In this investigation, students will explore basic thermodynamic concepts, including spontaneity, entropy, and enthalpy through a series of guided questions and procedures. Show ObjectiveGiven prior knowledge of the thermodynamic terms entropy, enthalpy, and spontaneous processes, students will gain a deeper understanding of how ΔG = ΔHsys – T ΔSsys expresses the second law of thermodynamics by exploring energy transfer between system and surroundings as salts dissolve. Safety
Materials for Each Group
Time RequiredOne class period, approximately 45–50 minutes. Lab TipsThis lab is designed for students to work together, discussing and answering the questions posed while proceeding through the step-by-step treatment of the second law. Pre-Lab DiscussionWhat does spontaneous mean? What kinds of processes in your experience happen spontaneously? Are there any differences between them? Incorporating into the CurriculumThis investigation could be incorporated into a unit on chemical changes or thermodynamics.
Thermodynamics is a way of describing energy transformations when a system changes from one state to another. The entire architecture of thermodynamics is built on carefully defined terms, many of which have an everyday meaning that is not exactly what chemists mean when they use the term. For example, one way that chemists state the second law of thermodynamics is that in any spontaneous change, the entropy of the universe increases. The underlined words have a very particular meaning that we need to know before we can understand the second law. The second law of thermodynamics may be expressed in many ways, and it has been used by chemists to understand everything from the work of a steam engine to the direction of time. It grew in the nineteenth century out of observations made about big things like steam engines, and today it is often used to illuminate the conceptual, chemical world of tiny things like atoms, ions, and molecules. In this activity we will use careful observations of the process of dissolving salts in water to more deeply understand the second law. What is a Spontaneous Change?A spontaneous change is any change that happens freely in time. For example, you can drop a ball from above your head and it falls to the floor (spontaneous) but you need to provide energy to the ball to place it over your head again. Being able to predict what processes will be spontaneous is how we apply the second law. Which of the following processes are spontaneous?
Good to know: spontaneous ≠ instantaneous! Even if a change is spontaneous, this doesn’t necessarily mean that it happens quickly. The second law tells us that all diamonds are spontaneously turning into coal, but this process is so slow we will never observe it taking place. System, Surroundings, UniverseThe system is the specific part of the universe we are considering, where a change is taking place. It can be any size—a test tube, a beaker, a human body, or an ocean. The surroundings are everything outside the system. The universe consists of the system and the surroundings together. Fill in the blanks below. Label the regions with the terms system, surroundings, and universe.
Entropy is a mathematically defined property in thermodynamics. It can often help to understand it as a measure of the possible arrangements of the atoms, ions, or molecules in a substance. The symbol for entropy is S, and a change in entropy is shown as “delta” S or ΔS. If the entropy of a system increases, ΔS is positive. If the entropy of a system decreases, ΔS is negative.
But more is going on than just ions leaving the solid and moving about more freely. Note in the figure above that the polar water molecules are attracted to and oriented around the dissolved ions. The ions are solvated. This orientation of a lot of the water molecules reduces their freedom to move about in the liquid, so the number of possible arrangements of the water molecules is reduced when the ions are present.
The change in entropy for the reaction system ΔSsys has to include both the positive change for the ions and the negative change for the water molecules. Which one predominates? For most salts with single charges on their cations and anions, like NaCl, KBr, or LiNO3, the positive change in entropy for the ionic solid separating into its ions in solution will predominate.
Looking at Entropy Changes in the Surroundings by Defining EnthalpyWe will now consider entropy changes in the surroundings by looking at another thermodynamic term, enthalpy. The enthalpy of a system has a definition in thermodynamics that relates to its internal energy, the pressure on the system, and the volume of the system. It is useful in understanding the second law, however, because at constant pressure and volume, a change in enthalpy is the same as the thermal energy transferred from the system to the surroundings, or from the surroundings to the system. The symbol for the enthalpy of a system is H, and a change in enthalpy is shown as “delta” H or ΔH. If thermal energy transfers from the system to the surroundings during a physical or chemical change, the ΔH is negative and the change is exothermic. If thermal energy transfers from the surroundings to the system during a change, the ΔH is positive and the change is endothermic. Case one for enthalpy
Case two for enthalpy
We are going to look at how entropy changes in the surroundings, depending on the sign of ΔH of the system. Before we do, however, let’s review.
Now consider what happens in the surroundings during the process of thermal energy transfer.
Looking at entropy changes in the universe and the second lawIn any spontaneous change, the entropy of the universe increases, or ΔS universe > 0. We have seen that we can consider the entropy of the system and the entropy of the surroundings separately. Since the universe is composed of the system and the surroundings, then we will consider both when determining the change in entropy of the universe: ΔS universe = ΔS surroundings + ΔS system
Complete the following:
Thermodynamics also defines a term known as free energy, G; the change, ΔG, when a system undergoes a change, is often considered the energy of the system available to do work. Free energy also relates to the second law, since in any spontaneous process the change in free energy (ΔG) for the system is negative. The definition for the change in free energy summarizes our discussion of entropy changes in the system and in the surroundings: ΔG = ΔHsys – T ΔSsys We have seen that whenever ΔHsys is negative (exothermic) the ΔSsur increases, or is positive.
This famous relationship of free energy change to changes in enthalpy and entropy shows us the balance between entropy changes in the system and the surroundings, and how that balance depends on temperature. Note that T stands for the absolute temperature in Kelvin, so its value is always positive. Use the relationship ΔG = ΔHsys – T ΔSsys to complete the table below:
The mysteries of bread making began to be simplified in the 1800s with the use of baking soda. Although it was also combined with sour milk to lighten the texture of heavy bread, it could produce a lightening effect in bread dough on its own when it decomposed:
Ozone (O3) is an unstable form of oxygen that is formed in the stratosphere. The ozone layer in the upper atmosphere protects life on the earth’s surface from high-energy ultraviolet light from the sun. Ozone is produced from oxygen gas:
If you have ever used hydrogen peroxide to disinfect an open cut, you may have seen bubbles form when the hydrogen peroxide decomposes:
Reflecting on the Investigation
Demo
Now that we’ve observed the qualitative relationships between ΔH, ΔS, and ΔG, let’s calculate the quantitative value of ΔG°rxn for the dissolution of solid ammonium nitrate in water at 25 °C:
Remember that ΔH°rxn = Σ n ΔH°f (products) − Σ n ΔH°f (reactants) and ΔS°sys = Σ n So (products) − Σ n So (reactants). Then use ΔG = ΔHrxn – T ΔSsys to find the change in free energy. Pay attention to units as ΔH°rxn will be calculated in kJ and ΔS°sys will be calculated in J. Be sure to reconcile units before finding ΔG.
Do your lab observations support your results? Explain. |