First Law of Thermodynamics - Video Tutorials & Practice Problems

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 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 or 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? No. 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 I 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 a chemist wishing to determine the final temperature of 30 g of a metal or places it into an insulated beaker containing 615.5 g of water. At 42.18 degrees Celsius. It is determined that the metal gains 19 points. 11 killed joules of energy from the information provided determine the system and the surroundings. All right, so the chemist wishes to determine the final temperature off the metal, or so it's trying to determine its final temperature. Is trying toe. Analyze that metal or so the metal war is what's of interest to us, so it must represent our system. It is our substance being analyzed. And then what else is being talked about? Well, they're talking about the water, right? So the water itself is around. That's what submerges the middle. Or but we're not concerned with it. So the water itself must represent our surroundings. So just remember, if we're trying to find some information on an object or chemical reaction, if we're 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. Now Heat uses the variable off. Q. Heat represents just the flow off thermal energy from a higher temperature. Object to a lower temperature object. So he is moving from something hotter to something colder. And we're gonna say here work, which is represented by W. It's just the movement of reacting molecules against gravity or an opposing force. If you're moving against, um, 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 when heat applications, it transfers heat from a hotter object to a colder object. Let's assume that the spirit on the on the left is at a higher temperature and then the square on the 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 release or gives off heat, then the sign of Q would be negative on the other side, the heat is going towards the um 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. Alright, 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 gonna 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 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 = –