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Wednesday, 8 February 2017

Thermodynamics III

1. Enthalpy Change of a reaction: The enthalpy change accompanying a reaction is called the reaction enthalpy. The enthalpy change of a chemical reaction is given by the symbol rH.
rH = (sum of enthalpies of products) (sum of enthalpies of reactants).
rH = ∑ai HProducts - ∑bi HReactants

2. The standard enthalpy of reaction is the enthalpy change for a reaction when all the participating substances are in their standard states. The standard state of a substance at a specified temperature is its pure form at1 bar. Standard conditions are denoted by adding the superscript o to the symbol
H, e.g., H o

3. The enthalpy change that accompanies melting of one mole of a solid substance in standard state is called standard enthalpy of fusion or molar enthalpy of fusion,
 fusHo.e.g. H2O(s)→H2O(l);ΔfusH0 =6.00 kJ mol−1

4. Amount of heat required to vaporize one mole of a liquid at constant temperature and under standard pressure (1bar) is called its standard enthalpy of vaporization or molar enthalpy of vaporization,
vapH0. e.g. H2O(l)→H2O(g);ΔvapH0 =40.79 kJ mol−1

5. Standard enthalpy of sublimationsubHis the change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1bar). Sublimation is direct conversion of a solid into its vapour. Solid CO2 (dry ice) sublimes at 195K with
subH 0=25.2 kJ mol–1; naphthalene sublimes slowly and for this subH 0=73.0 kJ mol-1

6. The standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation (also known as reference states) is called Standard Molar Enthalpy of Formation.
Its symbol is f H0

7. Standard enthalpy of combustion is defined as the enthalpy change per mole (or per unit amount) of a substance, when it undergoes combustion and all the reactants and products being in their standard states at the specified temperature.

8. Enthalpy of atomization (symbol: aH0) It is the enthalpy change on breaking one mole of bonds completely to obtain atoms in the gas phase. In case of diatomic molecules, like dihydrogen the enthalpy of atomization is also the bond dissociation enthalpy. In some cases, the enthalpy of atomization is same as the enthalpy of sublimation.

9. Bond Enthalpy (symbol:  bond  H0):
    rH = ∑ bondenthapies reactants - ∑ bondenthapies products

10. Enthalpy of Solution (symbol solH0 ): Enthalpy of solution of a substance is the enthalpy change when one mole of it dissolves in a specified amount of solvent. The enthalpy of solution at infinite dilution is the enthalpy change observed on dissolving the substance in an infinite amount of solvent when the interactions between the ions (or solute molecules) are negligible.

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