Standard enthalpy change defined

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 most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

The superscript "o" or degree sign on AH designates a standard enthalpy change, that is, an enthalpy change when the reactants start out in standard states and the products end up in standard states. Standard states are defined as follows ... 

The strength of a chemical bond is often known as the bond dissociation energy. It is defined as the standard enthalpy change of the reaction in which the bond is broken. Let us assume that a silicon wafer is to be cleaned. The slurries used for CMP have either A1203 or SiOz. There are most likely four types of chemical bonds involved in chemical bonding, as shown in Table 8.2.14 In this table, we also list the value of bond length. Here we do not consider the angle or the orientation of the bonds. 

To evaluate the integral one needs to have the standard enthalpy change as a function of temperature. This information can be computed from the partial molar heat capacities. The partial molar heat capacity for component j is defined by... 

Defines different types of standard enthalpy change... 

The magnitude of any enthalpy change depends on the temperature, pressure, and state (gas, liquid, or solid crystaUine form) of the reactants and products. To compare enthalpies of different reactions, we must define a set of conditions, called a standard suite, at which most enthalpies are tabulated. The standard state of a substance is its pure form at atmospheric pressure (1 atm) and the temperature of interest, which we usually choose to be 298 K (25 °C). The standard enthalpy change of a reaction is defined as the enthalpy change when all reactants and products are in their standard states. We denote a standard enthalpy change as AH°, where the superscript ° indicates standard-state conditions. 

Equation 7.3.8 is applicable to both reactive and non-reactive systems. For the latter the standard state enthalpies and enthalpies of formation cancel because input and output flowrates are equal. A more convenient form of equation 7.3.8 for reactive systems is obtained by introducing the enthalpy change for the reaction. For a reaction described by the general stoichiometric equation (equation 7.2.7), the standard enthalpy change is defined as... 

Plan We can use standard enthalpies of formation to calculate AH for the reaction. We can then use Le ChateUer s principle to determine what effect temperature will have on the equilibrium constant. Recall that the standard enthalpy change for a reaction is given by the sum of the standard molar enthalpies of formation of the products, each multipUed by its coefficient in the balanced chemical equation, less the same quantities for the reactants. At 25 C, AHj for NH3( ) is —46.19 kj/mol. The AHJ values for H2(g) and N2(g) are zero by definition, because the enthalpies of formation of the elements in their normal states at 25 C are defined as zero (S tion 5.7). Because 2 mol of NH3 is formed, the total enthalpy change is... 

Most ion/molecule equilibrium studies have been devoted also to the derivation of extensive scales of BDEs. From the temperature dependence of the equilibrium constants (Van t Hoff equation), enthalpy changes have been obtained. The BDEs are defined as the standard enthalpy changes at 0 K for the reaction (Eq. 1.6). These can be calcnlated using Kirchhoff s equation ... 

The quantity usually tabulated in primary thermodynamic tables is Afff (r), the standard enthalpy of formation of the substance B at temperature T (usually 298.15 K or 0). The standard enthalpy of formation at temperature T is defined as the standard enthalpy change of the reaction in which the substance B is formed from its elements each in the state most... 

Dissociation Energy— the measure of the strength of a chemical bond, defined by the standard enthalpy change of the bond being broken. 

The standard enthalpy changes of hydration and solution can be defined in terms of one mole ofa specified compound reacting completely. 

Define the term standard enthalpy change of formation. [3]... 

The gas-phase basicity GB and the proton affinity PA of a base B are defined as the standard Gibbs energy change and the standard enthalpy change, respectively, of the formal deprotonation reaction 1.12 ... 

The measured enthalpy change for a reaction has a unique value only if the initial state (reactants) and final state (products) are precisely described. If we define a particular state as standard for the reactants and products, we can then say that the standard enthalpy change is the enthalpy change in a reaction in which the reactants and products are in their standard states. This so-called standard enthalpy of reaction is denoted with a degree symbol, AjH . 

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