First Law of Thermodynamics: Study with Video Lessons, Practice Problems & Examples

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred between a system and its surroundings. In this context, heat (q) is the flow of thermal energy from hot to cold, while work (w) involves the movement of molecules against opposing forces. A system losing heat has negative q, while gaining heat has positive q. If the system does work, w is negative; if the surroundings do work on the system, w is positive. Understanding these concepts is crucial for analyzing chemical reactions and energy changes.

First Law of Thermodynamics

Now the first law of thermodynamics says that energy cannot be created nor destroyed. What happens instead is that it's transferred between our system and its surroundings. Now when we say system, the system represents the chemical reaction because we're in chemistry, represents a chemical reaction or a substance that is being studied or analyzed. The surroundings are everything else that is not that substance or not that chemical reaction. So if we take a look here at this image, we have a container. Inside of this container, we have gas molecules. Let's suppose that the gas molecules are what I am studying and observing. The gas molecules represent my system. The container is just what holds my system. It itself is not the system. I'm only examining the gases, not the container. So the container and everything outside the container, including you, me, the universe, would be our surroundings. Both of these ideas together deal with the first law of thermodynamics. So just remember, you can't create energy, you can't destroy energy. It just changes from one form to another, and changing from one form to another means the transferring of energy between systems and surroundings.

First Law of Thermodynamics Example 1

Now a chemist wishing to determine the final temperature of 30 grams of a metal ore places it into an insulated beaker containing 615.5 grams of water at 42.18 degrees Celsius. It is determined that the metal gains 19.11 kilojoules of energy. From the information provided, determine the system and the surroundings.

Alright. So, the chemist wishes to determine the final temperature of the metal ore, trying to analyze that metal ore. Therefore, the metal ore is what is of interest to us; it must represent our system, as it is the substance being analyzed. What else is being talked about? Well, they are discussing the water, which submerges the metal ore, but we are not primarily concerned with it. Thus, the water itself must represent our surroundings.

Just remember, if we aim to find some information on an object or chemical reaction—if we are trying to analyze them—they represent our system. Everything that is not the system has to be our surroundings.

First Law of Thermodynamics

Now to understand the transferring of energy between systems and surroundings, you first have to understand the idea of heat versus work. Heat uses the variable of q. Heat represents just the flow of thermal energy from a higher temperature object to a lower temperature object. So heat is moving from something hotter to something colder. And we're going to say here work, which is represented by w, is just the movement of reacting molecules against gravity or an opposing force. If you're moving against an opposing force or against gravity, some work has to be done on your part. So just remember, heat and work have to do with this transferring of energy between system and surroundings. Click on the next video and let's see what happens to the signs of q and w under different circumstances.

First Law of Thermodynamics

Now remember, heat is the flow of energy, more specifically thermal energy between a hotter object towards a colder object. So, in heat applications, it transfers heat from a hotter object to a colder object. Let's assume that the sphere on the left is at a higher temperature, and then the sphere on the right is at a lower temperature. Heat naturally moves from a place that is hotter to a place that is colder. Now the system on the left is losing heat; this one here is gaining heat. Well, if you are losing, so if it loses, evolves, releases, or gives off heat, then the sign of q would be negative. On the other side, the heat is going towards the colder object so it is gaining heat. So if a system gains, absorbs, or takes any heat, then it has a positive q. So that's the way we observe the signs of q. If heat is being moved, whoever is gaining the heat is positive q, whoever is losing the heat is negative q.

Now work is a little bit different. Work is the force done by reacting molecules on a frictionless piston. All right. So we're going to say here, let's say we have our gas in this container and the piston here can move up or down. Let's say the gas molecules themselves want to be spread out even more from each other, and they decide to push up against the piston, so they're doing work on the piston here. As a result of doing work on the piston here, they are going to have a negative w. If the system does work on the surroundings, it is a negative w. The surroundings here would be the piston or the container. Conversely, let's say the gas molecules are just hanging around, not doing anything, and some outside force decides to push down on this piston. The piston again is our surroundings. It's going to come down and it's going to squeeze down on the gas molecules. In this case, the surroundings are doing work on the system. If the surroundings are doing work on the system and the system is doing nothing, then work will be positive. That's because the system is not working against an opposing force. It's just sitting back and letting it happen. So just remember, q and w can be positive or negative depending on situations. So just remember, if our system gains heat, it's positive q. If it loses heat, it's negative q. If the system does any type of work, it's going to be a negative w, and if the surroundings do work on the system, then it's positive.

What are the signs of q and w when a system loses heat while doing work on the surroundings?

• A. q = – , w = –
• B. q = + , w = +
• C. q = – , w = +
• D. q = + , w = –

Here’s what students ask on this topic:

What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred between a system and its surroundings. This means that the total energy of an isolated system remains constant. In mathematical terms, the change in internal energy (ΔU) of a system is equal to the heat (q) added to the system minus the work (w) done by the system on its surroundings. This can be expressed as:

$\mathrm{\Delta U}=q-w$

Understanding this law is crucial for analyzing energy changes in chemical reactions and physical processes.

How does heat transfer between a system and its surroundings?

Heat transfer between a system and its surroundings occurs due to a temperature difference. Heat (q) flows from a hotter object to a colder one. If a system loses heat, it has a negative q, and if it gains heat, it has a positive q. For example, if a hot gas in a container loses heat to the cooler surroundings, the gas has a negative q. Conversely, if the gas absorbs heat from the surroundings, it has a positive q. This transfer of thermal energy is a key concept in the first law of thermodynamics.

What is the difference between heat and work in thermodynamics?

In thermodynamics, heat (q) and work (w) are two ways energy can be transferred between a system and its surroundings. Heat is the flow of thermal energy from a higher temperature object to a lower temperature object. Work, on the other hand, involves the movement of molecules against an opposing force, such as gravity. For example, if gas molecules in a container push against a piston, they are doing work on the piston, resulting in a negative w. Conversely, if an external force pushes the piston down, compressing the gas, the surroundings are doing work on the system, resulting in a positive w.

How do the signs of q and w change under different circumstances?

The signs of q (heat) and w (work) depend on the direction of energy transfer. If a system gains heat, q is positive; if it loses heat, q is negative. For work, if the system does work on the surroundings, w is negative; if the surroundings do work on the system, w is positive. For example, if gas molecules in a container expand and push a piston upward, the system is doing work on the surroundings, resulting in a negative w. Conversely, if an external force compresses the gas by pushing the piston downward, the surroundings are doing work on the system, resulting in a positive w.

What is the significance of the first law of thermodynamics in chemical reactions?

The first law of thermodynamics is significant in chemical reactions because it helps us understand energy changes. During a chemical reaction, energy can be transferred as heat or work between the system (the reacting substances) and the surroundings. By applying the first law, we can calculate the change in internal energy (ΔU) of the system, which is crucial for predicting reaction behavior and designing energy-efficient processes. The law ensures that we account for all energy changes, making it a fundamental principle in thermodynamics and chemistry.