What is an enthalpy diagram?
An enthalpy diagram shows the change in enthalpy of the reaction as the chemicals move from reactants to products.
The change in enthalpy is a fancy term for the change in thermal energy of the system at constant pressure.
Typically we classify reactions as either endothermic or endothermic.
Endothermic means the thermal energy of the products is more than that of the reactants.
Exothermic means the thermal energy of the products is less than that of the reactants.
Endothermic means the reaction has a positive change in enthalpy.
Exothermic means the reaction has a negative change in enthalpy. The diagram just illustrates that.
Here is a video which discusses how to calculate the enthalpy change when 0.13g of butane is burned. The video includes an enthalpy diagram and discusses how the diagram relates to the problem.
Video from: Noel Pauller