Endothermic and Extothermic Reactions (Continue)

The energy absorption or release can be placed directly in the equation.



An exothermic reaction releases energy. The energy change that accompanies a reaction can be written in the equation. The reaction takes less energy to break the bonds than it gives off to form bonds.

An endothermic reaction uses energy as a reactant.
The energy change that accompanies a reaction can be written in the chemical equation. The reaction requires the addition of energy to the reactants to form the bonds in the products. 
 

Enthalpy of a reaction or energy change of a reaction DH, is the amount of energy or heat absorbed in a reaction. If the energy is required, DH is positive, and if energy is released, the DH, is negative.







_________ Products
    ­
    |
    | DH, positive for
    |         endothermic reaction
    |
__|______ Reactants

DH = SH_products - SH_reactants

Energy Calculations 

The return of the MOLE!!!
EXAMPLE
What is the energy change when 5 moles of oxygen and 5 moles of nitrogen react?
Use the energy change per mol from the equation
N2(g) + O2(g) + 43 kJ -----> 2 NO(g) .
Check the balanced equation mole ratio for moles of N2 to moles of O2.
mole ratio N2 to O2 is 1 mole N2 / 1 mole O2
You can verify the number of moles of N2
moles N2 = [ 5 moles O2] [1 mole N2 / 1 mole O2] = 5 moles N2 
Determine the kcal change.
[ 5 moles N2 ][43 kcal /mole N2 ] = 215 kJ
The reactants in this reaction must take energy from the surroundings to form products.