How does one know if a chemical reaction is exothermic or endothermic from the chemical equation for the reaction?
A chemical equation describes a reaction by showing the quantitative relationships between all of the reactants and products. Additional information is needed to indicate if a reaction is endothermic or exothermic.
Thermochemical equations include the enthalpy (heat) change in the chemical reaction, designating it as `Delta` H. If the `Delta` H of a reaction is negative the reaction is exothermic (releases heat), and if it's positive the reaction is endothermic (absorbs heat).
Here's why: The enthalpy change, or `Delta` H of a reaction is the sum of all of the energy absorbed in breaking bonds in the reactants minus the sum of all of the energy released when new bonds form in the products. When the energy aborbed exceeds the energy released there is a net absorption of energy and `Delta` H is positive. Conversely, it's negative for a net release of energy.
An equation that includes enthalpy is typically written like this:
`CH_4 + 2 O_2 -> CO_2 + 2 H_2O, DeltaH = 891 (kJ)/(mol)`
If enthalpy isn't included it can be calculated by looking up bond energies of the individual bonds in all of the reactant and product molecules. The enthalpy change is
`Delta` H = `Sigma` (bond energies of reactants)- `Sigma` (bond energies of products)