Standard enthalpy changes

Standard Heat of Reaction. This is the standard enthalpy change accompanying a chemical reaction under the assumptions that the reactants and products exist in their standard states of aggregation at the same T and P, and stoichiometric amounts of reactants take part in the reaction to completion at constant P. With P = 1 atm and T = 25°C as the standard state, AH (T,P) can be written as... 

The standard enthalpy change, Aff°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. 

Coffee-cup calorimeter Bomb calorimeter Standard enthalpy change First law of thermodynamics AH versus AE... 

The Arrhenius activation energy,3 obtained from the temperature dependence of the three-halves-order rate constant, is Ea = 201 kJ mol-1. This is considerably less than the standard enthalpy change for the homolysis of acetaldehyde, determined by the usual thermodynamic methods. That is, reaction (8-5) has AH = 345 kJ mol-1. At first glance, this disparity makes it seem as if dissociation of acetaldehyde could not be a predecessor step. Actually, however, the agreement is excellent when properly interpreted. 

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.

C06-0074. Determine the standard enthalpy change for each of the following reactions ... 

Both the integrated and differential forms show that a plot of log K against 1/T should yield a straight line with a slope equal to -AH0/2.303 R. Thus, a measured value of AH0 can be employed to calculate the equilibrium constant at temperatures other than that for which it is given. Conversely, it is possible to use measurements of the equilibrium constant at a number of temperatures to evaluate the standard enthalpy change for the reaction. 

Note A negative sign is necessary in equation 3.24 as Qr is positive when heat is evolved by the reaction, whereas the standard enthalpy change will be negative for exothermic reactions. Qp will be negative when cooling is required (see Section 3.4). 

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... 

The standard enthalpy change for the reaction at 25 °C may be taken as —23 kcal/g mole. 

The standard enthalpy change for this reaction at 200 °C is —126 kJ/mole for the reaction as written. Each CSTR has a volume of 0.2 m3. The feed rate is 66 cm3/sec. The following data on average heat capacities are available. 

Inlet temperature = 312 °K Standard enthalpy change on reaction at 300 °K = —42 kJ/mole D formed Liquid heat capacity = 2.0 J/cm3oK... 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

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

In all cases studied the standard enthalpy change accompanying the replacement of two water molecules by the chelating ligand is... 

Example Calculate the standard enthalpy change for the oxidation of ethanol (C2H5OH) to ethanal (CH3CHO), given the following standard enthalpies of combustion ... 

The standard enthalpy change of atomisation of an element, AHtt, is the enthalpy change when one mole of gaseous atoms is produced from the element in its standard state, under standard conditions. 

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. 

The acidity of a substrate AH in the gas phase is measured using the standard enthalpy change A//°cid for the heterolytic bond dissociation ... 

The numerical values of cell potentials and half-cell potentials depend on various conditions, so tables of standard reduction potentials are true when ions and molecules are in their standard states. These standard states are the same as for tables of standard enthalpy changes. Aqueous molecules and ions have a standard concentration of 1 mol/L. Gases have a standard pressure of 101.3 kPa or 1 atm. The standard temperature... 

As we mentioned, it is necessary to have information about the standard enthalpy change for a reaction as well as the standard entropies of the reactants and products to calculate the change in Gibbs function. At some temperature T, A// j can be obtained from Af/Z of each of the substances involved in the transformation. Data on the standard enthalpies of formation are tabulated in either of two ways. One method is to list Af/Z at some convenient temperature, such as 25°C, or at a series of temperatures. Tables 4.2 through 4.5 contain values of AfZ/ at 298.15 K. Values at temperatures not listed are calculated with the aid of heat capacity equations, whose coefficients are given in Table 4.8. 

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