How can bond strength be determined

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure of the strength of this attraction. 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.

Some texts use the equivalent but opposite convention, defining lattice energy as the energy released when separate ions combine to form a lattice and giving negative exothermic values.

Thus, if you are looking up lattice energies in another reference, be certain to check which definition is being used. In both cases, a larger magnitude for lattice energy indicates a more stable ionic compound. Thus, the lattice energy of an ionic crystal increases rapidly as the charges of the ions increase and the sizes of the ions decrease.

When all other parameters are kept constant, doubling the charge of both the cation and anion quadruples the lattice energy. Different interatomic distances produce different lattice energies. The compound Al 2 Se 3 is used in the fabrication of some semiconductor devices.

Which has the larger lattice energy, Al 2 O 3 or Al 2 Se 3? The O 2— ion is smaller than the Se 2— ion. Thus, Al 2 O 3 would have a shorter interionic distance than Al 2 Se 3 , and Al 2 O 3 would have the larger lattice energy. How would the lattice energy of ZnO compare to that of NaCl? It is not possible to measure lattice energies directly. However, the lattice energy can be calculated using the equation given in the previous section or by using a thermochemical cycle.

Figure 2 diagrams the Born-Haber cycle for the formation of solid cesium fluoride. Figure 2. The Born-Haber cycle shows the relative energies of each step involved in the formation of an ionic solid from the necessary elements in their reference states. We begin with the elements in their most common states, Cs s and F 2 g. In the next step, we account for the energy required to break the F-F bond to produce fluorine atoms.

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. We now have one mole of Cs cations and one mole of F anions. These ions combine to produce solid cesium fluoride. The enthalpy change in this step is the negative of the lattice energy, so it is also an exothermic quantity.

In this case, the overall change is exothermic. Table 3 shows this for cesium chloride, CsCl 2. Thus, the lattice energy can be calculated from other values. We begin with the elements in their most common states, Cs s and F 2 g. In the next step, we account for the energy required to break the F—F bond to produce fluorine atoms. 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.

We now have one mole of Cs cations and one mole of F anions. These ions combine to produce solid cesium fluoride. The enthalpy change in this step is the negative of the lattice energy, so it is also an exothermic quantity. In this case, the overall change is exothermic. Thus, the lattice energy can be calculated from other values. For cesium chloride, using this data, the lattice energy is:.

The Born-Haber cycle may also be used to calculate any one of the other quantities in the equation for lattice energy, provided that the remainder is known.

Lattice energies calculated for ionic compounds are typically much larger than bond dissociation energies measured for covalent bonds. Keep in mind, however, that these are not directly comparable values. For ionic compounds, lattice energies are associated with many interactions, as cations and anions pack together in an extended lattice.

For covalent bonds, the bond dissociation energy is associated with the interaction of just two atoms. The strength of a covalent bond is measured by its bond dissociation energy, that is, the amount of energy required to break that particular bond in a mole of molecules.

Multiple bonds are stronger than single bonds between the same atoms. The enthalpy of a reaction can be estimated based on the energy input required to break bonds and the energy released when new bonds are formed. For ionic bonds, the lattice energy is the energy required to separate one mole of a compound into its gas phase ions. Lattice energy increases for ions with higher charges and shorter distances between ions.

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