Search results
Results From The WOW.Com Content Network
The bond dissociation energy (enthalpy) [4] is also referred to as bond disruption energy, bond energy, bond strength, or binding energy (abbreviation: BDE, BE, or D). It is defined as the standard enthalpy change of the following fission: R—X → R + X. The BDE, denoted by Dº(R—X), is usually derived by the thermochemical equation,
The term bond-dissociation energy is similar to the related notion of bond-dissociation enthalpy (or bond enthalpy), which is sometimes used interchangeably.However, some authors make the distinction that the bond-dissociation energy (D 0) refers to the enthalpy change at 0 K, while the term bond-dissociation enthalpy is used for the enthalpy change at 298 K (unambiguously denoted DH° 298).
This method is based on Hess's law, which states that the enthalpy change is the same for a chemical reaction which occurs as a single reaction or in several steps. If the enthalpies for each step can be measured, then their sum gives the enthalpy of the overall single reaction. [11]
The change in enthalpy for a reaction can be calculated from the enthalpies of formation of the reactants and the products; Elements in their standard states make no contribution to the enthalpy calculations for the reaction, since the enthalpy of an element in its standard state is zero.
The Van 't Hoff equation relates the change in the equilibrium constant, K eq, of a chemical reaction to the change in temperature, T, given the standard enthalpy change, Δ r H ⊖, for the process. The subscript r {\displaystyle r} means "reaction" and the superscript ⊖ {\displaystyle \ominus } means "standard".
Hess's law is useful in the determination of enthalpies of the following: [2] Heats of formation of unstable intermediates like CO (g) and NO (g). Heat changes in phase transitions and allotropic transitions. Lattice energies of ionic substances by constructing Born–Haber cycles if the electron affinity to form the anion is known, or
Enthalpy of atomization is the amount of enthalpy change when bonds of the compound are broken and the component atoms are separated into single atoms ( or monoatom). Enthalpy of atomization is denoted by the symbol ΔH at. The enthalpy change of atomization of gaseous H 2 O is, for example, the sum of the HO–H and H–OH bond dissociation ...
These approximations account for the atomic, bond, and group contributions to heat capacity (C p), enthalpy (ΔH°), and entropy (ΔS°). The most important of these approximations to the group-increment theory is the second-order approximation, because this approximation "leads to the direct method of writing the properties of a compound as ...