We now have one mole of Cs cations and one mole of F anions. An exothermic reaction (ΔH negative, heat produced) results when the bonds in the products are stronger than the bonds in the reactants. Since the lattice energy is negative in the Born-Haber cycle, this would lead to a more exothermic reaction. acid is the strongest acid out of these four because hydroiodic acid has the lowest value for the pKa. The lattice energy of a compound is a measure of the strength of this attraction. So this bond right here must Because the bonds in the products are stronger than those in the reactants, the reaction releases more energy than it consumes: This excess energy is released as heat, so the reaction is exothermic. Thus, we find that triple bonds are stronger and shorter than double bonds between the same two atoms; likewise, double bonds are stronger and shorter than single bonds between the same two atoms. The enthalpy change in this step is the negative of the lattice energy, so it is also an exothermic quantity. Converting one mole of fluorine atoms into fluoride ions is an exothermic process, so this step gives off energy (the electron affinity) and is shown as decreasing along the y-axis. So the more stable the conjugate base, the stronger the acid. In such a lattice, it is usually not possible to distinguish discrete molecular units, so that the compounds formed are not molecular in nature. There may also be energy changes associated with breaking of existing bonds or the addition of more than one electron to form anions. Using the bond energies in [link], calculate the approximate enthalpy change, ΔH, for the reaction here: SolutionFirst, we need to write the Lewis structures of the reactants and the products: From this, we see that ΔH for this reaction involves the energy required to break a C–O triple bond and two H–H single bonds, as well as the energy produced by the formation of three C–H single bonds, a C–O single bond, and an O–H single bond. It can be obtained by the fermentation of sugar or synthesized by the hydration of ethylene in the following reaction: Using the bond energies in [link], calculate an approximate enthalpy change, ΔH, for this reaction. Explain your choice. In CsCl the coordination number is 8. For the ionic solid MX, the lattice energy is the enthalpy change of the process: Note that we are using the convention where the ionic solid is separated into ions, so our lattice energies will be endothermic (positive values). Note: Mg, Which compound in each of the following pairs has the larger lattice energy? These ions combine to produce solid cesium fluoride. (a) a large radius vs. a small radius for M+2, (b) a high ionization energy vs. a low ionization energy for M, (c) an increasing bond energy for the halogen, (d) a decreasing electron affinity for the halogen, (e) an increasing size of the anion formed by the halogen. The [latex]\Delta{H}_{s}^{\textdegree }[/latex] represents the conversion of solid cesium into a gas, and then the ionization energy converts the gaseous cesium atoms into cations. Ionic character in covalent bonds can be directly measured for atoms having quadrupolar nuclei (2H, 14N, 81,79Br, 35,37Cl or 127I). Average Bond Lengths and Bond Energies for Some Common Bonds. ZnO would have the larger lattice energy because the Z values of both the cation and the anion in ZnO are greater, and the interionic distance of ZnO is smaller than that of NaCl. Explain your choice. These ions combine to produce solid cesium fluoride. Lattice energies are often calculated using the Born-Haber cycle, a thermochemical cycle including all of the energetic steps involved in converting elements into an ionic compound. The lattice energy of LiF is 1023 kJ/mol, and the Li–F distance is 201 pm. The difference between the size of similar pairs of ions actually gets even smaller as you go down Groups 6 and 7. So let me go ahead and write anion instead of ion here.