Law of heat summation

If the heats of reaction at a given temperature are known for two separate reactions, the heat of reaction of a third reaction at the same temperature may be determined by simple algebraic addition. This statement is the Law of Heat Summation. For example, reactions (1.6) and (1.7) can be carried out conveniently in a calorimeter at constant pressure ... 

The superscript to the heat of formation symbol A// represents the standard state, and the subscript number represents the base or reference temperature. From the example for the Law of Heat Summation, it is apparent that the heat of formation of carbon monoxide from Eq. (1.8) is... 

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. 

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... 

Thus, irrespective of the mechanism of removing carbon from the surface, the main considerations are as follows (1) only CO must form on the surface, and (2) the flux of oxygen from the quiescent atmosphere must be stoichiometric with respect to CO formation, regardless of the intermediate reactions that take place. This result is the same as that reached earlier, where the two primary reactions were added according to the 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. 

In 1840, G. H. Hess (1802-1850) published his law of heat summation, which he derived on the basis of numerous thermochemical observations. 

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. 

The Indirect Method. 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 AHf can be determined by an indirect approach, which is based on the law of heat summation (or simply Hess s law). 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, hi other words, if we can break down the reaction of interest into a series of reactions for which AH can be measured, we can calculate AT/ xn for the overall reaction. 

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). 

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. 

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)... 

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