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Enthalpy and Bond Strength



Changes in the enthalpy of a chemical system and the bond strength of the species that make up the system can be described by thermodynamics, which studies the flow of energy within systems and between a system and its surroundings. Internal energy, described as the sum of the kinetic and potential energy of the particles in a system, is a central concept in thermodynamics. Internal energy changes when heat is added to or removed from the system or when the system does work on the surroundings or the surroundings do work on the system. For a chemical reaction, the change in enthalpy is the change in internal energy and a change in the work needed to displace the environment to produce the components of the system. Enthalpies of reactions can be added to or subtracted from each other, as described by Hess's law. Scientists have experimentally measured various enthalpies, including enthalpies of formation, combustion, vaporization, and fusion. Hess's law allows known enthalpy values to be used in calculations of unknown enthalpies of reaction. Enthalpy changes associated with breaking a bond and breaking a lattice structure can also be determined.

At A Glance

  • Enthalpy (H) is equal to the internal energy of a system (U) plus the work (w), pressure times volume (w=PVw=PV), needed to displace the environment to produce the components of the system: H=U+PVH=U+PV.
  • The enthalpy of a reaction is equal to the total enthalpy of products minus the total enthalpy of the reactants: ΔH=HproductsHreactants\Delta H=H_{\rm{products}}-H_{\rm{reactants}}.
  • Hess's law states that if a reaction is carried out in multiple steps, the reaction enthalpy is equal to the sum of the reaction enthalpies of each state.
  • Enthalpy of formation is the enthalpy of the reaction in which a compound is formed from its elements.
  • The enthalpy of a reaction can be estimated by the bonds that are broken and bonds that are formed in a chemical reaction.
  • Lattice energies can be calculated by adding the energy changes involved in breaking and forming bonds as well as the energy changes that occur during changes of state.