In this critical review, the significance of the term 'activity' is examined in the context of the properties of aqueous solutions. The dependence of the activity of water(,) at ambient pressure and 298.15 K on solute molality is examined for aqueous solutions containing neutral solutes, mixtures of neutral solutes and salts. Addition of a solute to water(,) always lowers its thermodynamic activity. For some solutes the stabilisation of water(,) is less than and for others more than in the case where the thermodynamic properties of the aqueous solution are ideal. In one approach this pattern is accounted for in terms of hydrate formation. Alternatively the pattern is analysed in terms of the dependence of practical osmotic coefficients on the composition of the aqueous solution and then in terms of solute-solute interactions. For salt solutions the dependence of the activity of water on salt molalities is compared with that predicted by the Debye-Hü ckel limiting law. The analysis is extended to consideration of the activities of water in binary aqueous mixtures. The dependence on mole fraction composition of the activity of water in binary aqueous mixtures is examined. Different experimental methods for determining the activity of water in aqueous solutions are critically reviewed. The role of water activity is noted in a biochemical context, with reference to the quality, stability and safety of food and finally with regard to health science.

The thermodynamic background to three calorimetric techniques is discussed; (i) titration microcalorimetry, (ii) adiabatic calorimetry, and (iii) heat conduction calorimetry. Relevant equations for each technique are derived from a common equation for the enthalpy H of a closed system. General patterns which emerge in the measured parameters are presented for adiabatic and heat conduction calorimeters linked to applications of these techniques.

General equations are developed relating the dependence of temperature on time for a closed system undergoing chemical reaction. Equations are reported for thermally insulated systems and for systems where Newton's law of cooling describes the exchange of thermal energy between system and surroundings. These equations provide the basis for analysing and modelling thermal methods for screening combinatorial libraries. Key equations describe the impact of chemical reactions having different orders and molecularities. The potential of resulting differential equations is illustrated using numerical integration to yield temperature-time plots which highlight the roles of rate constants and enthalpies of reactions.

Heat conduction microcalorimetry offers an important technique in the context of physical organic chemistry for the study of chemical reactions in solution. The thermodynamic basis of the technique is discussed. Model reaction schemes are used in an examination of the dependence on time of the rate of heat production. The impact of rate constants and enthalpies of reaction on the recorded dependence is reviewed for first-and second-order reactions.

Theoretical chemistry Theoretical chemistry Z 0350 Three Important Calorimetric Applications of a Classic Thermodynamic Equation -[35 refs.]. -(BLANDAMER*, M. J.; CULLIS, P. M.; GLEESON, P. T.; Chem. Soc. Rev. 32 (2003) 5, 264-267; Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UK; Eng.) -Lindner 48-278