# Change in Enthalpy During Atomic or Molecular Changes:

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#### Change in Enthalpy During Ionization or Enthalpy of Ionization (ΔionizationH°):

• The enthalpy change that accompanies the removal of an electron from each atom or ion in one mole of gaseous atoms or ions is called its enthalpy of ionization.

#### Explanation:

• Consider following reaction

Na(g)   →   Na+(g)+ e ,    ΔionizationH  = 494  kJ  mol-1

• Thus, the equation indicates that when one mole of gaseous sodium atom ionizes to Na+(g) ion, the change in enthalpy is + 494 kJ. i.e. 494 kJ of energy is absorbed.

#### Change in Enthalpy During Atomization or Enthalpy of Atomization (ΔatomizationH°):

• The enthalpy change that accompanies the dissociation of all the molecules in one mole of gas phase substance into gaseous atoms is called its enthalpy of atomization.

#### Explanation:

• Consider following reaction

Cl2(g) → Cl(g)+ Cl(g),          ΔatomizationH  = 242  kJ mol-1

• Thus, the equation indicates that when one mole of gaseous chlorine molecule dissociates completely into its atomic form in gaseous state then the change in enthalpy is + 242 kJ. i.e. 242 kJ of energy is absorbed.

#### Enthalpy of Solution (ΔsolutionH°):

• Change in enthalpy of chemical reaction at a given temperature and Pressure, when one mole of a solution is dissolved in a specified quantity of solvent so as to form a solution of particular concentration is called as enthalpy of a solution.

#### Explanation:

• Consider following reaction

KOH(s)   +   H2O(l) → KOH(aq)        ΔsolutionH   = -58.57 KJ mol-1

• Thus, the equation indicates that when one mole of potassium hydroxide (solute) dissolves in one mole of water (solvent) to form one mole of potassium hydroxide solution in water then the change in enthalpy is -58.57 kJ. i.e. 58.57 kJ of energy is released

### Bond Enthalpy (Bond Energy):

• Chemical reactions involve the breaking and making of chemical bonds. Energy is required to break a bond and energy is released when a bond is formed.
• It is possible to relate heat of reaction to changes in energy associated with breaking and making of chemical bonds.
• With reference to the enthalpy changes associated with chemical bonds, two different terms are used in thermodynamics. (i) Bond dissociation enthalpy (ii) Mean bond enthalpy Let us discuss these terms with reference to diatomic and polyatomic molecules.

#### Diatomic Molecules:

• Consider the following process in which the bonds in one mole of dihydrogen gas (H2) are broken:

H2(g) →  2H(g) ;   ΔH–HHO = 435.0 kJ mol-1

• The enthalpy change involved in this process is the bond dissociation enthalpy of H–H bond.
• The bond dissociation enthalpy is the change in enthalpy when one mole of covalent bonds of a gaseous covalent compound is broken to form products in the gas phase. Note that it is the same as the enthalpy of atomization of dihydrogen. This is true for all diatomic molecules.

#### Polyatomic Molecule:

• In the case of polyatomic molecules, bond dissociation enthalpy is different for different bonds within the same molecule.
• Let us consider a polyatomic molecule like methane, CH4.  The overall thermochemical equation for its atomization reaction is given below:

CH4(g) →  C(g) + 4H(g) , ΔH = +1665 KJ.

• In methane, all the four C – H bonds are identical in bond length and energy. However, the energies required to break the individual C – H bonds in each successive step differ. In such cases, we use mean bond enthalpy

CH4(g) →  CH3(g) + H(g) , ΔH = +427 KJ.

CH3(g) → CH2(g) + H(g), ΔH = +439 KJ

CH2(g) → CH(g) + H(g), ΔH = +452 KJ

CH(g) → C (g) + H(g),   ΔH = +347 KJ

CH4(g) →  C(g) + 4H(g) , ΔH = +1665 KJ

• We find that mean C–H bond enthalpy in methane as 1664/4 =  416 kJ/mol.
• Using Hess’s law, bond enthalpies can be calculated.

#### Enthalpy of Reaction from Bond Enthalpy:

• The reaction enthalpies are very important quantities as these arise from the changes that accompany the breaking of old bonds and formation of the new bonds.
• We can predict enthalpy of a reaction in the gas phase if we know different bond enthalpies. The standard enthalpy of reaction is related to bond enthalpies of the reactants and products in gas phase reactions as:

ΔH = ∑ Bond enthalpies reactants  –     ∑ Bond enthalpies products

• Remember that this relationship is approximate and is valid when all substances (reactants and products) in the reaction are in gaseous state.
• The values of given bond enthalpy can be used to calculate bond enthalpies of specific bond in the molecule.

#### Crystal Lattice Energy (ΔLatticeH°):

• Crystal lattice energy is defined as the enthalpy change or energy released accompanying formation of 1 mole of crystalline solid from its constituent ions in the gaseous state at a constant temperature.
• Explanation:

M+(g)  + X(g) → M+X(s) ,  ΔH = – x KJ  mol-1

• Thus, the equation indicates that when one mole of ionic compound M+X-(s) is formed from its constituent ions the change in enthalpy is  – x kJ. i.e. x kJ of energy is evolved.
• Crystal lattice energy is always negative.
• The sequence of actions involved in the formation of 1 mole of ionic compound in its standard state from its constituent elements in their states at constant temperature and pressure is called Born-Haber cycle.
• #### Factors Affecting Crystal Lattice Energy:

• Crystal lattice energy depends on interionic distance in the crystalline solid. As the distance decreases, the crystal lattice energy increases.
• Crystal lattice energy depends on the charge of constituent cations and anions.Enthalpy of Hydration

#### Enthalpy of Hydration  (ΔHydrationH°):

• Enthalpy of hydration is defined as the enthalpy change or amount of heat evolved when one mole of gaseous ions dissolve in water by hydration to give infinitely dilute solution at constant temperature and pressure.

#### Explanation:

• Consider following reaction

Na+(g) + aq → Na+(ag) ,     ΔH = – 390 KJ mol-1

• Thus, the equation indicates that when one mole of sodium ion in the gaseous state is dissolved in water the change in enthalpy is – 390 kJ. i.e. 390 kJ of energy is released.
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