Updated: Oct 24, 2019
In a previous blog post named Chemical reaction we discussed what a chemical reaction is. We also talked about endothermic and exothermic reactions. Today we will build upon that knowledge.
Enthalpy is all energy in a system, including the energy stored in bonds, the energy that is heat etc. You can’t measure all the enthalpy in a system, only how it changes when a chemical reaction occurs. ΔH is the symbol for the enthalpy change.
Hess’s law states that the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs. It means that if reactant A turns into product B it doesn’t matter how many steps are taken, the enthalpy change is the still same.
The law of conservation of energy states that energy can neither be destroyed or created, only transfer between different forms of energy. These two laws are essential for calculating the enthalpy change. We will deal with calculations in my next post.
Exothermic reactions is when chemical energy transfers to heat energy. In other words an exothermic reaction release energy, most often heat energy. The released energy must come from inside the reactants. The enthalpy change is always negative because the products contain less energy than the reactants. Exothermic reactions are the norm. An example is photosynthesis.
Endothermic reactions is the opposite of exothermic reactions. It absorbs heat and make the surroundings cooler. Since the reactants absorb energy then the energy of the products must be higher. Therefore the enthalpy change is positive. Of course some activation energy is needed to make an endothermic reaction happen. An example is cell respiration.
Sources: Higher Level Chemistry by Mike Ford and Catrin Brown Image credit: Figure 1 and 2 - https://sites.google.com/site/ellesmerealevelchemistry/module-3-periodic-table-energy/3-2-physical-chemistry-1/3-2-1-enthalpy-changes/3-2-1-a-b-c-enthalpy-changes