Hess's Law

Enthalpy (H)

• The heat content of a system at constant pressure.

• A chemical equation that includes the amount of heat released or absorbed during the reaction.

• The heat released or absorbed during a chemical reaction.

• The heat released after burning 1 mole of a substance.

Chemical reactions that release energy. The ΔH reaction is negative.

• Specific Heat

• Endothermic Reaction

• Exothermic Reaction

• Enthalpy

(Hess's Law)Calculate the ∆H for the following reaction: 2H₂O₂ →  2H₂O  + 1 O₂ 
You are given these two equations:
2H₂  +  O₂ → 2H₂O            ∆H  =  -572 kJ
H₂  +  O₂  →  H₂O₂            ∆H  =  -188 kJ 

• ∆H  =  -948 kJ 
• ∆H  =  -196 kJ 
• ∆H  =  -384 kJ 
• ∆H  =  -188 kJ 

The Law of Conservation of Energy states:

• Energy can created or destroyed but not transformed 
• Energy cannot be created or destroyed, it can only transformed
• Energy can't be created, destroyed or transformed

Hess's Law states that...

• all reactions are reversible

• the enthalpy change of a reaction is independent of the route taken

• the direct route results in a greater enthalpy change

• the indirect route results in a lower enthalpy change as energy is lost in the second step

The diagram represents which type of reaction

• endothermic
• exothermic