Hess’ law of summation

Based on the assumption that the system is closed, which is usually the case in DSC and microcalorimetry, any reaction or change in state is independent of the path and can be subdivided into small reversible steps (Hess Law of Summation). The First Law of Thermodynamics states that energy may neither be created nor be destroyed. It defines the internal energy, dU, as the sum of the change in heat that has been transferred to the system, dq, and the work done on the system, dw. 

Bostic et al. (113) reported on a series of PCT fabrics treated with selected phosphorus- and halogen-containing flame retardants which were studied by static oxygen bomb calorimetry. The amount of heat evolved when these fabrics were burned in the open atmosphere was determined indirectly using calculations based on Hess law of summation. The heat evolution, when corrected for contributions due to burning of the flame retardant, appeared to correlate with the efficiency of the flame retardant treatment and was interpretable in terms of mechanisms of flame retardant action. 

If aprocess can occur in successive steps, AH for the overall process is equal to the sum of the enthalpy changes for the individual steps. This rule is Hess law or, more formally, Hess law of constant heat summation. 

Thus we see that heats of reaction may be added together in just the same way as the equations for chemical reactions. This fact is of use when the heat of a particular reaction is difficult to measure experimentally, but the reaction can be spht up into other reactions whose heats are more easily determined. This is the basis of Hesses law of constant heat summation. 

Use Hess s law of summation of enthalpies of reaction to calculate the enthalpy change for a reaction. 

This principle, known as Hess law of independent heat summation is a direct consequence of the enthalpy being a state function. Hess law is one of the most powerful tools of chemistry, for it allows the change in the enthalpy (and in other thermodynamic functions) of huge numbers of chemical reactions to be predicted from a relatively small base of experimental data. 

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

Many compounds cannot be directly synthesized from their elements. In some cases, the reaction proceeds too slowly, or side reactions produce substances other than the desired compound. In these cases A// can be determined by an indirect approach, which is based on Hess s law of summation for extensive thermodynamic quantities. 

Equation (8.13) corresponds to the standard enthalpy of formation of MX(s). The equations in the box represent a second pathway for the formation of MX(s) from its constituent elements. Equation (8.14) represents Hess s law of summation for these two pathways. 

Hess s law Sometimes called the law of constant heat summation, it states that the total heat change accompanying a chemical reaction is independent of the route taken in reactants becoming products. Hess s law is an application of the first law of thermodynamics to chemical reactions. 

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. 

The enthalphy change for this reaction at 25°C can be estimated from the algebraic summation of enthalphy changes for reactions that combine to give the same overall reaction (an application of Hess Law) ... 

Hess s law, or the law of constant heat summation, states that at constant pressure, the enthalpy change for a process is not dependent on the reaction pathway, but is dependent only upon the initial and final states of the system. The enthalpy changes of individual steps in a reaction can be added or subtracted to obtain the net enthalpy change for the overall reaction. 

Chemists can determine the enthalpy change of any reaction using an important law, known as Hess s law of heat summation. This law states that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products). The enthalpy change is independent of the pathway of the process and the number of intermediate steps in the process. It is the sum of the enthalpy changes of all the individual steps that make up the process. 

Explain how you used Hess s law of heat summation to determine AH of the combustion of magnesium. State the result you obtained for the thermochemical equation that corresponds to chemical equation (1). 

In this section, you learned how to calculate the enthalpy change of a chemical reaction using Hess s law of heat summation. Enthalpies of reaction can be calculated by combining chemical equations algebraically or by using enthalpies of formation. Hess s law allows chemists to determine enthalpies of reaction without having to take calorimetric measurements. In the next section, you will see how the use of energy affects your lifestyle and your environment. 

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... 

Hess s law phys chem The law that the evolved or absorbed heat in a chemical reaction is the same whether the reaction takes one step or several steps. Also known as the law of constant heat summation. hes-oz, 16 hetero- chem Prefix meaning different for example, a heterocyclic compound is one in which the ring is made of more than one kind of atom. hed-o-ro heteroatom org chem in an organic compound, any atom other than carbon or hydrogen. hed-3-ro,ad-3m ... 

The principle that different structural domains, moieties, or features of a molecular substance contribute separately and additively to a property of a substance. In 1840, G. H. Hess introduced the Law of Constant Heat Summation, a relation that allows one to calculate the heat of a reaction from collected measurements of seemingly different reactions, as long as the summation of a series of reactions yields the same overall chemical reaction as the one of interest. Thermodynamic additivity requires that if two components, A and B, contribute independently to some process, then the total change in free energy (or enthalpy or entropy) is the sum of components, AG = AGa + AGb. In view of its broad use in examining chemical and physical principles, Benson has even offered the view that additivity is the fourth law of thermodynamics. 

The terms AHj, L, AH yUnd i used in Fig. 7.1 are all enthalphy changes defined as follows AHi is the heat of immersion of the solid into the liquid, L is the latent heat of condensation, AH yis the heat of adsorption when the solid is equilibrated with saturated vapor, and i is the heat liberated when solid in equilibrium with saturated vapor is immersed into liquid. Using Hess s law of heat summation... 

Hess was a Swiss-born Russian chemist and doctor. He became professor at Saint Petersburg Technological Institute in 1830. He formulated Hess s Law, an early principle of thermochemistry, which is also known as the law of constant heat summation., ... 

Hess law ( law of constant heat summation ) Reaction enthalpies add together as do the associated chemical reactions. 

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