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5.5: Thermodynamics and Systems

  • Page ID
    52341
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    The study of how energy in its various forms moves through a system is called thermodynamics. In chemistry specifically it is called thermochemistry. The first law of thermodynamics tells us that energy can be neither created nor destroyed but it can be transferred from a system to its surroundings and vice versa.99 For any system, if we add up the kinetic and potential energies of all of the particles that make up the substance we get the total energy. This is called the system’s internal energy, abbreviated as E in chemistry.100 It turns out that it is not really possible to measure the total internal energy of a system. But we can measure and calculate the change in internal energy represented as ΔE (we use the Greek letter Δ to stand for change). There are two ways that the internal energy of a system can change: we can change the total amount of thermal energy in the system (denoted as q), or the system can do work or have work done to it (denoted as w). The change in internal energy is therefore:

    ΔE = q + w

    At the molecular level, it should now be relatively easy to imagine the effects of adding or removing thermal energy from a system. However work (usually defined as force multiplied by distance) done on or by the system is a macroscopic phenomenon. If the system comes to occupy a larger or smaller volume, work must be done on the surroundings or on the system, respectively. With the exception of gases, most systems we study in chemistry do not expand or contract significantly. In these situations, ΔE = q, the change in thermal energy (heat). In addition, most of the systems we study in chemistry and biology are under constant pressure (usually atmospheric pressure). Heat change at constant pressure is what is known as a state function, it is called enthalpy (H). A state function is a property of a system that does not depend upon the path taken to get to a particular state. Typically we use upper case symbols (for example, H, T, P, E, G, S) to signify state functions, and lower case symbols for properties that depend on the path by which the change is made (for example, q and w). You may be wondering what the difference between a state and path function is. Imagine you are climbing Mount Everest. If you were able to fly in a balloon from the base camp to the top you would travel a certain distance and the height change would be 29,029 feet. Now in contrast if you traveled on foot, via any one of the recorded paths – which wind around the mountain you would travel very different distances – but the height change would still be 29,029 feet. That is the distance travelled is a path function and the height of Mt Everest is a state function. Similarly, both q and ΔH describe thermal energy changes but q depends on the path and ΔH does not. In a system at constant pressure with no volume change, it is the change in enthalpy (ΔH) that we will be primarily interested in (together with the change in entropy (ΔS), which we examine shortly in greater detail).

    Because we cannot measure energy changes directly we have to use some observable (and measurable) change in the system. Typically we measure the temperature change and then relate it to the energy change. For changes that occur at constant pressure and volume this energy change is the enthalpy change, ΔH. If we know the temperature change (ΔT), the amount (mass) of material and its specific heat, we can calculate the enthalpy change:

    ΔH (J) = mass (g) x specific heat (J/g oC) x ΔT (oC).101

    When considering the enthalpy change for a process, the direction of energy transfer is important. By convention, if thermal energy goes out of the system to the surroundings (that is, the surroundings increase in temperature), the sign of ΔH is negative and we say the process is exothermic (literally, “heat out”). Combustion reactions, such as burning wood or gasoline in air, are probably the most common examples of exothermic processes. In contrast, if a process requires thermal energy from the surroundings to make it happen, the sign of ΔH is positive and we say the process is endothermic (energy is transferred from the surroundings to the system).

    Questions to Answer

    • You have systems (at 10 oC) composed of water, methanol, ethanol, or propanol. Predict the final temperature of each system if equal amounts of thermal energy (q) are added to equal amounts of a substance (m). What do you need to know to do this calculation?
    • Draw a simple sketch of a system and surroundings. Indicate by the use of arrows what we mean by an endothermic process and an exothermic process. What is the sign of ΔH for each process?
    • Draw a similar diagram and show the direction and sign of work (w) when the system does work on the surroundings (expands), and when the surroundings do work on the system (contracts).
    • Draw a diagram to show the molecular level mechanism by which thermal energy is transferred in or out of a system. For example how is thermal energy transferred as an ice cube melts in a glass of water?

    Questions to Ponder

    • What does the difference in behavior of water, methanol, ethanol, and propane tell us about their molecular behavior/organization/structure?

    References

    99 In fact, we should say mass-energy here, but because most chemical and biological systems do not operate under the high-energy situations required for mass to be converted to energy we don’t need to worry about that (for now).

    100 Or U if you are a physicist. This is an example of how different areas sometimes seem to conspire to make things difficult by using different symbols and sign conventions for the same thing. We will try to point out these instances when we can.

    101 One important point to note is that this relationship only works when the thermal energy is used to increase the kinetic energy of the molecules—that is, to raise the temperature. At the boiling point or freezing point of a liquid the energy is used to break the attractions between particles and the temperature does not rise.


    5.5: Thermodynamics and Systems is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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