Standard enthalpy change of reaction

Fig. 1.2 Standard enthalpy changes of (a) the complexing of lanthanide ions in aqueous solution by EDTA" ( left-hand axis) (b) the standard enthalpy change of reaction 2, the dichloride being a di-f...

Table 1.3 Esti mated values of the four components of the contribution made by ligand field stabilization energy to the lattice enthalpy of KsCuFe, to the hydration enthalpy of Ni (aq), AH (Ni, g), and to the standard enthalpy change of reaction 13.

AG = Gibbs energy change of reaction (1), J/mole A AH° = Standard Enthalpy change of reaction (1), J/mole A AS° = Standard Entropy change of reaction (1), J/mole A K r = Cell efficiency, E/E... 

Here AH is the standard enthalpy change of reaction AS the standard reaction entropy change R the gas constant T the temperature /(eq the equilibrium constant for the reaction, given simply by the product of concentrations (activities in reality) of all the products to the power of their stoichiometric coefficients over the same product for reactants m the number of products / / the forward rate constant kr the reverse rate constant n, the stoichiometric coefficient of species i and 1 the number of reactants. A AG value below zero indicates a reaction with an equilibrium point where there is an excess of products over reactants, a... 

The enthalpy change of a reaction when the reactants and products are in their standard states is called the standard enthalpy change of reaction, AH (with the superscript- - sign being read as standard ) ... 

The standard enthalpy change of reaction (1.16) could be written as... 

The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions. The reactants and products must be in their standard states. 

The symbol for standard enthalpy change of reaction is AH. Enthalpy changes of reaction can be exothermic or endothermic. 

Enthalpy changes of processes depend only on the end states. Normally the enthalpy change of reaction is known at some standard tem-... 

However, each of the individual reactions involves the formation of a compound from its elements or the decomposition of a compound into those elements. The standard enthalpy change of a reaction that involves the formation of a compound from its elements is... 

AH° is the standard enthalpy change of a reaction that occurs at 1 atm pressure. Unless otherwise stated, the reaction temperature is 25°C. 

Heat effects accompanying chemical reaction influence equilibrium constants and compositions as well as rates of reaction. The enthalpy change of reaction, AHr, is the difference between the enthalpies of formation of the participants. It is positive for endothermic reactions and negative for exothermic ones. This convention is the opposite of that for heats of reaction, so care should be exercised in applications of this quantity. Enthalpies of formation are empirical data, most often known at a standard temperature, frequently at 298 K. The Gibbs energies of formation, AGfl likewise are empirical data. 

Enthalpy changes of processes depend only on the end states. Normally the enthalpy change of reaction is known at some standard temperature, Tb = 298 K for instance. The simplest formulation of the heat balance, accordingly, is to consider the reaction to occur at this temperature, to transfer whatever heat is required and to raise the enthalpy of the reaction products to their final values. 

We obtain the standard enthalpy change on reaction A H as a sum of the molar enthalpies of each chemical participating in the reaction ... 

The bond-dissociation energy is defined as the standard enthalpy change of the reaction in which the bond is broken R X - R —X. 

Figure 2-8 The equilibrium constant of Reaction 2-79 as a function of temperature in InK versus lOOO/T plot. The rough straight line means that the standard state enthalpy change of Reaction 2-79 is constant. Solid circles are 1-atm data from Zhang et al. (1997a) and open circles are 500-MPa data from Zhang (unpublished data).

In choosing a reference state, we are allowed to make a choice for each element, because elements cannot be transformed into each other by chemical means. The choice usually made for the reference state of an element is the form in which it is stable at temperature T and the standard pressure =1.0 bar. For example, at most temperatures, for 02 this would be gaseous diatomic molecules for iron, it would be the solid metal, and for bromine, it would be the diatomic in the liquid state below 59°C and in the gaseous state above 59°C. We call the standard enthalpy change of the reaction in which 1 mol of compound i is formed from its component elements in their reference states the heat of formation of compound i, A H°(T). The heat of reaction is related to heats of formation as... 

The standard enthalpy change of a reaction is equal to the sum of the standard molar enthalpies of formation of the products, each multiplied by its coefficient, n, in the balanced equation, minus the corresponding sum of the standard molar enthalpies of formation of the reactants. 

The enthalpies of formation of the alkali metal selenides were re-evaluated by the review from the enthalpy changes of Reactions 1 and 2 exemplified for lithium in Table A-3. The temperature of the experiments was about 288 K. The reactions combine to Reaction 3 from which the enthalpy of formation of the alkali metal selenide was calculated with selected auxiliary data at 298.15 K. A correction for the formation of some Se in Reaction 2 cannot be made due to lack of data. This also pertains to the correction of the data from the experimental temperature to 298.15 K. The results are assumed to be valid at standard conditions within the error limits and the following val-... 

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