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In the synthesis of urea from ammonia and carbon dioxide, the two reactants first combine to form ammonium carbamate as follows,  2 NH3 + CO2 (NH3)2CO2, with the liberation of a considerable quantity of heat. The present study was undertaken with the object of obtaining an accurate measure of this heat for use in such connections as the design and control of urea synthesis apparatus. Experimental determinations of this value have previously been and in addition there are available vapor-pressure data3 from which a value may be calculated, but examination of this material revealed discrepancies which would impair its usefulness. The experimentally determined values range from 37,700 cal. to over 42,000 cal. per mole at constant pressure.