Heat summation, Hess’s law of Constant

An important corollary of this postulate is known as Hess s law of constant heat summation (1840) The overall heat of a chemical reaction is the same whether the reaction occurs in a single step or multiple steps. 

This generalization was first proposed in the year 1840 by G. H. Hess on the basis of his experimental measurements of reaction heats. It is sometimes called Hess s Law of Constant Heat Summation. 

Acmally, this question was answered on empirical grounds long before thermodynamics was established on a sound basis. In courses in elementary chemistry, students become familiar with Hess s law of constant heat summation, which was enunciated in 1840. Hess pointed out that the heat absorbed (or evolved) in a... 

If a process can be imagined to occur in successive steps, AH for the overall process is equal to the sum of the enthalpy changes for the individual steps. This rule, sometimes called Hess s law of constant heat summation, has many applications in thermochemistry. 

It is important to realize that the change in energy of a system depends on the functions of state (U and U2) not on the path of the transformation. This is known as Hess s Law of Constant Heat Summation. 

This procedure, which is an example of Hess s law of constant heat summation, relies upon the fact that AH for any reaction depends only on the final and initial states, and illustrates a convenient method for calculating AH values that would be difficult to measure experimentally. 

The type of construction given in Fig. 10a, which utilizes the Hess s law of constant heat summation, can serve as a means of quantitatively analyzing the thermodynamics of solvation. Further, this view of the solvation process provides a method for considering different standard states. For nonionic species a commonly used standard state is infinite dilution. Although activities become infinite for ions in this limit, it is still a useful reference state because the analytic Debye-Hiickel limiting law is valid in this regime.168... 

The change in state of a system produced by a specified chemical reaction is definite. The corresponding enthalpy change is definite, since the enthalpy is a function of the state. Thus, if we transform a specified set of reactants to a specified set of products by more than one sequence of reactions, the total enthalpy change must be the same for every sequence. This rule, which is a consequence of the first law of thermodynamics, was originally known as Hess s law of constant heat summation. Suppose that we compare two different methods of synthesizing sodium chloride from sodium and chlorine. 

Quantity of water 982° [F.] Experiments in which steam was passed into a worm immersed in cold water gave 920° F. He preferred 960° F., i.e. 535° C. (modern value 536). Cavendish also measured the difference in heat evolved on neutralising (i) alkali carbonates and (ii) caustic alkalis with acids, to find the cold generated by the emission of fixed air , which really assumes Hess s law of constant heat summation. ... 

Hess s Law of Constant Heat Summation. To describe the standard enthalpy change for the formation of N02(g) from N2(g) and 02(g),... 

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