Stability Of Ionic Systems Using Born Haber Cycles

Standard enthalpy of sublimation is the enthalpy change that occurs when one mole of substance in solid state is converted directly into gaseous state. This conversion should take place at constant temperature and pressure. It is expressed in kilo joule per mole.

Enthalpy of sublimation is always greater than enthalpy of fusion and enthalpy of vaporization. In fact, enthalpy of sublimation is equal to the sum of enthalpy of fusion and enthalpy of vaporization. Enthalpy of sublimation of carbon dioxide is 32.3kj/mol.

Standard enthalpy of evaporation is the amount of energy required to convert one mole of substance in liquid state into gaseous state at constant temperature and pressure. It is expressed in kilo joule per mole.It is also known as heat of vaporization. Enthalpy of evaporation of Acetone is 31.3 kj/mol.

Standard enthalpy of fusion is the enthalpy change that results by providing energy required to convert one mole of a substance in solid state to liquid state. The term fusion means melting. The temperature at which the transition from solid to liquid occurs is melting point. There is no change in temperature during the process.

For example, Enthalpy of fusion of ice is 6.02 kj/mol. This means that six point zero two kilo joule per mole of heat is required to convert one mole of ice into liquid at melting point of zero Celsius. Standard enthalpy of fusion of sodium chloride is twenty eight point eight kilo joule per mole 28.8 kj/mol.

In chemistry, Atomization means breaking the bonds between atoms of a compound to covert it into individual atoms in gaseous state. So what is the enthalpy of atomization? Standard enthalpy of atomization is the change in enthalpy when an element in its standard state is atomized to produce one mole of atoms in gaseous state. It is denoted by ΔatH.

For example, standard enthalpy of atomization of hydrogen is 218 kj/mol.

Enthalpy of first ionization is the enthalpy change when first electron is removed from valence shell of an isolated gaseous atom.The enthalpy change occurs when one mole of electrons are removed from one mole of isolated gaseous atoms. Remember that, from each isolated gaseous atom only the first valence electron is removed. This is the way standard enthalpy of first ionization is calculated. It is expressed in kilo joule per mole.

Standard ionization enthalpy of sodium is 496 kj/mol. This means that four hundred ninety six kilo joule of enthalpy is absorbed to when one mole of electrons are removed from one mole of isolated gaseous atoms. This is the first ionization enthalpy of sodium.

The enthalpy change when an electron is added to an isolated gaseous atom is called Enthalpy of electron gain.When an electron is added to an isolated gaseous atom, energy is either released or absorbed.When one mole of isolated gaseous atoms gain electron, then enthalpy change during the process is calculated. This enthalpy change is called Standard enthalpy of electron gain.

Standard enthalpy of electron gain of oxygen is -141 kj/mol.As we can see, for the addition of another electron to O⁻ the enthalpy change is positive. This is because of repulsion between electrons due increased number of electrons. Apart from that, after adding one electron to an isolated gaseous atom, adding another electron requires energy.

The enthalpy change when one mole of an ionic compound dissociates into its ions in gaseous phase is called Standard lattice enthalpy of an ionic compound.We can also define lattice enthalpy as enthalpy change when one mole of ionic compound is formed from its gaseous ions.Enthalpy change when one mole of an ionic compound dissociates into its ions is called lattice enthalpy of dissociation. Enthalpy change when one mole of an ionic compound is formed from its gaseous ions is called lattice enthalpy of formation.

In the given illustration, sodium chloride dissociates into its ions in gaseous state. These ions are sodium ion and chloride ion. Its standard lattice enthalpy of dissociation is 787 kj/mol.Standard lattice enthalpy of formation of sodium chloride is -787 kj/mol.

Lattice enthalpy can not be measured directly. To calculate lattice enthalpy, we use Born-Haber cycle.In Born-Haber cycle we use Hess’ Law to calculate lattice enthalpy of ionic compounds. You need to be familiar with enthalpy of formation, ionization enthalpy, enthalpy of atomization, electron affinity, enthalpy of dissociation and lattice enthalpy.As we have discussed all these terms, we shall now proceed to calculation of lattice enthalpy of sodium chloride using Born-Haber cycle.

In the given illustration, we can see that a cycle is drawn showing all the mentioned enthalpies. The first step shows the enthalpy of formation of sodium chloride from its constituents in their elemental state. These constituents are sodium in solid state and chlorine in gaseous state.Enthalpy of formation of sodium chloride is -410.9 kj/mol.

The second step indicates the formation of sodium atoms and chlorine atoms in gaseous state. So we shall calculate the enthalpy of atomization of Sodium which is a metal and enthalpy of dissociation of chlorine which is a non metal.Enthalpy of atomization of sodium is 107 kj/mol. Enthalpy of atomization of chlorine is 122 kj/mol.

The third step indicates the formation of sodium ions and chloride ions in gaseous state from sodium atoms and chlorine atoms in gaseous state. We shall determine the ionization energy of sodium and electron affinity of chlorine. This is because electron is removed from sodium to make sodium ion. And electron is added to chlorine to make chloride ion.Ionization enthalpy of sodium is 495 kj/mol | positive four nine five kilo joule per mole. Electron affinity enthalpy of chlorine is -349 kj/mol.

The fourth step indicates the formation of sodium chloride from its gaseous ions. As we know that the enthalpy change that occurs when ionic compound is formed from its gaseous ions is called standard lattice enthalpy. We shall now calculate this lattice enthalpy.As we can observe in the illustration that sodium and chlorine in their elemental state are converted into solid sodium chloride in first step. Alternatively, sodium and chlorine in their elemental state are first atomized, then ionized and then converted into solid sodium chloride by going through a series of steps.

We can say that first step is equal to the sum of second step, third step and the fourth step. More precisely, we can say enthalpy of formation is equal to sum of enthalpy of atomization, ionization energy of metal and electron affinity of non metal and lattice enthalpy of sodium chloride.By rearranging the given formula in the illustration we can find lattice enthalpy of ionic compound.

So the lattice enthalpy of sodium chloride is calculated in the illustration.Lattice enthalpy of formation of sodium chloride is -785.9 kj/mol.