Hesss Law

Most of the processes take place at constant pressure. In 1840, Hess found that the total heat, which the system releases or consumes at a chemical reaction, is equal regardless of whether the reaction runs in one step or stepwise. This is the principle of Hess s law. The Hess s law can be demonstrated on the oxidation of sulfur 

Enthalpy is the function of state and its value depends only on the starting and final state of the system Qp = H2- H ). In the case of a chemical reaction, its value depends only on the state of the chemical reactants and the reaction products. 

The molar heat capacities at constant volume and constant pressure are defined by the relations 

Heat capacities depend on temperature and pressure, or on temperature and volume. For practical use, the temperature dependence of Cp is given in tables in the form of empirical equations of the type 

These equations are valid in the temperature interval in which the heat capacity is measured. The total amount of heat needed for heating 1 mol of a substance is 

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N2 O4 N2(g) + 2 02(g) N2 04(g) The pathway shown in Figure 6-19 is not how the reaction actually occurs, but enthalpy is a state function. Because the change of any state function is independent of the path of the reaction, we can use any convenient path for calculating the enthalpy change. Hess law summarizes this feature ... 

The similarity arises because both equations describe similar processes and are consequences of Hess law. For Equation, we imagine a process in which reactants decompose into elements in their standard states, which then recombine to give products. For Equation, we imagine a process in which reactants break up entirely into gaseous atoms, which then recombine to give products. 

C14-0133. The enthalpy of sublimation of Ice at 273.15 K Is not the simple sum of the enthalpies of fusion and vaporization of water, but it can be calculated using Hess law and an appropriate path that Includes fusion and vaporization. Devise such a path, show it on a phase diagram for water, and carry out the calculation, making reasonable assumptions If necessary (C(liquid water) = 75.3 7 mol K , and C(water vapor) = 33.6 K ). 

This relationship (sometimes called Luther s law) for the transfer of several electrons permits us to calculate one redox potential if the others are known. Obviously, this is an analogy of the Hess law in thermodynamics. Equation (3.2.24) is not restricted to the case where the lowest oxidation state is a metal. 

This formula, known as Hess Law, allows us to determine the magnitude of enthalpy associated with a process and whether that process is endothermic or exothermic. 

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) ... 

KEY TERMS oxidation-reduction disproportionation Hess Law exothermic reaction... 

Hess law states that the enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final states. ... 

Once more, we start by drawing a Hess-law cycle with the elements at the bottom of the page. This time, it is not convenient to write the reaction of interest along the... 

Extremely high selectivities are frequently interpreted as "ion fixation", which suggests an irreversible phenomenon. This is the case for exchanges of Cs, Rb and K in illite clay minerals (95-96) as well as for Cu(NHj) exchange in fluorhectorite (66). However, reversibility was verified from the Hess law for adsorption of Cs, Rb and K on the high affinity sites in illite (91) and modified montmorillonites (101) as well as for the exchange of transition metal complexes (29, 75). 

Chain Reactions in]. Nevertheless, the energy evolved depends only on the initial and final states and not on intermediate ones. Once the reaction is completed, the net heat evolved is exactly the same as if the reactant molecules were first dissociated into their atoms, and then reacted directly to form the final products (Hess Law). If a compd be formed directly from the atoms, the heat of atomization (Qa.) which was required to generate them from the molecules... 

Work, heat, AU, AH, Isothermal, isobaric, isochoric, adiabatic Hess law, calorimetry, T-dependence of enthalpy Second Law 5 lectures... 

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

Excellent measurements 14> of each of these terms in several solvents show that only the last mixing term is constant with respect to the degree of polymerization P, whereas the other two terms are P-dependent. Hence, applying Hess law, (Hs — Hg)rev = AH8(gel + sol) = AHs(glass -+ sol) — AHs(glass - gel), and comparing the result with Eq. (7), the thermodynamic meaning of both terms of Eq. (7) is obtained The second term... 

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. 

EXAMPLE 4 Hess law provides us with the application based on the following ... 

The addition of voltages in part (a) of Example 7 is really a form of addition of Gibbs free energy (Hess law). Because in the overall cell chemical reaction, the electrons cancel, both electrode reactions must involve the same... 

Sometimes, the terms heat of explosion and heat of detonation get mixed-up and are often (wrongly ) used as if they were equivalent. The heat of explosion and the heat of detonation are both heats of reaction but are obtained under different conditions (by applying Hess law). In the detonation run, the heat of reaction corresponds to the heat of reaction at the C-J point , and is called heat of detonation. The heat of reaction obtained under constant volume condition combustion is usually called the heat of combustion or heat of explosion . There is one additional term which is worth mentioning here and that is the heat of complete combustion . This term is usually used to denote constant volume combustion in an oxygen atmosphere. 

We can apply the Hess Law because the molecular N2 in eq. 10.15 only appears in eqs. 10.13 and 10.14 (as a product), we can add them (each multiplied by one half). Note Physical states are omitted for simplicity. 

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