Thermochemistry: The 3 Types of Systems

Start by defining a system versus the surroundings. In thermochemistry, the "system" is simply the specific part of the universe we are focusing on. Everything else around it is the "surroundings." The imaginary line separating them is the boundary.

Now, let's look at the most common setup you interact with daily: the open system. In an open system, both matter (physical stuff) and energy (like heat or work) can flow freely across the boundary.

Think of a hot cup of coffee sitting on your desk without a lid. Heat is actively escaping into the air, which is a transfer of energy. At the same time, steam is evaporating, which is a transfer of matter.

Because it loses both heat and physical mass to the room, your coffee mug is the perfect everyday example of an open system. Biological organisms, including humans, are also open systems because we constantly exchange energy and matter with our environment!

What happens when we put a lid on our system? Welcome to the closed system. In this setup, the rules change slightly: energy can still cross the boundary, but matter is completely trapped inside.

Imagine a pot of soup cooking on the stove with a heavy, tightly fitted lid. As the stove burner transfers heat into the pot, the soup gets hotter. That is energy easily entering the system.

However, because the lid is sealed, the steam cannot escape. The soup might boil, but the water vapor hits the lid, condenses, and drips right back in. No physical mass is lost or gained.

Closed systems are incredibly useful in chemistry because they allow scientists to study temperature changes and energy transfer without worrying about losing any of the actual chemicals they are testing. You keep the ingredients, but you change the heat!

Now let's take things to the extreme with the isolated system. In an isolated system, the boundaries are completely locked down. Neither matter nor energy can enter or exit.

To picture this, think of a perfectly insulated, high-quality thermos flask. If you pour hot soup into it and seal the lid, no steam escapes (zero matter transfer). Because of the vacuum insulation, the heat doesn't bleed out into your hands (zero energy transfer).

In reality, a truly perfect isolated system is nearly impossible to build on Earth, as a tiny bit of heat will eventually leak out over time. The only perfectly isolated system we know of is the entire universe itself!

However, chemists use heavily insulated containers called calorimeters to mimic isolated systems. This allows them to measure the exact amount of heat released by a chemical reaction without outside room temperatures ruining the experiment.