Sunday, February 20, 2011

Exothermic and Endothermic (Mandy)

ENDOTHERMIC: absorb energy to break bonds
EXOTHERMIC: release energy to join bonds

Energy diagrams allow one to observe endothermic or exothermic reactions.

in above diagram, "starting products" are called "energy of reactants", which indicates the total potential energy of all reactants. The "transition state" is also known as "energy of activated complex".

one thing that the above diagram did not mention is the "change in enthalph" (delta H). The change in potential energy during the reaction. It is the energy of products - energy of reactants.

Therefore, if "delta H" is positive, it is an ENDOTHERMIC reaction.
                 if "delta H" is negative, it is an EXOTHERMIC reaction.

Lets look at this concept from another point of view:

CH4 + 2O2 -> CO2 + 2H2O + 812kJ

Endothermic: reactants have the energy term on the left hand side and positive 'delta H'
Exothermic: reactants have the energy term on the right hand side and negative 'delta H'

Time for some examples!! --->
In this reaction, the total energy of the reactants is 80 kJ mol-1, the total energy of the products is -90kj mol-1 and the activation energy for the forward reaction is 120 kj mol-1.
a. Draw a diagram of the energy profile for this reaction. Label the diagram.
b. State whether the reaction is endothermic or exothermic.
c. Calculate the change in enthalpy.

ans: b. endothermic reaction
       c. 10kJ

an interesting video :) ENJOY.

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