# Thermochemical Equations Practice Problems

in this video we're going to focus on

## thermochemical

### equations

so what exactly is a## thermochemical

equation it's a chemical equation that relates the amount of thermal heat that can be absorbed or released so what you need to understand is the relationship between the coefficient and this number so for example if four moles of iron metal reacts in its reaction negative 1652 kilojoules of heat energy will be released if three moles of oxygen gas is consumed in this reaction the sameamount of heat energy will be released and if two moles of iron iii oxide is produced in this reaction the same amount of heat energy will be released so you need to understand the relationship between this amount of thermal energy and the moles of the reactants and products so now let's focus on this problem iron metal reacts with oxygen gas to produce iron iii oxide according to the reaction shown below how much heat is released when 9 moles of fe reacts with excess oxygen gas so let's

start with what we're given and that's 9 moles of fe our conversion factor is that 4 moles of fe will generate 1652 kilojoules of heat energy so we want the units moles of fv to cancel so we're going to put that in the bottom and put negative 16 52 kilojoules on top so these units will cancel and so the answer is going to be nine times sixteen fifty two divided by four three thousand seven hundred seventeen kilojoules of thermal energy will be released if nine moles of fe reacts so

that's the answer to part a now let's move on to part b how much heat is released when 24 grams of o2 is consumed in the reaction well let's find out so let's start with what we're given that's 24 grams of o2 so if you're given a mass what you need to do is you need to convert the grams to moles using the molar mass found in the periodic table and then using the

## thermochemical

equation you can convert moles to kilojoules and so that's what we need to do in thisproblem so oxygen has an atomic mass of 16 therefore o2 is going to be 16 times 2 which is 32 grams per mole so now that we have the moles of o2 we can use the fact that three moles of o2 would generate that amount of heat energy so i'm going to put three moles of o2 on the bottom so that the unit moles of o2 will cancel and then 1652 kilojoules on the top of the fraction so it's 24 divided by 32 which is 0.75 times 16 52 divided by 3. so it's negative 413 kilojoules of thermal

energy so that's how much heat will be released if 24 grams of o2 reacts in this chemical reaction now let's move on to part c how many grams of iron iii oxide will be produced if 4 500 kilojoules of heat energy is released so this time we're given the amount of kilojoules we need to convert it back to moles and then to grams so let's start with what we're given and that is 4 500 kilojoules and because this energy is released it's negative now we can convert from

kilojoules to moles using the fact that two moles of iron iii oxide corresponds to 1652 kilojoules so negative 1652 kilojoules will be released when two moles of iron oxide are produced so now what we need to do is find the molar mass of fe2o3 so we have two fe atoms and three oxygen atoms so the atomic mass of iron is 55.85 and for oxygen is 16. 2 times 55 plus 3 times 16 that will give us a molar mass of 159.7 grams per mole so one mole of fe2o3 has a mass of 159.7 grams so now it's going

to be 4500 times 2 divided by 1652 and then take that result multiply by 159.7 so the answer is about 870 grams of iron oxide so make sure you know how to convert between grams and kilojoules when you're given a thermal chemical a problem like this one now let's move on to this problem propane reacts with oxygen gas to produce carbon dioxide and water as shown in the reaction below how much energy in kilojoules will be released if 500 kilograms of propane is burned in the air so

we're given the kilograms of c3h8 we need to convert it to grams and then use the molar mass to convert it to moles and then use the equation to convert it to kilojoules so let's start with 500 kilograms of propane now one kilogram is equivalent to a thousand grams and the molar mass of propane c3h8 it's going to be 3 times 12.01 plus 8 times 1.008 so you should get this number so i'm going to put one mole of propane on top and on the bottom 44.094 grams and the substance is c3h8

now notice the ratio between c3h8 and the 22 21 kilojoules so the coefficient is one one mole of propane will yield 2 21 kilojoules of heat energy so now we're going to multiply 500 by a thousand and then divide that by 44.094 and then multiply by two thousand two hundred and twenty one so you should get two point five two times ten to the seventh power kilojoules and it's negative so the exact answer is about 25 million 184 832 kilojoules now what about part b how many kilograms of

propane is need to generate 5 megajoules of energy if the process is 40 efficient so we need to generate 500 megajoules of energy and 40 of the energy that's released will be in its usable form the other sixty percent will be lost maybe due to heat or something it could be lost as friction it could escape into the environment but only forty percent of the energy that's going to be released that's the amount that we're going to capture and do something useful with it so how much

do we need to begin with so forty percent of what number is five to find that answer you have to take five and divide it by forty percent so five divided by point four is twelve point five so we need to determine the amount of propane that we need to burn to generate 12.5 megajoules of heat once we generate that amount of heat we can capture 40 of it which represents 5 the other sixty percent which is twelve point five times point six that's going to be lost to the environment so that's

gonna be waste energy so we need to convert this value to kilograms of propane so let's start with 12.5 megajoules of energy now before we can use this number we need to convert megajoules into kilojoules one megajoule is equivalent to a thousand kilojoules so now that we have the energy in kilojoules we can convert it to the moles of propane so for every 2 221 kilojoules that we have one mole of propane needs to be consumed so now let's convert moles of propane into grams so the molar

mass was 44.094 grams per 1 mole of propane now the last thing we need to do is convert grams to kilograms so there's a thousand grams per kilogram and so that's it for the conversion so now all we got to do is just plug in the values that we have so it's going to be 12.5 times a thousand divided by twenty two twenty one multiplied by forty four point zero nine four divided by a thousand so this is going to be .248 kilograms of propane so that's how much kilograms of propane that

we need to burn in order to generate 5 megajoules of energy if the process is forty percent efficient you