Energy can take various forms, but each form can be measured in a way that makes it possible to keep track of how much of one form is converted into another. Whenever the amount of energy in one place or form diminishes, the amount in another place or form increases by an equivalent amount.
Thus, if no energy leaks in or out across the boundaries of a system, the total energy of all the different forms in the system will not change, no matter what kinds of gradual or violent changes actually occur within the system.
But energy does tend to leak across boundaries. In particular, transformations of energy usually result in producing some energy in the form of heat, which leaks away by radiation or conduction such as from engines, electrical wires, hot-water tanks, our bodies, and stereo systems. Science for All Americans , p.
Different energy levels are associated with different configurations of atoms in molecules. Some changes in configuration require additional energy, whereas other changes release energy. For example, heat energy has to be supplied to start a charcoal fire by evaporating some carbon atoms away from others in the charcoal ; however, when oxygen molecules combine with the carbon atoms into the lower-energy configuration of a carbon dioxide molecule, much more energy is released as heat and light.
Or a chlorophyll molecule can be excited to a higher-energy configuration by sunlight; the chlorophyll in turn excites molecules of carbon dioxide and water so they can link, through several steps, into the higher-energy configuration of a molecule of sugar plus some regenerated oxygen. Later, the sugar molecule may subsequently interact with oxygen to yield carbon dioxide and water molecules again, transferring the extra energy from sunlight to still other molecules.
Energy conservation is a difficult concept for students to grasp because it is counter-intuitive to their everyday experiences with heat, sound, light, and other forms of energy.
It is important, therefore, to go through each step of this lesson carefully so that students understand that the energy released from hydrocarbon combustion comes directly from the stored energies within the bonds. As a follow-up, the energy that is released from the combustion of hydrocarbons also is not lost; rather, it is used to make our cars run, radios produce sound, bulbs light, and so forth.
In order for students to do this lesson, as well as the other lessons in this series, they need to have prerequisite knowledge of the basics of atoms and their structure. Basic information about atoms can be found at The Atom. Students also should be comfortable drawing molecular structures and determining stoichiometrically correct chemical equations. Because organic chemistry usually concludes a year-long, high-school chemistry course, the lesson also includes some concepts of thermodynamics, such as bond energies, and endothermic and exothermic reactions.
These bond energies incorporate units in joules and moles so students should be comfortable using calculations involving moles. Most advanced chemistry textbooks and curricula include mathematical methods of evaluating the entropy change in a reaction, which in turn is used to calculate the amount of energy that is available for work from a reaction i.
Before beginning the main portion of this lesson, it is important to determine any misconceptions that students may have about the necessity of oxygen to support combustion reactions. To figure out what students already know about fuel needing oxygen, light a small candle, such as a tea light, as a demonstration activity.
Place a small, clear drinking glass or cylinder over the tea candle and allow students to make observations. The flame will extinguish as all the oxygen is utilized. Ask students:. Re-cap for students that oxygen was used by the candle. Each time a substance combines with oxygen, oxidation occurs. Sometimes, substances react with oxygen quickly, and at other times, very slowly over time.
Ask students for some examples of substances that react with oxygen quickly and slowly. Students may suggest iron reacting with oxygen to form rust or wood reacting with oxygen in order to burn and release energy.
For this reason, fossil fuel resources are often referred to as hydrocarbon resources. Although impurities exist in fossil fuels, hydrocarbon combustion is the primary process in the burning of fossil fuel. An example of hydrocarbon combustion is illustrated in Figure 1. See simulation at the bottom of the page for more examples. Regardless of the type of hydrocarbon, combustion with oxygen produces 3 products: carbon dioxide, water and heat, as shown in the general reaction below.
The energy required to break the bonds in the hydrocarbon molecules is substantially less than the energy released in the formation of the bonds in the CO 2 and H 2 O molecules. For this reason, the process releases significant amounts of thermal energy heat.
This thermal energy can be used directly perhaps to heat a home or else it can be converted to mechanical energy, using a heat engine. However, this is subject to efficiency losses, resulting in necessary significant energy losses as waste heat governed by the second law of thermodynamics. The resulting useful mechanical energy will be a lot less than the initial thermal energy provided by the hydrocarbon combustion.
Note that CO 2 is always produced in hydrocarbon combustion; it doesn't matter what type of hydrocarbon molecule. Producing CO 2 and H 2 O is actually how useful energy is obtained from fossil fuels. Question e2ea6. Question 08b What is the thermochemical equation for the combustion of benzene? Why are chemical reactions reversible? Why are chemical reactions important?
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