Calorimeter values were obtained for the standard enthalpy change for the following reactions at 10,25, and 40 degrees C.: 1. The dissaciation of the amino acid, glycine, in the zwitterion form (HA) to give the basic form (A-) and a proton. 2. The reactions of copper (II) ion with glycine to form the Cu gly+ and Cu (gly)2 complexes. 3. The reactions of nickel (II) ion with glycine and alanine to form Ni gly+, Ni (gly)2, Ni al+, and Ni (al)2 complexes. For calorimetric enthalpy calculations, the initial and final concentrations of all species in the calorimeter must be known; consequently, thermodynamic equilibrium constants were determined for this purpose. The equilibrium constants were calculated from measurements of pH versus the volume of standard acid or base added. Thermodynamic equilibrium constants were determined for the following types of reactions at 10, 25, and 40 degrees C.: 1. The dissociation of the acidic amino acid (H2A+) to the zwitterion (HA) and a proton for glycine and phenylalanine. 2. The dissociation of the amino acid zwitterion (HA) to the amino acid base (A-) and a proton for glycine and phenylalanine. 3. The formation of the complexes Cu gly+, and Cu (gly)2 from the reaction of copper (II) ion with glycine. 4. The formation of the complexes Ni gly+, Ni (gly)2, Ni al+, Ni (al)2, Ni Øal+, and Ni (Øal)2 from the reaction of nickel (II) ion with glycine, alanine, and phenylalanine. The thermodynamic functions ΔH°, ΔF°, and Δs° were calculated from the above constants. A form of the Gibbs-Helmholtz equation, d ln K/d(l/T) = -ΔH°/R , was used for calculating ΔH° from the cariation of the equilibrium constants with temperature. ΔH° was calculated from the slope of a straight line fit for the plot log K versus 1/T. The ΔH° so calculated was compared with ΔH° determined from calorimeter measurements. The difference between the values varied from about 0.1 kilocalorie to 2 kilocalories, or about 1 to 20%. Since the heat of the reaction is measured directly in the calorimeter, the calorimetric method should give the better value. it was concluded, therefore, that the variation of euilibrium constants with temperature may not be a reliable method for determining the standard enthalpy change. The reliability of the method probably depends upon the number of points taken and the amount of variation of constants with temperature. It was further concluded that log K does not, in general, vary in an exact linear manner with 1/T. A linear relationship implies that ΔH° is constant with temperature change. All but one of the calorimetric results show a substantial ΔH° change with temperature.



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



Date Submitted


Document Type





Calorimetry, Amino acids, Solution (Chemistry)



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Chemistry Commons