First Law of Thermodynamics - Online Tutor, Practice Problems & Exam Prep

The First Law of Thermodynamics, a fundamental concept in chemistry, states that energy cannot be created or destroyed, only transferred between a system and its surroundings. The system typically refers to the substance or chemical reaction under study, while the surroundings encompass everything else. Energy transfer can occur in two forms: heat ($Q$) and work ($W$). Heat is the flow of thermal energy from a higher temperature object to a lower temperature object, with the sign of $Q$ being positive when the system gains heat and negative when it loses heat. Work involves movement against gravity or an opposing force, with $W$ being negative when the system does work on the surroundings and positive when work is done on the system by the surroundings. Understanding the signs of $Q$ and $W$ is crucial for analyzing energy changes during chemical reactions.

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 is everything else that is not that substance or not that chemical reaction. So if we took 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 would represent my system.

The container is just what holds my system. It itself is not the system I'm only examining. The gas is not the container though. 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 the 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 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 is everything else that is not that substance or not that chemical reaction. So if we took 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 would represent my system. The container is just what holds my system. It itself is not the system I'm only examining.

The gas is not the container though. 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 the 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

Now to understand the transferring of energy between systems and surroundings, you first have to understand the idea of heat versus work. Now 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. It's just the movement of reacting molecules against gravity or an opposing force. If we'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 & 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. When 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 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, it's 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 in 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 molecules 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&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 W.

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 = –