Ch 7 1st Law of Thermodynamics

Added: 2026-05-18

[00:00:00]hi everybody this is just practice problem and a review from chapter 7 you cover chapter 7 chem 1a so we're just reviewing that I know I put some lecture material on that on canvas and it's a lot so this is just summarizing it going over important things that you need to know for 1b so the first law of thermodynamics is the law of conservation of energy which says energy is neither created nor destroyed it's only converted from one form into another and one way we express that mathematically is by writing changing internal energy of a system is equal to the heat added or deducted from the system plus the work that's done on the system or done by the system so internal energy of the system refers to the total kinetic and potential energy of the system and it's very difficult to measure in absolute terms so we usually measure the change in it and there are two ways you can change it by either adding heat or deducting heat from the system so that's one way energy can go in and out in terms of heat from the system the other is by work by you know doing work on the system or having the system do work on the surroundings so just a quick review system is the part of the universe that one is in is interested in so cylinder could be your system or chemical reaction could be your system anything outside of that is considered surrounding and system plus surrounding is considered to be universe so the changing internal energy of the system is equal to the heat released by the system or absorbed by the system and we use the sign to indicate the direction of open Fe so if it's released by the system is negative we always look at things from in chemistry from point of view of the system so if your system is losing energy losing heat that's maybe it's kind of like you losing it's negative if it's absorbing it then it's positive same with work if the work done is by the system that's negative because the system is doing the world same thing if the work is done on the system then it's positive so on the system it's positive by the system it is negative so now let's look at the work part so the P Delta V so what we you can measure working fix for example is cylinder like here is by taking the pressure the external pressure that the system is pushing against and I won't find that by the change in the volume so if the system is for example expanding the external pressure that is pushing against times the change in volume will be to work that the system is doing on the surrounding and if the system is pushing against astronomies doing work on the surroundings so that should be negative Delta being that case comes up to be positive so for the size to work we have to put it negative here so the work comes out to be and so let's do an example it's a cylinder absorb three kilojoules of heat from surroundings and increase its volume from one liter to five liter pushing against two atmosphere of external pressure calculate the change in the internal energy of that cylinder so this is your cylinder you're putting in three kilojoules of heat into it from strongly so if it's going into the system that's positive is positive and then as a result the gas in here expands increasing the volume from one liter to five liter so it's doing work on the surrounding against two atmosphere pressure so we want to calculate the change energy of the system as a result of that process so to do that firstly calculates first we are going to calculate the amount of work that this system did on surroundings so the work is equals to minus P Delta V pressures to atmosphere changing volume is four liters so that comes out to be eight liter atmosphere leader atmosphere is the unit energy and one liter atmosphere is equal to 101 Joule if you remember we went over that ideal gas constant R we said it's point zero eight to one liter atmosphere over Kelvin mole if you express it in liter atmosphere if you don't express it in joules it's 8.314 joules per Kelvin mole those two are expressing the same value in different units it's like saying okay this is 12 inches or one foot it's the same length expressed in different units so oneness Libre atmosphere is 101 Joule so if I convert it into joules and in kilojoules and that is minus point 808 kilojoules the reason it's negative is because we're doing the system's doing work on the surroundings so what's going on at a system energy in the form of wort so therefore it's negative so the Delta e is equal to Q plus W we're putting three kilojoules of heat energy into the system in the form of heat we took out point eight kilojoules of energy on an assistant in terms of work so the internal energy of the system actually increased because it took in more energy in the form of heat than it put out in the form of work and so if the internal energy increased especially since that increase was in the form of P then the cylinder will be hotter than before so as a result of this the cylinder will be hotter than it was if the all thing he was negative then the cylinder would be holder than before