When does the breaking of chemical bonds release energy?
Category: Chemistry Published: June 27, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
The breaking of chemical bonds never releases energy to the external environment. Energy is only released when chemical bonds are formed. In general, a chemical reaction involves two steps: 1) the original chemical bonds between the atoms are broken, and 2) new bonds are formed. These two steps are sometimes lumped into one event for simplicity, but they are really two separate events. For instance, when you burn methane (natural gas) in your stove, the methane is reacting with oxygen to form carbon dioxide and water. Chemists often write this as:
CH4 + 2 O2 → CO2 + 2 H2O + energy
This balanced chemical equation summarizes the chemical reaction involved in burning methane. The reactants are on the left, the products are on the right, and the arrow represents the moment the reaction happens. But there are a lot of interesting things happening that are hidden behind that arrow. A more detailed equation would look something like this:
CH4 + 2 O2 + a little energy → C + 4 H + 4 O → CO2 + 2 H2O + lots of energy
The first line of the equation contains the original reactants: methane molecules and oxygen molecules. The first arrow represents the breaking of the bonds, which requires energy. On the middle line are the atoms, now broken out of molecules and free to react. The second arrow represents the forming of new bonds. On the last line are the final products. It takes a little energy, such as the spark from the igniter in your stove, to get the reaction started. That is because bonds must be broken before the atoms can be formed into new bonds, and it always takes energy to break bonds. Once the reaction has started, the output energy from one burned methane molecule becomes the input energy for the next molecule. Some of the energy released by each bond that is formed in making carbon dioxide and water is used to break more bonds in the methane and oxygen molecules. In this way, the reaction becomes self-sustaining (as long as methane and oxygen continue to be supplied). The igniter can be turned off. If breaking bonds did not require energy, then fuels would not need an ignition device to start burning. They would just start burning on their own. The presence of spark plugs in your car attests to the fact that breaking chemical bonds requires energy. (Note that the combustion of methane actually involves many smaller steps, so the equation above could be expanded out into even more detail.)
The textbook Advanced Biology by Michael Roberts, Michael Jonathan Reiss, and Grace Monger states:
Biologists often talk about energy being made available by the breakdown of sugar, implying that the breaking of chemical bonds in the sugar molecules releases energy. And yet in chemistry we learn that energy is released, not when chemical bonds are broken, but when they are formed. In fact, respiration supplies energy, not by the breaking of bonds in the substrate, but by the formation of strong bonds in the products. However, the overall result of the process is to yield energy, and it is in this sense that biologists talk about the breakdown of sugar giving energy.
The total energy input or output of a reaction equals the energy released in forming new bonds minus the energy used in breaking the original bonds. If it takes more energy to break the original bonds than is released when the new bonds are formed, then the net energy of the reaction is negative. This means that energy must be pumped into the system to keep the reaction going. Such reactions are known as endothermic. If if takes less energy to break the original bonds than is released when new bonds are formed, then the net energy of the reaction is positive. This fact means that the energy will flow out of the system as the reaction proceeds. This fact also means that the reaction can proceed on its own without any external energy once started. Such reactions are known as exothermic. (Endothermic reactions can also proceed on their own if there is enough external energy in the form of ambient heat to be absorbed.) Exothermic reactions tend to heat up the surrounding environment while endothermic reactions tend to cool it down. The burning of fuels is exothermic because there is a net release of energy. Cooking an egg is endothermic because there is a net intake of energy to make the egg cooked. The bottom line is that both endothermic and exothermic reactions involve the breaking of bonds, and both therefore require energy to get started.
It makes sense that breaking bonds always takes energy. A chemical bond holds two atoms together. To break the bond, you have to fight against the bond, like stretching a rubber band until it snaps. Doing this takes energy. As an analogy, think of atoms as basketballs. Think of the energy landscape of chemical bonds as a hilly terrain that the basketballs are rolling over. When two balls are placed near a round hole, gravity pulls them down to the bottom where they meet and stop. The two balls now stay close together because of the shape of the hole and the pull of gravity. This is like the chemical bond uniting atoms. To get the balls away from each other (to break the bonds), you have to roll them up opposite sides of the hole. It takes the energy of your hand pushing the balls to get them up the sides of the hole and away from each other. The energy you put into the system in order to pull apart the balls is now stored as potential energy in the balls. Atoms don't literally roll up and down hills, but they act like they are moving in an energy landscape that is very similar to real hills.