Chemical Bonding (Part-1)

Subject: Chemistry Class: B.Sc - I (Hons)

Paper Code/Name: Paper IB (Inorganic Chemistry) Topic: Chemical Bonding (Part-1) Faculty Name: Dr. Rupali Gupta

College Affiliation: M. M. Mahila College, Ara Date: 27/05//2020

What is an ionic bond?

Definition: The electrostatic force of attraction which holds the two oppositely charged ions together is called the ionic bond. • A chemical bond is formed between two atoms by the complete

transfer of one or more electrons from one atom to the other as a result of which the atoms attain their nearest inert gas configuration.

• There are primarily three ways in which two atoms combine

together to lose energy and to become stable. • One of the ways is by donating or accepting electrons so as to

complete their octet configuration. • The bond formed by this kind of combination is known as an ionic

bond or electrovalent bond.

• An ionic bond is formed when the valence (outermost) electrons of one atom are transferred permanently to another atom.

• The atom that loses the electrons becomes a positively charged ion since the number of protons exceeds electrons and is called cation.

• While the one that gains those electrons becomes a negatively charged ion due to excess number of electron than proton and is called anion.

• The oppositely charged particles attract each other by electrostatic force of attraction and results in formation of ionic or electrovalent bond.

• A pictorial representation of ionic bond formation is shown for NaCl.

How an ionic bond is formed?

Electronegativity and Ionic Bonding

• An Ionic bond is the bond formed by the complete transfer of valence electron so as to attain stability.

• This type of bonding leads to the formation of two oppositely charged ions – positive ion i.e. cations and negative ions i.e. anions.

• The presence of two oppositely charged ions results in a strong attractive force between them. This force is an ionic or electrovalent bond.

• Ionic bonds form between atoms with large differences in electronegativity, whereas covalent bonds formed between atoms with smaller differences in electronegativity.

• The compound formed by the electrostatic attraction of positive and negative ions is called an ionic compound.

Ionic Bond Properties

Due to the presence of a strong force of attraction between cations and anions in ionic bonded molecules, the following properties are observed: (i) The ionic bonds are the strongest of all the bonds. (ii) The ionic bond has charge separation and so they are the most reactive of all the bonds in the proper medium. (iii) The ionic bonded molecules have high melting and boiling point. (iv) The ionic bonded molecules in their aqueous solutions or in the molten state are good conductors of electricity. This is due to the presence of ions which acts as charge carriers.

Energetics involved in ionic bond formation

• The formation of sodium chloride from sodium and chlorine is very exothermic, as indicated by the large negative enthalpy of formation value, Hf° = -410.9 kJ.

• What factors make formation of ionic compounds so exothermic? • The principal reason ionic compounds are stable is attraction

between ions of opposite charge.

• This attraction draws the ions together, releasing energy and causing ions to form a solid array, or lattice.

• A measure of how much stabilization results from arranging oppositely charged ions in an ionic solid is given by the lattice energy, which is the energy required to completely separate one mole of a solid ionic compound into its gaseous ions.

• Table 1 lists the lattice energies for a number of ionic compounds.

• The large positive values indicate that the ions are strongly attracted to one another in ionic solids.

• The energy released by the attraction between ions of unlike charge more than makes up for the endothermic nature of ionization energies, making the formation of ionic compounds an exothermic process.

• The strong attractions also cause most ionic materials to be hard and brittle with high melting points: for example, NaCl melts at 801 °C.

Table 1: Showing Lattice energies of some ionic compound

Born Haber Cycle

• Born Haber process or more commonly referred to as Born Haber cycle is a method that allows us to observe and analyze energies in a reaction.

• It mainly helps in describing the formation of ionic compounds from different elements.

• The methodology further enables us to understand the overall reaction process through a series of steps.

• Born-Haber cycle was introduced in the year 1919 by German scientists named Fritz Haber and Max Born. Born Haber cycle is mainly used to calculate the lattice energy.

• It also involves several steps or processes such as electron affinity, ionization energy, sublimation energy, the heat of formation and dissociation energy.

Considerations

• The reaction of electropositive metals with electronegative nonmetals produces ionic solids.

• Alkali and alkaline earth metals react with chalcogen or halogen family elements to form compounds, which are crystalline ionic solids.

• Ionic compounds being stabilized by the electrostatic force of attraction between positive and negative charges are expected to have similar physical properties.

• But physical properties like stability, the water solubility of these ionic compounds differ much.

• The difference is, attributed to the difference in an enthalpy called ‘Lattice energy’, between the ionic solids.

• Lattice energy is the energy that keeps together the cations and anions of the compound in fixed positions in a crystalline solid state.

• Lattice energy can be defined as either energy released when gaseous ions form one mole of a solid ionic compound or as the energy required to convert one mole ionic solid into its gaseous ions.

• There is no way to measure experimentally this lattice energy. Hess law of heat summation is the only indirect way of estimating the lattice energy.

• Application of Hess law of heat summation to the formation of solid ionic compounds involve enthalpy of all processes that are necessary for the formation of the solid ionic compound from the elemental state of the constituent atoms, in a form cycle such that the total energy on summation is zero.

What is Born Haber Cycle?

• Born Haber cycle is a cycle of enthalpy change of process that leads to the formation of a solid crystalline ionic compound from the elemental atoms in their standard state and of the enthalpy of formation of the solid compound such that the net enthalpy becomes zero.

• Let us study the example of NaCl for calculation of its Lattice energy.

Born Haber cycle of sodium chloride NaCl, (or any AB-type Mono-valent ionic solid)

• The heat of formation of sodium chloride (ΔHf0) from the sodium

metal and chlorine gas can be experimentally measured. Na (s) + 1/2Cl2 (g) → NaCl(s) ΔHf

0 = -411kJ/mol

• The formation of ionic solid sodium chloride form solid sodium metal and gaseous chlorine is not a single step process but goes through several processes.

• Heat changes of all the processes except the lattice energy can be experimentally measured.

The processes or steps in the formation of sodium chloride are: 1. Solid sodium atom sublimes to gaseous atom by absorbing heat energy (∆Hsub). Na (s) → Na (g), Sublimation energy ΔHsub = + 107kJ/mol 2. Gaseous sodium atom absorbs the ionization energy to release one electron and forms gaseous sodium ion. Na(g) → Na+(g) + 1e– Ionization energy ∆HIE = +502kJ/mol

3. Diatomic gaseous chlorine breaks into two individual atoms by absorbing bond energy, such that each chlorine atom absorbs half of the bond energy of chlorine molecule. Cl2(g) → 2Cl(g) Bond dissociation energy of chlorine = 1/2 ∆Hdiss= 1/2×242 = +121kJ/mol 4. Chlorine atom accepts an electron to form chloride ion and releases energy equivalent to electron affinity. Cl(g) + 1e– → Cl– (g) Electron affinity = ∆HEA = -355kJ/mol

5. Gaseous sodium ion and gaseous chloride ion combine to form solid sodium chloride molecule and releases energy equivalent to lattice energy. Na+(g) + Cl– (g) → Na+Cl– (s)

Lattice energy = ∆HLE = U = ? • Summation of enthalpy of all the processes (from step 1 to step 5)

give the net enthalpy of formation of solid crystalline sodium chloride from sodium and chlorine in their standard conditions of solid and gas respectively.

• This should be equal to the experimentally measured enthalpy of formation of solid sodium chloride.

• The enthalpies are represented as a cycle in the figure.

So, • ΔHf

0 = (ΔHsub + ∆HIE +1/2∆Hdis + ∆HEA + U or ΔHf0) – (ΔHsub + ∆HIE + 1/2

∆Hdis + ∆HEA + U) = 0 • 411 + 107 + 502 +121 -355 +U = 0 • Here, except lattice energy, all other enthalpies can be

experimentally measured. • Lattice energy of the sodium chloride solid: U = ΔHf

0 – (ΔHsub + ∆HIE + ½ ∆Hdis + ∆HEA). = -411 -107 -502 -121 +355 = – 786kJ/mol

REFERENCES

(i) Basic Inorganic Chemistry, F. A Cotton, G. Wilkinson, and Paul L. Gaus, 3rd Edition

(1995), John Wiley & Sons, New York.

(ii) Concise Inorganic Chemistry, J. D. Lee, 5th Edition (1996), Chapman & Hall,

London.

(iii) Recent Aspects in Inorganic Chemistry, R. C. Aggarwal, 1st Edition (1987), Kitab

Mahal, Allahabad.

(iv) Principles of Inorganic Chemistry: Puri, Sharma and Kalia.

(v) Modern approach to fundamentals of inorganic chemistry by Biltu Singh, Kiran

Prakashan.

(vi) NCERT Chemistry, Class XI