Chapter 7.2, Problem 13SSC

Interpretation Introduction

Interpretation:

The energy change and its stability associated with formation of ionic compound.

Concept introduction:

Chemical compounds are formed when the atoms, ions or molecules of two substances are attracted towards each other forming a chemical bond. This bond can be electrostatic when two oppositely charged ions are attracted towards each other and this bond is said to be an ionic bond. In other words, ionic compounds are made up of ions.

Answer to Problem 13SSC

The formation of ionic compound is sum of all the energies according to Hess’s Law.

Explanation of Solution

Energy changes for ionic compounds are depicted using ‘Born Haber cycle’, It takes place in 4 steps.

1. Formation: It is the conversion of elements in standard state to ionic compounds. The ionic compounds are stable and therefore enthalpy change is usually exothermic.
2. Atomisation: It involves the formation of gaseous atoms of individual elements. As energy is required to break bonds within the elements to convert to gaseous form.
3. Ionisation: This involves the removal of electrons from one atom and add to the other atom. The removal of electrons requires energy called as ionization energy. The addition of electrons is electron affinity which releases energy.
4. Lattic energy: This stage involves the formation of ionic compound from gaseous ions. The lattice energy is exothermic as strong bond is formed.

Using Hess’s Law, the expression $\Delta {\text{H}}_f=\Sigma \Delta {\text{H}}_{rest}$ stands valid for all ionic compounds

Conclusion

The formation of ionic compound is sum of all the energies according to Hess’s Law.