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 the basis 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 (l)
NH4+ (aq)
+
NO3- (aq)
H = 25 kJ / mol
NH4Cl (s)
+
H2O (l)
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 ammonium 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 litre 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)
Ba(OH)2.8 H2O
(s)
Ba(OH)2.8 H2O
(s)
+
2 NH4Cl
BaCl2
(s)
(aq)
+ 2NH4SCN
(s)
+ 2NH4NO3
(s)
Ba(SCN)2
+
(aq)
+
+
2NH3
+
2NH3
(aq)
10 H2O
(l)
(aq)
+
10 H2O
(l)
(aq)
(aq)
Ba(NO3) 2
2NH3
+
10 H2O
(l)
H =+60
kj/mole
H =+60
kj/mole
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 o 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. The experiment also works with citric acid.
(NH4) 2CO3
+
(s)
(NH4) 2CO3
+
(s)
2CH3COOH
2CH3COONH4
(aq)
(aq)
2C2H5COO
2C2H5COONH4
H
(aq)
+
CO2
+
(g)
+
CO2
+
(g)
H2O
H =+?
(l)
kj/mole
H2O
H =+?
(l)
kj/mole
H2O
H =+?
(l)
kj/mole
(aq)
(NH4) 2CO3
(s)
+
2C3H7COO
2C3H7COONH4
H
(aq)
+
CO2
(g)
+
(aq)
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 and Cold Packs Feb 1987: pp7-11