Enthalpy

• ENTHALPY

  During a chemical reaction, the chemical bonds may be formed or may be broken. There is an energy release whenever bonds are formed and there is an energy consumed whenever bonds are broken
• The difference between the energy released by bond formation and the energy consumed by bond cleavage during a chemical reaction is called enthalpy.

  It is denoted by H and the change in enthalpy has a symbol of △H.
• Enthalpy Calculation

  
• Types of Enthalpy Change

  Heat of formation

  Heat of combustion

  Heat of neutralization

  Heat of Dissolution
• Heat of formation

  The enthalpy change that takes place when one mole of compound is formed from its elements
• Heat of combustion

  The enthalpy change that takes place when one mole of compound is completely burnt in excess oxygen
• Heat of neutralization

  The enthalpy change that takes place when the equivalent of an acid is completely neutralized with the equivalent base
• Heat of Dissolution

  The enthalpy change that takes place when one mole of substance is dissolved in specified quantity of solvent in given temperature
• HESS’S LAW

  Many compounds cannot be directly synthesized from their elements. In some cases, the reaction proceeds too slowly, or side reactions produce substances other than the desired compound
• HESS’S LAW

  • In these cases, △Hof can be determined by an indirect approach, which is based on Hess’s law of heat summation, or simple Hess’s Law, named after Swiss chemist Germain Hess
  • It can be stated as “When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps”