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This article envisages bond enthalpies in the form of bond energies and looks at some simple calculations involving them. One of the most confusing things about this is the way the words are used. These days, the term "bond enthalpy" is normally used, but we will also find it described as "bond energy" - sometimes in the same article. An even older term is "bond strength". So we can take all these terms as being interchangeable. As we will see below, though, "bond enthalpy" is used in several different ways, and we might need to be careful about this. Explaining the terms Bond dissociation enthalpy and mean bond enthalpy Simple diatomic molecules A diatomic molecule is one that only contains two atoms. They could be the same (for example, Cl 2 ) or different (for example, HCl). The  bond dissociation enthalpy  is the energy needed to break one mole of the bond to give separated atoms - everyth

There is no denying the fact that Chemical reactions can either release energy to their surroundings, exothermic , or energy can be transferred to them from the surroundings, endothermic. Exothermic reactions are most common, however, an important example of an endothermic reaction is photosynthesis in plants, where the energy supplied is from sunlight. Law of conservation of energy: Energy cannot be destroyed or created but only transferred from one form to another. The total energy of a system of reacting chemicals and surroundings remains constant. Enthalpy change is the term used to describe the energy exchange that takes place with the surroundings at a constant pressure and is given the symbol ΔH. Enthalpy is the total energy content of the reacting materials. It is given the symbol, H. ΔH = ΔH products - ΔH reactants The units are kilojoules per mole (kJmol -1 ) An exothermic enthalpy change is always given a negative value, as energy is lost to the

 we   can clearly see that we   need an input of energy to get the reaction going. Once the activation energy barrier has been passed, we   can also see that we   get even more energy released, and so the reaction is overall exothermic. If we   had an endothermic reaction, a simple energy profile for a non-catalyzed reaction would look like this: Unfortunately, for many reactions, the real shapes of the energy profiles are slightly different from these, and the rest of this page explores some simple differences. What matters is whether the reaction goes via a single transition state or an intermediate. We will look at these two different cases in some detail. Energy profiles for reactions which go via a single transition state only This is much easier to talk about with a real example. The equation below shows an organic chemistry reaction in which a bromine atom is being replaced by an OH group in an organic compound. The starting compound

Discussion Questions How lattice energy is estimated using Born-Haber cycle? How is lattice energy related to crystal structure? Lattice Energy The Lattice energy, U, is the amount of energy required to separate a mole of the solid (s) into a gas (g) of its ions. MaLb(s) a Mb+(g) + b Xa- (g)     U kJ/mol This quantity cannot be experimentally determined directly, but it can be estimated using Hess Law in the form of Born-Haber cycle. It can also be calculated from the electrostatic consideration of its crystal structure. As defined, the lattice energy is positive, because energy is always required to separate the ions. For the reverse process, the energy released is called energy of crystallization, Ecryst. a Mb+(g) + b Xa- (g) MaLb(s)     Ecryst kJ/mol Therefore, U = - Ecryst Values of lattice energies for various solids have been given in literature, especially for some common solids. Some are given here. Comparison of Lattice Energies (U i