Heat Changes Associated with Chemical Reactions, Hess’ Law, and Reaction Profiles

Energy is what causes all of the work around you. When you look at a toaster and it is browning your bread, a series of events has led to this. First, electrical energy is transferred through the cord to the toaster. The toaster converts the electrical energy to heat energy which passes through its heat coils. The heat from the heat coils goes into the bread (surface first, of course) and reacts with the molecules in the bread. Some of the simple carbohydrate molecules on the surface of the bread are heated enough to actually react, and your bread begins to brown.

If a process is releasing energy, the surroundings would feel warm. If a process is absorbing energy, the surroundings would feel cool. Chemicals store energy within their bonds. The amount of energy stored within a compound depends on the bond types and the number of bonds within the molecule. When reactions occur to break bonds, the energy is released. Usually, reactions can occur to reform new bonds, which absorb energy. The enthalpy change of the entire reaction depends on the energy released and absorbed at each step. This is known as Hess’s law.