic reactions are not common but 3 differing endothermic reactions are described below.

1. Dissolving ammonium salts in water.

If 200 mls of water are added to 200g of ammonium nitrate in a vacuum flask the temperature drops down to

around -5o C. This example of a spontaneous endothermic reaction is thebasis of a commercial product, an ice pack that

doesn't have to be kept in the freezer. These ice packs contain a small quantity of ammonium nitrate (NH4NO3) or

ammonium chloride (NH4Cl), which is separated from a sample of water by a thin membrane. When the pack is struck

with the palm of the hand, the membrane is broken, and the salt dissolves in the water.

NH4NO3(s)
+
H2O
NH4+(aq)
+
NO3-(aq)

H = 25 kJ/mol
NH4Cl(s)
+
H2O
NH4+(aq)
+
Cl-(aq)

H= 20 kJ/mol

Because the reaction is endothermic, it absorbs heat from its surroundings, and the ice pack can get cold enough to treat

minor athletic injuries.

2.Reaction of solid barium hydroxide with ammomium salts

If 32g of solid barium hydroxide octahydrate are mixed with one of the following

  • 11g of solid ammonium chloride
  • 16 grams ammonium thiocyanate
  • 17g of ammonium nitrate

in a 1 L round bottomed flask a vigorous endothermic reaction occurs and ammonia vapour is produced.

Stir the chemicals together by twirling them in the flask. The two solids will gradually react as follows:

Ba(OH)2 .8 H2O

(s)
+
2 NH4Cl

(s)
BaCl2

(aq)
+
2 NH3 (aq)
+
10 H2O

(l)

H =

+60 kj/mole
Ba(OH)2 .8 H2O

(s)
+
2 NH4SCN

(s)
Ba(NO3)2 (aq)
+
2 NH3 (aq)
+
10 H2O

(l)

H =

+60 kj/mole
Ba(OH)2 .8 H2O

(s)
+
2 NH4SCN

(s)
Ba(SCN)2 (aq)
+
2 NH3 (aq)
+
10 H2O

(l)

H =

+60 kj/mole

There is a sharp temperature drop and if the flask is wet on the outside frost forms. The temperature may drop to -20 0 C.

  • When solids become liquid, set flask on moist piece of wood.
  • Hold until water freezes and flask sticks to wood.
  • Lift flask to show its hold on the wood.

3. Reaction of solid ammonium carbonate with carboxylic acids

If 1g of solid ammonium carbonate is added to 10mls of either

  • 1.0M ethanoic acid
  • 1.0M propanoic acid
  • 1.0M butanoic acid (saturated)

a vigorous endothermic reaction occurs.

(NH4)2CO3 (s)
+
2 CH3COOH(aq)
2 CH3COONH4(aq)
+
CO2 (g)
+
H2O(l)
+ ? kj/mole
(NH4)2CO3 (s)
+
2 C2H5COOH(aq)
2 C2H5COONH4(aq)
+
CO2 (g)
+
H2O(l)
+?

kj/mole
(NH4)2CO3 (s)
+
2 C3H7COOH(aq)
2 C3H7COONH4(aq)
+
CO2 (g)
+
H2O(l)
+? kj/mole

References

Chemical Demonstrations: Vol 1 pp8: Chemical Cold Pack: B.Z Shakhashiri

Chemical Demonstrations: Vol 1 pp10: Endothermic reactions of hydrated barium hydroxide. B.Z Shakhashiri.

Chemistry Matters: American Chemical Society. Hot & Cold Packs Feb 1987: pp7-11