in this video we're going to focus on thermochemistry and some of the equations that you need for this chapter so let's begin let's start with internal energy the change in the internal energy of a system is equal to Q + W now Q represents the amount of heat energy that enters or leaves the system so here's a question for you let's say if if Inside the Box represents the system and let's say the outside anything outside the box is the surroundings now let's say the temperature of the system is 100° C but the surroundings is only 50° C why should let's say 25 now will Q be positive or negative Heat flows from hot to cold so heat is going to flow out of the system into the surroundings so Q relative to the system is going to be positive I mean negative excuse me Q is negative whenever heat is released from the system is considered to be an exothermic process now for the surroundings the surroundings absorb heat energy so Q is positive for the surroundings so it's endothermic for the surrounding anytime heat is absorbed Q is positive typically Q is measured in Jewels now you need to know that 1 K is equal to 1,000 Jew and a calorie a lower case calorie that's 4.184 Jew a capital calorie is a th000 lowercase calories so those are some other conversion factors you need to know W represents work and work is equal to P Delta V when work is done on a system W is positive whenever work done whenever work is done on a system now if work is done by the system W is negative now for this equation wals P Delta V when the change in volume is positive whenever gas expands the work is negative due to this negative sign and so whenever a gas expands it can do work it can exert a force to move a piston or something now to compress a gas you need to apply a force so whenever gas is compressed the change in volume is negative so the work overall is positive so as you apply a force to compress the gas here perform and work on a system so the internal energy of the system goes up whenever you do work on it but whenever the system does work or whenever work is done by the system the internal energy of the system goes down it loses potential energy because it's using up energy to do work so let's see if we can uh apply this information in approxim problem but before we do another thing you need to know is that 101.3 jewles is equal to 1 L * 1 18m so now let's say if 300 jewles of heat energy was absorbed by the system and also let's say the system which is a gas let's say the gas expands from 2 L to 3 l at a pressure of 5 ATM calculate the change in the internal energy of the system so the first thing we need to do is use this equation W is equal to P Delta V the pressure is 5 ATM the change in volume final minus initial it's 3 - 2 so so Delta well P Delta V is going to be5 and the units is going to be liters time ATM now in order to get Delta e we need to convert liters time ATM into Jews and as you mentioned earlier 1 liter * 1 atm is equal to 101.3 Jewels so the units liters cancel and ATM cancel as well so what's 5 * 101.3 5 * 100 is 500 5 * 1 is 5 and 5 * 3 if 5 * 3 is 15 5 * 3 is 1.5 so this is going to be negative 506.com that's why W is negative because the gas expanded and the gas perform worked so the internal energy of the system went down so now we can calculate everything let's plug in what we have into uh the Delta e equation so since heat energy is absorbed it's endothermic Q is POS 300 and W is- 56.5 so so 300 minus 56.5 that's going to be equal to -206 Jew so that's how much energy the system lost overall so now there's some other equations that you need to know Q can be calculated using equation m c delta T M is the mass C is the specific heat capacity and in the case of water it's uh 4.184 jewles per gram per Celsius so what this means is that if you have a one gram sample of water it takes 4.18 Jew to heat up that 1 gr sample of Water by 1° C delta T is the change in temperature which could be in kelvin or Celsius so let's say if you want to calculate you know the amount of energy that's required to heat up let's say 50 grams of water from 25° C to 75° C how much energy is required so the mass is 50 the heat capacity of water is 4.184 and a change in temperature final minus initial 75 minus 25 that's 50 so if you have a calculator 50 * 50 is 2500 and if you multiply that by 4.184 you should get 10,460 jewles so that's how much heat energy is required to heat up 50 grams of water from 25 degrees C to 75 degre Cs and if you divide this by a th you could convert your answer into kles if you want to so whenever you want to calculate the amount of heat energy absorbed or released whenever there's a temperature change use the equation qals MC now whenever you want to calculate the amount of heat energy that's absorbed or released whenever there's a base change you can use the equation q = m * Delta H this could be enthropy of fusion or vaporization or you can use Q which is equal to n * Delta H it really depends on what the units of Delta H is and personally I think it's easier if you just convert it so let me give you an example let's say if you have 54 gram of ice at 0 degrees Celsius and you want to find out how much heat energy is required to melt that ice into liquid water at the same temperature whenever you melt ice while ice is melting into liquid water the temperature it remains constant so this is a phase change we want to go from a solid into a liquid so it's easier if you simply convert it now you need to know the heat of fusion for water the heat of fusion for water is about 6 KJ per mole so what that means is that if you have one mole of water you requires 6 K to melt ice it requires 6 K of heat energy to melt one mole of ice into liquid water so what we need to do is convert grams to moles and then moles to kilog so let's begin so we have 54 gram of H2O in a solid form and if you look at the periodic table oxygen has an atomic mass of 16 and hydrogen has a mass of one time two so the molar mass for water is 18 gram per one mole and so the units grams cancel and now we can convert moles to kles we know that 6 K of heat is required per one mole of ice so let's see what this is equal to 54 / 18 is 3 and 3 * 6 well that's it's going to be 18 so there's 18 K of heat energy that's required to melt 54 G of ice into liquid water now sometimes you might get what is known as a thermochemical equation so consider the combustion reaction of propane with oxygen gas is going to produce CO2 and water go ahead and balance this reaction and let's say this reaction releases 12200 KJ of heat energy notice that there's no Delta h symbol if you see the K of heat energy on the right side it's exothermic if you see it on the left side it's endothermic now whenever you want to balance a combustion reaction balance the carbon atoms first so let's put a three in front of CO2 now this eight hydrogen atoms on the left so we need a four in front of water and so we have six oxygens from the 3 CO2 molecules four from H2O 6 + 4 is 10 if you divide it by this number two you're going to get five so we need to put a five in front of o2 so now this reaction is balanced so how many kilj of heat energy will be released if 64 G of oxygen reacts how would you answer that question how would you find the answer so just like before we're going to convert grams into moles and then moles into kles so let's start with 64 gram of oxygen let's convert it to moles anytime you want to go from GRS to moles you need to use the M Mass found in a periodic table the atomic mass for oxygen is 16 so for O2 16 * 2 is 32 so there's 32 G of o2 per 1 mole of o2 and so as we can see the units grams of o2 cancel now you need to understand what this number means so if one mole of propane reacts 1200 k of heat energy will be released now if five moles of o2 reacts 1,200 kles of heat energy will be released if three moles of CO2 is produced well that's going to correspond to 1200 KJ so what you need is the K per mole ratio so since we're dealing with oxygen the five is important to us so for every 5 moles of o2 that reacts 1,200 K of heat energy will be released and so we can eliminate moles of o2 so now we can do the math so let's see if we can do this without a calculator so 64 / 32 is 2 and so we have 2 * 1200 and a five on the bottom so 1,00 is basically 12 * 100 and 100 is 20 * 5 so we can cancel the fives so what's 20 * 2 20 * 2 is 40 and now what's 40 * 12 well 4 * 12 is 120 so 40 I mean I take that back 4 * 12 is 48 so 40 * 12 is 480 you can see it this way 40 is 4 * 10 so if you multiply 10 and 12 you get 120 120 * 4 is 480 so let's go back to this example now let's say if 3600 K of heat energy was released in this reaction how many gr of CO2 was produced so we want to go backwards you want to convert kles to moles and moles to CO2 so let's start with 3600 K now the K per Mo ratio there's 1,200 k for every M of CO2 so we're going to use that to go from K to mol so for every 1,200 K of heat energy that was released 3 moles of carbon dioxide was produced so the units kles cancel and now the last thing we need to do is convert moles to grams so the molar mass for CO2 is going to be 12 for carbon plus 32 for the two oxygen atoms so that's going to be 44 G of CO2 per one mole and so these units cancel so 12 goes into 36 three times you can cancel the zeros if you want so we have 3 * 3 * 44 so 3 * 3 is 9 so what's 9 * 44 9 * 44 is basically 10 - 1 * 44 so 44 * 10 that's 440 44 * 1 is -44 so 44 440 minus 44 how much is that so 440 minus 40 is 400 and if you subtract another four from that you should get 396 and so there's going to be 396 gr of CO2 that's produced in this reaction consider this reaction methane reacts with oxygen gas to produce CO2 in water and let's balance it so we need a two in front of H2 we already have one carbon atom on both sides now we have four oxygen atoms on the right side so we need a two in front of o2 so now let's say if you're given the Heats of formation for cl2 for H2O and CH4 let's say for CH4 it's netive 785 and this is Delta H with a f on the bottom and a circle on top that's the enthropy of formation the enthropy of formation is the enthropy of the heat that's absorbed or released or the enthropy of the reaction whenever a compound is produced from its elements and it's natural standard State and you have to produce only one mole of that compound that's the definition for the anthropy of formation if you want to know but we don't have to worry about that for this example but let's say for CO2 it's about 393 and for water it's - 286 how can you use this information to calculate or estimate the enthropy of this entire reaction if you want to find the enthropy of the reaction using the heat of formation it's the sum of the enthalpies for the products minus the sum of the heat of formation for the reactants that's how you can calculate it so on the product side which is the stuff on the right side we have cl2 plus 2 H2O and for the reactants uh excuse me the reactants that's the stuff on the left side that's we have CH4 plus 202 now the value for CO2 that's uh 393 and for H2 it's going to be 2times the value for H2 which is - 286 and then minus that for CH4 which is 785 now O2 is a Pure Element in its natural state so it's zero for um any such elements so now we just got to do the math so we have - 393 2 * 286 that's I'm going to use a calculator for that that's a576 and 785 that's plus 785 so if you add these numbers up you should get -84 so that's how you can estimate the eny of the reaction using the heat information and typically these values are going to be in KJ per mole so this is going to be 84 K per mole now let's say if we have this equation 2 a + 2 B turns into D plus e how can you estimate the enthropy of this reaction if you're given the enthropy of two other reactions let's say a plus b turns into C and the enthropy for that reaction is 200 and let's say we have D plus e turns into 2 C and Delta H is400 for that reaction so how can we use those two reactions to find the enthropy of this reaction the one on top so we got to use something called hes law which means we got to modify equations one and two such that when we add them we can get equation three and if that happens we can add the enies of number one and two to estimate the enthropy of the reaction for number three so let's focus on a notice that in the net reaction we need 2 a so that tells us that we need to double equation one so let's multiply equation one by two so it's going to be 2 a + 2 B and that turns into 2 C so we got to multiply the enthropy of the reaction as well so it's going to be 400 now now notice that D and E are on the right side of the equation that means we got to reverse equation two so it's going to be 2 C which turns into D plus e and so whenever you reverse it the reaction has to change sign so it's going to go from 400 to posi 400 so now let's add these two equations so notice that we have 2 C on the left and 2 C on the right so these two are going to cancel so everything else is going to drop down on the left side we have 2 A plus 2 B so that's going to fall down and on the right side we have D plus e so notice that when we add these two equations we get equation three which means that we can now add the enthropy of these two equations so the enthropy of reaction for equation three is going to be 800 and so that's how you can get it using hess's law so that is it for this video thanks for watching and have a great day