Ionic Bond

IONIC BOND Common mono-atomic ions of some groups of the representative elements:

Some Lewis Structure of ionic compounds are given below

Energy changes during formation of ionic bond:

Ionic bond formation from gaseous atoms involves three steps: 1) Formation of gaseous cation – It involves loss of electrons by the gaseous atoms and is an endothermic process. Energy required for this process is ionization energy.

2) Formation of gaseous anion – It involves the gain of electrons by the gaseous atoms and is an exothermic process. The energy released in this process is called electron gain enthalpy.

The above process will become endothermic if divalent and trivalent anions are formed because second and successive electron gain enthalpies are positive.

3) Combining of oppositely charged ions to form ionic compound – It involves the formation of crystal lattice by combining oppositely charged ions.It is an exothermic process and energy released is called Lattice Enthalpy (∆LH0).

For the stable ionic bonding the net effect of above three processes should be exothermic i.e. total energy released is > total energy required Factors that favor the formation of ionic bond:

(a) Ionization Enthalpy (Energy): Lesser the value of ionization enthalpy, greater the tendency of the atom to form cation. For example, alkali metals form cations easily because of the low value of ionization energies.

(b) Electron gain enthalpy: Greater the value of electron gain enthalpy, more the tendency of the atom to form anion .For example ,halogens have high electron gain enthalpies within the respective periods and form ionic compounds easily .

(c) Lattice enthalpy: It is the energy released when the close packing of the gaseous ions of the opposite charge forms one mole of ionic solid. Magnitude of lattice energy gives an idea about the inter-ionic forces and it also gives the measure of the stability of the ionic compound which depends upon the following factors.

(d) 1) Size of the ions—Smaller the size of the ion s, lesser the inter nuclear distance and greater the inter ionic interaction, hence, larger the magnitude of lattice energy

2) Charge on the ions--- Larger the magnitude of the charge on the ions greater will be the attractive forces and higher the negative value of lattice energy. Lattice Enthalpies of Halides of Alkali Metals (kJ mol-1)

In short the conditions are 1) Ionization enthalpy of the cation forming elements should be low. 2) Electron gain enthalpy of the element forming anion should be of high negative value. 3) Lattice energy should also have high negative value. Ionic Compounds: The compounds that contain ionic or electrovalent bond are called ionic compounds. And ionic bond is the force of attraction holding the oppositely charged ions i.e. positive and negative charges. Properties of ionic compounds: 1) Physical state These exist in crystalline solids. Ionic crystals do not exist in the form of molecules but exist in the form of ions. Ions are arranged in a well defined geometry which constitutes ionic crystals. For example in Sodium chloride crystal each sodium ion is surrounded by six chloride ions and each chloride is surrounded by six sodium ions as shown in the figure.

BLUE–CHLORIDE IONS

RED- SODIUM IONS

2) High melting and boiling point Because of strong electrostatic forces of attraction ionic compounds have high melting and boiling point. The variation in melting point depends on the charges on the ions and the ionic radii. Closer the ions in the crystal, stronger will be the force of attraction and higher will be the melting point. e.g. NaF to NaI. NaF (1270K), NaCl (1073K), NaBr (1023K), and NaI (924K).

3) Solubility Ionic compounds are soluble in water and other polar solvents . This is due to the strong electrostatic interactions between the ions and polar solvents.

4) Elecrtrical conductivity Ionic compounds conduct electricity when in solution form or in molten state. This is because ions are free to move in molten state not in solid form.

5) Ionic reactions When dissolve in water ionic compounds split up into oppositely charged ions.

6) Non-Directional Since each ion is surrounded by oppositely charged ions uniformly all around the ion so electric field is non–directional. Thus ionic bond is called non–directional. Lattice enthalpy of ionic crystals: It is defined as the energy released when one mole of ionic crystal is formed from it’s constituent ions in the gaseous state. It is represented as

The negative sign with lattice enthalpy shows that energy is released in the process. During the formation of ions the process is reversible and lattice enthalpy will have +ve sign because energy is absorbed in the process. It is represented as

Lattice enthalpy can also be defined as the energy required to completely separating one mole of solid ionic compound into gaseous ionic constituents. The magnitude of lattice enthalpy depends upon: Size of the ion: smaller the size of the ion, lesser is the internuclear distance, higher will be the inter ionic attraction and larger will be the lattice enthalpy. Charge on the ions: Larger the magnitude of charge on the ions, greater will be the attractive forces between the ions and higher will be the lattice enthalpy Lattice enthalpies of some ionic solids:

BORN-HABER CYCLE FOR LATTICE ENTHALPIES BORN and HABER devised a method to relate the lattice enthalpy of ionic solids to other thermodynamic data. This is called BORN-HYBER cycle. For example: During the formation of sodium chloride from metallic sodium and chlorine gas, the energy changes during this process and BORN-HABER cycle are as follows:

The overall process is considered to take place in the following steps: Conversion of metallic sodium into gaseous sodium atoms: The sodium atom can loose it’s electron in the gaseous state only. Thus, it has to be sublimated. The energy required for sublimation i.e. converting one mole of metallic sodium into gaseous sodium atom is called enthalpy of sublimation (S)

Dissociation of chlorine molecule into chlorine atoms: Energy required for converting gaseous chlorine molecule into chlorine atoms is called enthalpy of dissociation (D)

Thus the enthalpy required for producing one mole of gaseous chlorine atoms is D/2

Conversion of gaseous sodium atom into sodium ions: The amount of energy required for converting one mole of gaseous sodium atom into sodium ions in the gaseous state is called ionization enthalpy (I.E.)

Conversion of gaseous chlorine atoms into gaseous chlorine ions: For converting one mole of gaseous chlorine atoms into chloride ions the energy required is called electron gain enthalpy (∆ egH)

Combination of oppositely charged ions to form ionic solid crystal: The oppositely charged Na+ and Cl- ions attract each other and form solid NaCl with the release of energy. The amount of energy released when one mole of solid crystalline compound is formed from gaseous ions of opposite charges is called lattice enthalpy (U)

The overall energy change may be expressed as:

The enthalpy change for the above reaction is called enthalpy of formation (∆ fH) All the above steps can also be represented in the form of BORN-HABER CYCLE as shown below.

According to Hess’s Law, the enthalpy of formation of one mole of NaCl should be the same whether the reaction has taken place in one step or through a number of steps.

Lattice enthalpy can be calculated by knowing the values of various enthalpies. While calculating the value of Lattice enthalpy (+) sign should be used for enthalpies absorbed and (-) sign for enthalpies released. For example:- Lattice enthalpy for NaCl can be calculated as shown below. 1) Enthalpy of sublimation of sodium (S) = 108.5 kJmol-1 2) Enthalpy of dissociation of Cl2 (D)=243.0 kJmol-1 3) Ionization enthalpy of sodium (IE)= 495.2 kJmol-1 4) Electron gain enthalpy of chlorine (∆ egH) = - 348.3 kJmol-1 5)Enthalpy of formation of NaCl (∆ fH)= - 381.8 kJmol-1 Substituting the various values in the equation

Applications of BORN-HABER Cycle: In addition to lattice enthalpies this cycle is also used for calculating other properties. 1) Calculation of enthalpy of formation: With the help of BORN-HABER cycle enthalpies of formation of unknown compounds can be calculated. If the calculation for the enthalpy of formation is coming out to be negative then the reaction is exothermic and the compound formed will be stable. If enthalpy of formation comes out to be positive then the reaction will be endothermic and the compound formed highly unfavorable energetically. For example – NaCl2 is not formed because enthalpy of formation calculated by BORN-HABER cycle is highly positive although it has higher lattice enthalpy than NaCl. 2) Calculation of electron gain enthalpy: The BORN-HABER cycle can also be used for calculating the electron gain enthalpies of some elements which can’t be calculated easily otherwise. By using the following equations electron gain enthalpy can be calculated.