Sulphuric Acid, Water, Ice
Date: Summer 2009
When we add water to concentrated H2SO4 then it is
exothermic, but when we add concentrated H2SO4 to ice under
stirring, it is endothermic. How does it happens?
SAFETY ISSUE: You should NEVER add water to concentrated H2SO4 -- always THE
REVERSE -- acid into water. Adding water to acid invites spattering due to
the large evolution of heat.
These two processes are quite different. The water/acid experiment involves
the large heat of solution of sulfuric acid. The ice/acid experiment
involves the heat of solution is the acid, but superimposed upon that
process is the "freezing point depression" of water due to the presence of
the solute, sulfuric acid. The freezing (melting) point of pure water (ice)
is the temperature (273.15 Kelvins) at which ice and liquid water are in
equilibrium. When you add a solute to the water, you alter the temperature
at which the ice and THE SOLUTION are in equilibrium. It turns out that the
temperature at which ice and the solution come to equilibrium is LESS
than the melting temperature of the pure ice. Quantitatively the relation
is: 1/T - 1/To = - R*ln(x)/DHfusion.
Here T is the absolute temperature (i.e. kelvins), To = 273.15 Kelvins, the
freezing point of pure water, R = gas constant = 1.987k-cal/mol Kelvins; "x"
< 0 is the mole fraction of solute -- actually it is the number of
"particles" present, so for H2SO4 "x" is 3x the molar concentration of H2SO4
(approximately), and (DHfusion) is the heat of fusion of water. If you play
around with this equation you will find some interesting consequences: For
example, since always (DHfusion) > 0, but ln(x) < 0, ALWAYS T< To. That is,
there is a freezing point depression (the temperature T is lower than To),
never a freezing point elevation. The value of T = 237 kelvins for a 30
weight % solution of H2SO4. (DHfusion) = 1436 cal/mol.
You can study this further in any textbook on "Physical Chemistry" or on
The reason is that when you add sulfuric acid to ice, TWO reactions
occur, one exothermic and one endothermic. The exothermic is the
dissolution of sulfuric acid. The endothermic one is the melting of ice.
When you add concentrated sulfuric acid to ice, a lot of the ice
melts. You then have a solution of sulfuric acid in water. Since
this is not a pure solution, its freezing point is lower than the
freezing point of pure water. If no heat is added to the solution
from the environment, it will come to equilibrium at the freezing
temperature of the solution, which is lower than the initial
temperature of this ice.
This is basically the same idea, but with a nastier substance, as
adding salt to ice to make a low-temperature slush cold enough to
freeze ice cream.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012