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.
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What is a Spontaneous Change?
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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?
- Ice melts when dropped in a cup of warm water.
- Water evaporates when it is spilled on a hot surface.
- Water in a glass on your desk decomposes to hydrogen and oxygen.
- Iron rusts in air.
- The smell of perfume spray spreads throughout a room.
- Equal volumes of olive oil and vinegar dissolve together to make a salad dressing.
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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, Universe
The 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.
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