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System and Surroundings: System and Surroudings in a Chemical Reaction

When boiling water, the water molecules can be treated as the system. Everything else is the surroundings. When energy transfers from the system to the surroundings, the reaction are exothermic. When energy transfers from the surroundings to the system, the reaction are endothermic.

System and Surroundings for Chemical Reactions

When a chemical reaction occurs it is important to consider the system and surroundings. The system is the collective substances in the reaction such as the reactants and products. The surroundings are everything around the reaction such as the reaction flask and the room. During a reaction, energy is transferred between the system and surroundings. This energy is a sum of all potential and kinetic energies.

Exchange of Energy Between System and Surroundings

Since, energy can move between a system to the surroundings, it is important to measure the amount of energy and energy changes. This energy change is described as: $$\Delta E\ =\ E_{final}\ -\ E_{initial}$$ When energy moves out of the system to the surroundings, $\Delta E$ is negative. When energy moves into the system from the surroundings, $\Delta E$ is positive.

Isolated System

If a system was closed off from its surroundings it is known as a closed or isolated system. In an isolated system, the amount total energy of the system remains the same. This is described in the first law of thermodynamics which states energy is a closed system is conserved and remains constant, it cannot be created or destroyed and can only change forms. However, most systems are not isolated, they are open systems, which means that there will be energy moving between the system and surroundings.

(videos) System and Surroundings: How to Chemistry: System and surroundings

By ShowMe App

In this video, you will learn what are system and surroundings. In addition, the video explains what is the difference between endothermic and exothermic reactions.

(video) System and Surroundings: Open System, Closed System and Isolated System - Thermodynamics & Physics

By The Organic Chemistry Tutor

In this video, you will learn about three different types of systems. And how the systems interact with the surroundings.

We've started our study of thermodynamics. So far, we've been building the foundation by defining some of our terms. Let's go through a quick refresher on what we've studied so far.

We started with internal energy. It was the sum of all the kinetic and potential energies. We know that atoms and molecules have various types of movement. They can translate, rotate, and vibrate. All of these play into kinetic energy. But, a chemical system also contains electrostatic interactions. We see it most prominently in chemical bonds. But, Van der Waal's forces also impact the potential energy.

Next, we studied how energy gets transferred. We started by defining work. It was the mechanical transfer of energy. So, our example was a gas lifting a piston or the piston compressing the gas. We learned that classical thermodynamics measures physical changes to understand chemical change. In our piston example, the volume change occurred because energy got transferred. The physical properties we often measure are volume, pressure, and temperature. We also investigated nonmechanical energy transfers. We saw that atoms and molecules could transmit energy via collisions. We called it heat. Our physical property for measuring heat was the temperature change.

In this lesson, we have to look at one more definition. But once we've made it through, we'll have our complete thermodynamics foundation. Then, we can apply it to chemical reactions!

System and Surroundings

Let's say you walked into class, and your teacher told you to perform a chemical reaction. Let's say it was the decomposition of hydrogen peroxide using a manganese catalyst. The balanced chemical equation is $2H_{2}O_{2(aq)}\rightarrow 2H_{2}O_{(l)} + O_{2(g)}$. What would you do? You would probably grab a beaker, pour some hydrogen peroxide and drop in a chunk of manganese. Success! You did it!

But what if we wanted to learn some thermodynamic information about this reaction? How would you set up your experimental design? First, we see that one of our products is a gas. That's a clue we need to measure mechanical work. So, we could perform the reaction in a piston apparatus. In any chemical reaction, it's a good idea to measure heat transfer. So, we'd also need a thermometer.

What would we find if we performed our reaction with our new set-up? As the reaction proceeds, the oxygen gas lifts the piston. Our starting pressure should be atmospheric pressure. If you're not at sea level, remember to use a barometer. Anyway, the pressure remains constant. So, by measuring the volume change, we could calculate the pressure-volume work. We would also see the temperature elevate. The decomposition of hydrogen peroxide is an exothermic reaction. So, it gives off heat.

So, why did we take this longing winding road? This reaction helped us to illustrate the system and the surroundings. In chemistry, we define the system as our chemical reaction. That includes the reactants, products, catalysts, transition states, intermediates, solvents, and anything else. The surroundings are everything else. In this case, it includes the piston, container, thermometer, the air in the room, and the rest of the universe.

Why do we care about defining the system and surroundings? It helps us to express how energy gets transferred. In our example, the oxygen gas lifted the piston. So, the system did work on the surroundings. The system also heats the surroundings. That's why the temperature increased. Eventually, we'll learn to calculate the heat from the temperature change via enthalpy. Once we know how to do that, we can calculate the change in internal energy.

It's critical to have our system and surroundings well defined. Why? If we don't, the situation quickly becomes unclear. Imagine we represented the system as our chemical reaction and the piston apparatus. We could still calculate the pressure-volume work. The piston pushed up against the surrounding air. So, it did work on the surroundings. But this is already less intuitive. It's easier to think of the oxygen gas pushing the piston. Oxygen transfers some of its kinetic energy into the piston's movement. Let's say the piston dropped. It's easier to think of the piston compressing the gas. It's more confusing to think of the atmosphere compressing the piston and gas. So, it's essential that we define our system well.