Hess’s Law of Heat Summation The

Hess s law of heat summation the scientific law stating that the enthalpy change of a physical or chemical process depends only on the beginning conditions (reactants) and the end conditions (products) and is independent of the pathway of the reaction or the number of intermediate steps in the reaction (5.3) heterogeneous catalyst a catalyst that exists in a phase that is different from the phase of the catalyzed reaction (6.4)... 

Hess s Law of heat summation The enthalpy change for a reaction is the same whether it occurs in one step or a series of steps. 

This application is based on Hess s law of heat summation the enthalpy change of m overall process is the sum of the enthalpy changes of its individual steps. To use Hess s law, we imagine an overall reaction as the sum of a series of reaction steps, whether or not it really occurs that way. Each step is chosen because its AH is known. Because the overall AH depends only on the initial and final states, Hess s law says that we add together the known AH values for the steps to get the unknown AH of the overall reaction. Similarly, if we know the AH values for the overall reaction and all but one of the steps, we can find the unknown AH of that step. 

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. 

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... 

In situations such as this, you can calculate AH for the formation of sulfur trioxide using Hess s law of heat summation. Hess s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction. Hess s law enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known. 

The excited-state reduction potential, °( Cr3+/Cr2+), can be estimated using an analysis similar to Hess s law of heat summation (Fig. 8.5). Using the emission maximum (730 nm) in the luminescence spectrum and converting units yields an excited-state energy of 164 kJ mol-1 for [Cr(phen)3]3+. That means that relaxation of the 2E excited state to the ground state involves AG° = 164 kJ mol-1 or a one-electron electro-... 

Hess s law of heat summation If a reaction goes through two or more steps, the enthalpy of reaction is the sum of the enthalpies of all the steps the enthalpy change is the same regardless of the number of steps. 

Because H is a state function, we can find AH of any reaction by imagining that it occurs as the sum of other reactions whose AH values we know or can measure (Hess s law of heat summation). 

Hess s law of heat summation A law stating that the enthalpy change of an overall process is the sum of the enthalpy changes of the individual steps of the process. (192) heteroatom Any atom in an organic compound other than C or H. (459)... 

Knowing the standard enthalpies of formation of reactants and products enables us to calculate the enthalpy of a reaction. We will discuss ways to determine these quantities either by the direct method from the elements or by the indirect method, which is based on Hess s law of heat summation. (6.6)... 

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, AH°f can be determined by an indirect approach, which is based on Hess s law of heat summation, or simply Hess s law, named after the Swiss chemist Germain Hess. Hess s law can be stated as follows When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. In other words, if we can break down the reaction of interest into a series of reactions for which can be measured, we can calculate A//°xn for the overall reaction. Hess s law is based on the fact that because // is a state function, AH depends only on the initial and final state (that is, only on the nature of reactants and products). The enthalpy change would be the same whether the overall reaction takes place in one step or many steps. 

Enthalpy is a state function. Its change is therefore independent of the pathway by which a reaction occurs. We do not need to know whether the reaction does, or even can, occur by the series of steps used in the calculation. The steps must (if only on paper ) result in the overall reaction. Hess s Law lets us calculate enthalpy changes for reactions for which the changes could be measured only with difficulty, if at all. In general terms, Hess s Law of heat summation may be represented as... 

Hess s law of heat summation for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps. (6.7)... 

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. ... 

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