Thermochemical Equations Practice Problems

In this video we will focus on

thermochemical

equations

. So what exactly is a

thermochemical

equation? It is 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, for example, if four moles of metallic iron react in your reaction, 1652 negative kilojoules of thermal energy will be released, if three moles of gaseous oxygen are consumed in this reaction, the same amount of thermal energy will be released and if two moles of iron iii In this reaction oxide is produced, the same amount of thermal energy will be released, so it is necessary 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: the 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 react with excess oxygen gas so let's start with what we are given and There are 9 moles of faith.

Our conversion factor is that 4 moles of fv will generate 1652 kilojoules of thermal energy, so we want the mole units of fv to cancel, so we're going to put that down and put negative 16 52 kilojoules up so that these units cancel out, so the answer will be nine times. Sixteen fifty-two divided by four three thousand seven hundred and seventeen kilojoules of thermal energy will be released if nine moles of faith react, 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 are consumed? In the reaction, let's figure it out, so let's start with what we're given, which is 24 grams of O2, so if they give you a mass, what you need to do is convert the grams to moles using the molar mass found in the table periodic and then using the thermochemical equation you can convert moles to kilojoules and that is what we have to do in this problem, so oxygen has an atomic mass of 16, therefore o2 will 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 thermal energy so I'm going to put three moles of o2 in the bottom so that the unit moles of o2 cancel out and then 1652 kilojoules at the top of the fraction, so that's 24 divided by 32, which is 0.75 times 16 52 divided by 3. so that's negative 413 kilojoules of thermal energy, so that's the amount of heat that will be released if 24 grams of O2 react 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 thermal energy are released? So this time they give us the number of kilojoules we need to convert back to moles and then to grams. Let's start with what we are given and it is 4500 kilojoules and because this energy being released is negative we can now convert from kilojoules to moles using the fact that two moles of iron iii oxide correspond to 1652 kilojoules so they will be released 1652 negative kilojoules. when two moles of iron oxide are produced, now what we have to do is find the molar mass of fe2o3, so we have two atoms of fe and three atoms of oxygen, so the atomic mass of iron is 55.85 and that of 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 will be 4500 times 2 divided by 1652 and then we'll multiply that result 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 given a thermal chemical.

A problem like this now let's move on to this problem. Propane reacts with oxygen gas to produce carbon dioxide and water as The following reaction shows how much energy in kilojoules will be released if 500 kilograms of propane are burned in air, thus giving us the kilograms of c3h8 that we need to convert them 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 equal to one thousand grams and the molar mass of propane c3h8 will 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 look at the relationship between c3h8 and 22 21 kilojoules, so the coefficient is one. One mole of propane will produce 2 21 kilojoules. of thermal energy, so now we're going to multiply 500 by a thousand and then divide it by 44,094 and then multiply it by two thousand two hundred and twenty-one, so you should get two point five two times ten to the seventh power of kilojoules and it's negative. So the exact answer is approximately 25 million 184,832 kilojoules. Now what about part b? How many kilograms of propane are needed to generate 5 megajoules of energy if the process has an efficiency of 40?

So we need to generate 500 megajoules of energy and 40 of the energy that is. 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 will be released, that's the amount that Let's capture it and do something useful with it. So how much do we need to get started? So, forty percent of what number is five. To find the answer, you must take five and divide it by forty percent, so five divided by the point four is twelve. point five, then we need to determine the amount of propane we need to burn to generate 12.5 megajoules of heat.

Once we generate that amount of heat, we can capture 40, which is 5, the other sixty percent, which is twelve point five times point six. that will be lost to the environment so it will be a waste of 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 convert megajoules to kilojoules, one megajoule is equivalent to one thousand kilojoules, so now that we have the energy in kilojoules we can convert it to moles of propane, so for every 2 221 kilojoules that we have, it is necessary to consume one mole of propane, so Now let's convert the moles of propane to grams, so the molar mass was 44.094 grams per 1 mole of propane.

Now the last thing we have to do is convert grams to kilograms, so there are a thousand grams per kilogram and that's it for the conversion, so now all we have to do is just plug in the values ​​that we have, so it will 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 it will be 0.248 kilograms of propane, so that is the number of kilograms of propane that we need to burn to generate 5 megajoules of energy if the process is forty percent efficient.