A Level: Enthalpy Changes part 1

 

A Level  chemistry 2/5/2014

Enthalpy Changes  Part 1

 students should:

1.     know that reactions can be endothermic or exothermic

2.     understand that enthalpy change (ΔH) is the heat energy change  measured under conditions of constant pressure

3.      know that standard enthalpy changes refer to standard conditions,   i.e. 100 kPa and a stated temperature (e.g. ΔH298)

4.      be able to recall the definition of standard enthalpies of combustion  (ΔHc ) and formation (ΔHf )

                   Exothermic and endothermic reactions

     If a reaction produces heat (increases the temperature of the surroundings)  it is exothermic

     If the temperature of the reaction mixture decreases (i.e. heat is absorbed) then the reaction is endothermic.

Exothermic a reaction which produces heat (ΔH has a negative value by convention, -ve)

Endothermic a reaction which absorbs heat (ΔH has a positive value by convention, +ve)

Enthalpy of reaction: The change in internal (chemical) energy (H) in a reaction   = ΔH. 

    The most stable state is where all energy has been released.

    When going to a more stable state, energy  will be released, and when going to a less stable state, energy will be gained (from the surroundings).

    On an enthalpy level diagram, higher positions will be less stable (with more internal energy)  therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if is higher, heat is gained (less stable, ΔH is + ve).

    

If