Methods of Communicating Enthalpy Changes

Chemists have different ways to express the enthalpy change of a reaction. Which would be?

1. Stating molar enthalpy of a specific reactant of the reaction;
2. Stating the enthalpy change for a balanced equation.
3. Including the enthalpy change as a part of the equation, being reactant or product, depending on the type of the reaction;
4. Representing by a diagram.

These four methods that were quoted have cons and pros, so why do we study them all?

They will provide us a better understanding on the topic. 

Which is?

The change in the energy of substances after a reaction occur. 

Which ones of these four methods are we can trust the most?

The first three ones, since they are empirical description. The fourth one, for being a diagram, is more of a theoretical description.

Let's talk about Method 1:

Well, the molar enthalpy is the measure of released or absorbed energy from or to a substance per mole. The substance and the reaction must be specified. 

Do we have standards: Yes, we do. Standard molar enthalpy are expressed with a * subscript. It means that it represents the molar enthalpy of substance under ideal conditions.

Method 2:

Just write the enthalpy change next to the balanced equation. Iif we change the coefficient or the order of the equation, then the enthalpy change will also change.

Method 3:In endothermic reactions the energy is on the reactant side. in exothermic reaction the energy is on the product.