Enthalpy, H

•The heat content of a system or total energy

  in the system

•Enthalpy, H of a system cannot be measured when there is a change in the system.

•Example: system undergoes combustion or ionisation.

 

                                  Enthalpy of Reaction, ∆H and Standard Condition

 

                     •Enthalpy of reaction:

–The enthalpy change associated with a chemical reaction.

•Standard enthalpy, ∆Hº

–The enthalpy change for a particular reaction that occurs at 298K and 1 atm (standard state)

 

                                     Thermochemical Equation

•The thermochemical equation shows the enthalpy changes.

            Example : H2O(s)   →    H2O(l)      ΔH  =  +6.01 kJ

•1 mole of H2O(l) is formed from 1 mole of H2O(s) at   0°C, ΔH  =  +6.01 kJ

•However, when 1 mole of H2O(s) is formed from 1 mole of H2O(l), the magnitude of ΔH  remains the same with the opposite sign of it.     

         H2O(l)   →    H2O(s)      ΔH  =  -6.01 kJ

                                  Types of Enthalpies