These old iron chains give off a small amount of heat as they rust. The rusting of iron is a chemical process. It occurs when iron and oxygen go through a chemical reaction similar to burning, or combustion. The chemical reaction that occurs when something burns obviously gives off energy. You can feel the heat, and you may be able to see the light of flames. The rusting of iron is a much slower process, but it still gives off energy. It's just that it releases energy so slowly you can't detect a change in temperature.
A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms as a result of a chemical reaction is called a product. During the reaction, the reactants are used up to create the products. Another example of a chemical reaction is the burning of methane gas, shown in Figure \(\PageIndex{2}\). In this chemical reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of chemical bonds. Chemical bonds are forces that hold together the atoms of a molecule. Bonds occur when atoms share electrons. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and water.
Matter rusting or burning are common examples of chemical changes. Chemical changes involve chemical reactions, in which some substances, called reactants, change at the molecular level to form new substances, called products. All chemical reactions involve energy. However, not all chemical reactions release energy, as rusting and burning do. In some chemical reactions, energy is absorbed rather than released.
All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure \(\PageIndex{5}\). Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces so the molecules can come together and react also takes energy.
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Watch the video below to learn more about activation energy. LICENSED UNDER Category: Chemistry 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: 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 energyThe 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:
Burning propane requires an igniter to get the reaction started because chemical bonds must be broken before new ones can be formed, and breaking bonds always requires energy. Public Domain Image, source: Christopher S. Baird. 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. |