Hess s law

Born-Haber cycle A thermodynamic cycle derived by application of Hess s law. Commonly used to calculate lattice energies of ionic solids and average bond energies of covalent compounds. E.g. NaCl ... 

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. 

The small value of the entropy change reflects the fact that only liquids are involved in tlris reaction. The heat balance in canying out tlris reaction may be calculted, according to Hess s law, by calculating tire heat change at room temperarnre, and subtracting tire heat required to raise the products to the hnal teirrperamre. The data for tlris reaction are as follows ... 

We use Hess s law to break down the equation into a composite of CeHe -b 6CH4 3C2H6 -b 3C2H4 I... 

Coneerning the enthalpies of formation, whieh are routinely obtained from semiempirieal ealeulations sueh as MNDO or AMI, they are diffieult to ealeulate ab initio and, in general, only relative values are ealeulated for eompounds of the same moleeular eomposition by means of isodesmie reaetions (98JCP692). This preeludes earrying out a ealeulation for the two tautomers and eomparing the ealeulated values with the experimental ones. Some authors have used Hess s law to overeome this diffieulty [97JPC(A)283]. 

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. 

Applications. (1) Heats of formation data of reactants and products can be used to calculate the standard heat of a chemical reaction by applying Hess s law. Thus,... 

Very frequently AH data are available for inorganic substances but not for organic compounds for which AHj values are more readily available. Because AH of hydrocarbons are not easily measurable, they are often deduced by Hess s law from known AH of the hydrocarbon and known AH values of the products of combustion. 

This relationship is referred to as Hess s law, after Germain Hess (1802-1850), professor of chemistry at the University of St. Petersburg, who deduced it in 1840. Hess s law is a direct consequence of the fact that enthalpy is a state property, dependent only on initial and final states. This means that, in Figure 8.6, AH must equal the sum of AH, and AH2, because the final and initial states are the same for the two processes. 

Hess s law is very convenient for obtaining values of AH for reactions that are difficult to carry out in a calorimeter. Consider, for example, the formation of the toxic gas carbon monoxide from the elements... 

This relation can be regarded as the free energy equivalent of Hess s law (Chapter 8). To illustrate its application, consider the synthesis of CuCl2 from the elements... 

Hess s law A relation stating that the heat flow in a reaction that is the sum of two other reactions is equal to the sum of the heat flows in those two reactions, 207... 

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. 

Heat of vaporization, 66 see also Vaporization Helium, 91 boiling point, 63 heat of vaporization, 105 interaction between atoms, 277 ionization energy, 268 molar volume, 60 on Sun, 447 source, 91 Hematite, 404 Hemin, structure of, 397 Hess s Law, 111 Heterogeneous, 70 systems and reaction rate, 126 n-Hexane properties, 341 Hibernation, 2 Hildebrand, Joel H.. 163 Holmium, properties, 412 Homogeneous, 70 systems and reaction rate, 126 Hydration, 313 Hydrazine, 46, 47, 231 Hydrides of third-row elements, 102 boiling point of. 315 Hydrocarbons, 340 unsaturated, 342... 

The author has found the following method of applying Hess s law very simple ... 

To use Hess s law to hnd the enthalpy of a given reaction, wc find a sequence of reactions with known reaction enthalpies that adds up to the reaction of interest. A systematic procedure simplifies that process. 

Section 6.11, when we calculated the enthalpy change for an overall physical process as the sum of the enthalpy changes for a series of two individual steps. The same rule applied to chemical reactions is known as Hess s law the overall reaction enthalpy is the sum of the reaction enthalpies of the steps into which the reaction can be divided. Hess s law applies even if the intermediate reactions or the overall reaction cannot actually be carried out. Provided that the equation for each step balances and the individual equations add up to the equation for the reaction of interest, a reaction enthalpy can be calculated from any convenient sequence of reactions (Fig. 6.30). 

As an example of Hess s law, let s consider the oxidation of carbon as graphite, denoted C(gr), to carbon dioxide ... 

EXAMPLE 6.9 Sample exercise Using Hess s law Consider the synthesis of propane, C H, a gas used as camping fuel 3 C(gr) + 4 H2(g) C3H8(g)... 

The lattice enthalpy of a solid cannot be measured directly. However, we can obtain it indirectly by combining other measurements in an application of Hess s law. This approach takes advantage of the first law of thermodynamics and, in particular, the fact that enthalpy is a state function. The procedure uses a Born-Haber cycle, a closed path of steps, one of which is the formation of a solid lattice from the gaseous ions. The enthalpy change for this step is the negative of the lattice enthalpy. Table 6.6 lists some lattice enthalpies found in this way. 

Calculate an overall reaction enthalpy from the enthalpies of the reactions in a reaction sequence by using Hess s law (Toolbox 6.1 and Example 6.9). 

Hess s law A reaction enthalpy is the sum of the enthalpies of any sequence of reactions (at the same temperature and pressure) into which the overall reaction can be divided, heterogenous alloy See alloy. heterogeneous catalyst See catalyst. heterogeneous equilibrium An equilibrium in which at least one substance is in a different phase from the others. Example AgCI(s) Ag+(aq) + Cl "(aq). heterogeneous mixture A mixture in which the... 

Again students are expected to realise that this does not represent a stand-alone chemical process, and electrons are not found free under usual conditions, and so this process would need to be coupled with one that provides a place for the electron to go. Students may meet this process as part of a simple redox process (say with the reduction of a less reactive metal), or as one component of the analysis of a more complex process using Hess s law to find an enthalpy change by aggregating the enthalpy terms of an indirect route. 

There are two important relationships in thermochemistry which are very useful in the calculation of enthalpies of reactions. These are known as Hess s law and Kirchoff s equation. 

Hess s law states that the overall change in enthalpy in a reaction is the same whether the reaction takes place in one step or through a number of intermediate steps. This law can also be regarded as a consequence of the fact that enthalpy is a state function so that the enthalpy difference between the final state (products) and the initial state (reactants) is the same, irrespective of the reaction path (sequence in which the reaction takes place). As an example, let the following reaction be considered,... 

The overall enthalpy change is the same as that indicated earlier. An useful consequence of Hess s law is that thermochemical equations can be added and subtracted just like algebraic equations. This facilitates the calculation of enthalpy changes for reactions which cannot be studied experimentally. 

Pursuing either of the two ways described, reactants from a given initial state ultimately reach the same final state. Then, according to Hess s law, the heat required in both the ways should be the same. In other words,... 

This process can be represented as taking place in a series of steps, each of which has a well-known enthalpy associated with it. The application of Hess s law provides a useful way to obtain the enthalpy for the overall process because it is independent of the path. 

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