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Name: Devising
Status: student
Grade: other
Country: India
Date: Summer 2009


Question:
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.



Replies:
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 "Thermodynamics".

Vince Calder


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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