Hess' Law

When you leave you house to go the gym, you may take root 1, or maybe you can take root 2. It does not matter how you get to the gym, as long as you do. Same applies to chemical reaction and enthalpy.

For hess' law, you must know to things:

- Whenever you switch products with reactants and vice versa, you must change your enthalpy sign and well.

-If you multiply the coefficient of the equation, you multiply the enthalpy change value as well. 

Hess' law modified 

Hess determined that you can actually find the enthalpy change of a reaction, by using the molar enthalpy of the components of the reaction.

When using the method, remember that: 

- A balanced equation is necessary;

- Molar enthalpy of formation of elements are equal to 0

- Water vapour is always produces, unless we are talking about photosynthesis, cellular respiration and closed system combustion. Which in the case, liquid water will be formed. 

r Ho

 =        

f Ho

pr Σoducts  ­        

f Ho Σ reactant