T - Energy Transfer and the First Law of Thermodynamics (Lesson)
Energy Transfer and the First Law of Thermodynamics
Energy changes associated with chemical reactions are an integral part of our everyday experience. As mentioned in the module overview, gasoline is burned in the engines of cars and trucks to harness the energy given off during this process. A related example would be our daily consumption of food which releases the energy stored in food and allows our bodies to function. Likewise, plants harness energy from the sun to fuel metabolic processes related to the production of glucose and other macromolecules.
These examples reveal the importance of understanding what happens during these chemical changes that allow for energy to flow from one source to another. The most common type of energy associated with these processes comes in the form of heat and is included in a field of chemistry known as thermochemistry. However, in order to understand this aspect of chemistry more fully, it is necessary to understand more broadly what the term energy means.
Energy
To borrow an idea from physics, energy is the ability to do work. Furthermore, energy comes in two forms referred to as potential energy (energy related to an object's position) and kinetic energy (energy related to an object in motion). For example, the ball perched atop the building below would have potential energy due to its high position whereas the race car in the image to the right has kinetic energy because it is in motion.
The question then is how do these concepts relate to chemical systems? How can chemical reactions have potential and kinetic energy? As will be shown later, chemical reactions can potentially do work, but this definition does not fully encompass what is meant by energy from a chemistry standpoint. A more applicable definition of energy would then be the ability to supply heat or do work. In chemical terms, potential energy is stored in the actual chemical bonds of substances before a reaction occurs. During a chemical reaction, these bonds are broken, and potential energy is converted into a form of kinetic energy as allowed by the first law of thermodynamics. In other words, there is no gain or loss of energy during this process but rather a conversion from one type of energy to another. The form of kinetic energy in this case is referred to as thermal energy which is the kinetic energy associated with particles on the atomic or molecular level. An increase in thermal energy leads to faster moving particles and vice versa. The flow of this type of energy from one object to another is referred to as heat.
System vs. Surroundings
In this module, there will be certain terms that are used whose definitions are important to grasp and understand. As mentioned above, heat is the flow of thermal energy, but the direction of this flow is just as important as knowing how much energy is flowing. To address this issue the terms system and surroundings are used.
In thermochemistry, a sample of matter that is being studied is referred to as the system, and energy is either flowing into or out of the system; essentially, the system is the reference point. Anything that does not belong to the system, by definition, is referred to as the surroundings. Older textbooks use the terms system and universe, so the term surroundings literally means everything else. A simple example will help illustrate this idea. Consider a cup of water that has had an ice cube placed in it. In this scenario, the ice cube can be imagined as the system and the water (as well as everything else) would therefore be the surroundings. Because the ice cube is colder than the surrounding water, heat will flow into the ice cube. Thus, the system is gaining thermal energy while the surroundings are losing it, but there is no loss of energy.
Notice also from the above example that everything is in terms of the direction of heat flow. In thermochemistry, there is no such thing as the flow of cold. Even though ice is added to water in order to make the water colder, from a thermochemistry point of view, the ice is absorbing heat from the water in order to make it colder.
Internal Energy and Sign Conventions
To summarize some of the important points above, the flow of energy can be summarized as follows. During physical and chemical processes, energy can be exchanged between the system and the surroundings in two ways: heat (q) and work (w) as the equation below demonstrates.
ΔE=q+w
In order to distinguish which direction energy is flowing, certain sign conventions are used. It should be noted that these signs simply indicate whether the system is gaining or losing energy (e.g. there is no such thing as "negative heat"). The signs are always relative to the system, and if the system is gaining energy the sign is (+) whereas if the system is losing energy the sign is indicated as (-). A table showing these conventions for all three parameters is shown below.
| q (heat) | (+) system gains thermal energy | (-) system loses thermal energy |
|---|---|---|
| w (work) | (+) work done on the system | (-) work done by the system |
| (+) energy flows into the system | (-) energy flows out of the system |
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