0.13 Reaction energy and bond energy  (Page 2/7)

An important fact about state functions is the change in a state function from start to finish during a process depends only on the value of the state function at the start and at the finish. The concept of a state function is somewhat analogous to the idea of elevation. The elevation of a city depends only on where the city is located. So, consider the difference in elevation between Houston and Denver. This difference is clearly independent of any path we choose to get from Houston to Denver. We could drive a direct route, we could take the train through Chicago, we could take a non-stop or a multi-stop flight, or we could depart Houston on a plane and parachute into Denver. Each path produces exactly the same elevation gain, equal to the elevation in Denver minus the elevation in Houston.

Since energy is a state function, the energy of the products depends only on the state of the products and the energy of the reactants only depends on the state of the reactants. So the energy change in going from reactants to products is just the energy of the products minus the energy of the reactants. We could choose any process that takes us from reactant to product. We do not even need to know what process or processes actually happen. You can see how a state function like energy is reflected in our observation of Hess’ Law.

Chemists choose to define a particular measure of the energy called the enthalpy, designated H. The enthalpy is a state function, just like the energy. The reason that chemists study the enthalpy is that the difference between the enthalpy of the products, and the enthalpy of the reactants, H(products) - H(reactants) = ∆H, is equal to the heat transfer during a reaction carried out under constant pressure: ∆H = q . Since a great many chemical processes, including essentially all biological processes, happen under constant pressure, this is a very helpful thing to know. For this reason, ∆H is often called the enthalpy of reaction or the heat of reaction. We will measure and calculate ∆H frequently in this concept study.

Observation 1: enthalpy of formation

Remember that our task is to understand what determines whether a reaction is exothermic or endothermic and why the energy transfer might be large or small. To do so, we need to get inside the reaction and find out what energy changes are taking place. And for this, Hess’ Law and the state function H are extremely valuable. These allow us to choose any processes we want to carry out a chemical reaction, knowing that the energy transfer doesn’t depend on what processes we pick. This frees us to pick processes to observe which are interesting or revealing to us. We want to pick processes which help us compare energies of reactants and products.

To do this, it is helpful to think of a standard to compare against. This is like defining elevation relative to sea level, making sea level the standard which is equal to zero elevation. We could have picked the zero to be the highest point on earth, the top of Mt. Everest, or the lowest point on earth, the bottom of the Marianas Trench, or the northernmost point, the North Pole, or anywhere else. We just need to pick something convenient which allows easy comparisons. Once we know every city’s elevation relative to sea level, we can easily calculate the elevation change for every possible city to city trip without ever making any of those trips.