INTRODUCTION TO ENERGY BALANCE WITH REACTIONS

[00:00:00]This is the introduction to energy balance with chemical reactions.

[00:00:11]So, introduction to energy balance with chemical reactions.

[00:00:17]With chemical reactions.

[00:00:25]Okay, introduction to energy balance with chemical reactions.

[00:00:38]So, introduction to energy balance with chemical reactions. So, some textbooks will call this thermochemistry.

[00:00:43]Actually, this is thermochemistry because it's just the continuation of the So, we know from a fact that a chemical reaction some chemical reactions, not all, um will release energy and then some chemical reactions will absorb energy.

[00:01:02]Okay? So, chemical reactions that release energy are referred to as exothermic.

[00:01:08]So, if they release energy, then those chemical reactions are referred to as exothermic.

[00:01:14]So, I think you're quite familiar with that.

[00:01:16]Then, if they absorb energy, then that chemical reaction is referred to as endothermic. So, this is just like first year chemistry.

[00:01:27]But, this is chemical engineering.

[00:01:29]We will just simply adding on to chemistry.

[00:01:34]So, if reactions release energy, they're exothermic. If they absorb energy, then those reactions are endothermic.

[00:01:41]So, if you have a reactor, okay, you want to find out how much energy is released or absorbed by a particular reaction. So, this is a reactor.

[00:01:52]Okay? We know for a fact that okay, this is this reaction absorbs energy.

[00:01:57]And these are the reactants. You have A then they're producing C.

[00:02:03]Okay. So, they're going to It's going to release energy.

[00:02:07]Okay, for a fact, okay, or it will absorb energy. That's a fact. So, now, this energy that is absorbed or released by reaction is referred to as the heat of a reaction, okay? So, it will be usually in the form of heat.

[00:02:22]So, the heat added heat of reaction is usually in the form of enthalpy. That brings me back to my next point.

[00:02:30]Okay, that is that the change in enthalpy of a reaction is equal to the heat added or released by a reaction. So, the energy balance for a for a chemical reaction is simplified to the form of heat added or released is equal to change in enthalpy.

[00:02:48]Okay?

[00:02:49]And then this is delta Hrx to mean that this is the enthalpy change related to the reaction.

[00:02:56]So, this is the energy balance simplified when applied to chemical reactions.

[00:03:02]So, what you must put in mind is this equation here and the idea that the heat added or released by a reaction is called the heat of reaction.

[00:03:12]Now, this heat of reaction is because of different types of chemical reactions, it means there are different names.

[00:03:22]So, we're talking about the heat of a reaction.

[00:03:26]The heat of reaction.

[00:03:29]And I said the heat of reaction is equal to the enthalpy of reaction.

[00:03:37]So, the heat of reaction or the enthalpy of reaction is simply the heat added or absorbed during a chemical reaction.

[00:03:44]So, I said there are different types of chemical reactions. So, if you have a reaction that involves combustion, okay?

[00:03:51]If you take combustion reaction, then the heat of reaction will be called the standard heat of reaction of combustion.

[00:03:56]Okay? To be symbolized like this. So, if this is there's a dot here, it means it's standard. I'll explain what standard is.

[00:04:02]So, if the reaction involves formation of a substance from its elements, then that will be called the heat of reaction of formation.

[00:04:12]Okay?

[00:04:13]So, these are the two main types of heat of reactions. Okay? So, let's talk about the first, which is called standard heat of reaction.

[00:04:22]Standard heat of reaction.

[00:04:25]I won't provide textbook definition, but I'll I'll try to make you understand what it means.

[00:04:30]So, the standard heat of reaction is usually symbolized like this with a temperature down.

[00:04:36]So, this is the heat of reaction at standard conditions. So, the heat of reactions at standard conditions.

[00:04:44]At standard conditions.

[00:04:50]Okay?

[00:04:51]So, the heat of reaction at standard conditions, at standard conditions. So, what are those standard conditions? So, what is standard temperature and pressure?

[00:05:04]a pressure of So, if you have a reaction, for example, um nitrogen reacting with hydrogen to produce ammonia, then this reaction is balanced like this.

[00:05:21]Then down here they write delta H 298 is equals to -109.4 uh kilojoules per mole.

[00:05:32]Okay? So, this is the standard heat of reaction So, that is what we mean by standard heat of reaction. It's the heat of reaction at standard conditions. And by standard conditions, we mean at 298 Kelvin and at one atmosphere.

[00:05:55]Second type is the standard heat of formation.

[00:05:59]The standard heat of formation.

[00:06:08]So the standard heat of formation or the standard enthalpy of formation.

[00:06:13]So you have all those new things.

[00:06:15]Standard enthalpy of formation is the energy change or the enthalpy change.

[00:06:22]I'll use the enthalpy change or the energy change or the heat change um that occurs So this is the enthalpy change that occurs So enthalpy change that occurs that occurs when forming a substance standard conditions from its elements.

[00:06:56]elements.

[00:06:58]So the heat of formation, okay? So it's the same thing. It's the enthalpy change that occurs when Okay, if we If we have this scenario here, we combine hydrogen and oxygen to produce water. The enthalpy change will be referred to as the standard uh will be referred to as enthalpy of formation of water, okay? Why? Because the the enthalpy of formation is zero for natural elements. So the enthalpy of formation is zero for natural elements.

[00:07:45]That means oxygen has a enthalpy of formation zero, hydrogen has an enthalpy of formation zero because these are in their natural elements, in their natural state. They are not formed, okay? They exist in nature like that.

[00:07:57]Hence, the standard enthalpy of such will be zero.

[00:08:00]The standard enthalpy of nitrogen will be zero.

[00:08:04]So, elements that exist in their natural state, they have a standard enthalpy of formation of zero because they are not being formed.

[00:08:10]But, anything else, standard enthalpy of formation or enthalpy of formation is the enthalpy change that occurs when forming a substance from its elements.

[00:08:45]Then, the next one is standard enthalpy of combustion.

[00:08:57]>> [groaning] >> The standard enthalpy of combustion is symbolized like this.

[00:09:05]So, standard enthalpy of combustion is the enthalpy change when burning a substance or for combustion reactions, okay?

[00:09:14]So, this is the enthalpy change when burning a substance.

[00:09:17]So, for example, if you have a fuel from aldehydes, uh if it reacts with oxygen, for example, then it produces carbon dioxide and water. So, the energy change that is associated with this reaction is the enthalpy of combustion.

[00:09:32]Okay, if you put this, it means it will be standard conditions.

[00:09:35]But, if it is just like this, it's simply the end of the combustion.

[00:09:40]Okay? This is just the end of the combustion.

[00:09:45]Okay?

[00:09:48]Associated with energy changes.

[00:09:50]Okay? Chemical reactions associated with energy changes.

[00:09:54]Then the energy change of a chemical reaction is called the enthalpy of reaction or the heat of reaction. Okay?

[00:10:01]We call it the heat of reaction.

[00:10:05]Or the enthalpy of reaction.

[00:10:07]So that is delta H R and H. Then depending on the type of reaction that is happening, this has different names.

[00:10:14]Okay? You can call it um the enthalpy of combustion if it's a combustion reaction involved.

[00:10:22]Then you have this guy.

[00:10:24]Then if it's it involves formation of of a new substance then you have this guy.

[00:10:32]Okay? And if this happen at standard conditions then you have this. Okay? If it is the measure of the standard conditions then you have this.

[00:10:42]Okay? Because these are measured at standard conditions.

[00:10:45]All right.

[00:10:46]So but it's all under heat of reaction.

[00:10:49]Now here's the issue is that for some substances these things can be measured at standard conditions.

[00:10:59]Because they're easy to measure.

[00:11:02]But some reactions are quite complicated.

[00:11:06]Or they've not been experimentally determined.

[00:11:13]Of calculating the heat of reaction in reactions where it's hard to determine experimentally.

[00:11:20]So to do that we use two different methods. Okay?

[00:11:24]I'll call the first one the formula method.

[00:11:28]Okay?

[00:11:30]The formula method and the second one I'll call it um Hess's law.

[00:11:35]Okay?

[00:11:37]So, you can use Hess's law or you can use the formula method. Okay, but for this class, I'll use both Hess's law and the formula method and look at how we can we can do that. So, if you're using the formula method, there are two formulas of calculating the heat of reaction or the enthalpy of reaction.

[00:11:59]Okay, there are two different ways, two formulas depending on what what information you've been given.

[00:12:06]So, we're saying the enthalpy of a reaction is equals to summation of the enthalpy Actually, you've been given these at standard conditions.

[00:12:19]minus summation of enthalpy of formation for the reactants.

[00:12:28]Okay.

[00:12:30]So, summation of enthalpy of formation of products and summation of enthalpy of formation of reactants. Now, if you have A like this in this scenario, a reaction you have stoichiometric coefficients.

[00:12:43]So, these are stoichiometric coefficients, the numbers before a species.

[00:12:48]And in chemistry, we symbolize this with a V. Okay?

[00:12:52]So, you multiply the individual stoichiometric coefficients for each of your balances in a reaction.

[00:13:00]Then, you sum up the heat of formations and you subtract them.

[00:13:05]That is the first formula using heat of formation.

[00:13:08]Then, you can also calculate the heat of a reaction by using um VI.

[00:13:14]So, summation of combustion.

[00:13:17]Now, here you use reactants first minus VI summation of heat of combustion of products. So, that's the absolute difference between the two approaches you can use.

[00:13:35]So, you can either use this first formula or you can use the second.

[00:13:47]Okay, so here's an example of um of calculating the heat of reaction.

[00:14:01]So, we're saying calculate the heat of reaction of the following reaction. So, we've been given the the heat of formation.

[00:14:13]Let's calculate the heat of reaction.

[00:14:14]Remember our formula we said the change in heat of reaction equals to the summation of heat of formation of what? Of products.

[00:14:26]minus summation of heat of formation of reactants.

[00:14:33]Okay, then you multiply that by this.

[00:14:38]Okay.

[00:14:39]So, let's start with the products. Okay?

[00:14:41]So, we know that in the product side um for the products Let me indicate that. Products.

[00:14:50]We know that we have um we have this here.

[00:14:55]Okay, which is um formaldehyde if you're interested in the name.

[00:15:01]Okay.

[00:15:03]Uh this is also written in most analysis quite common. So, it's also written like this in a compressed form.

[00:15:14]So, I have water here.

[00:15:16]So, here the stoichiometric coefficient is one. So, just say one times what is the heat of formation of this guy?

[00:15:24]So, the heat of formation is -106 -106.108 sorry.

[00:15:31]So, this is like um -108.6 plus Okay?

[00:15:41]We also have water.

[00:15:43]So, water is -285.3.

[00:15:47]So, 1 * - 285.3.83 Okay.

[00:16:20]24.10.43 Okay. So, this will be kilojoules per mole.

[00:16:31]Okay.

[00:16:33]So, that is what we have for our heat of um heat of formation formation for the product side.

[00:16:40]Then, we do the same thing for the reactants.

[00:16:44]So, we do the same thing same thing for the reactants and measure it.

[00:16:49]So, for the reactants, we know that we have um So, here oxygen, remember this is its natural element. So, the heat of formation is zero. That's why we don't have it here.

[00:16:59]So, for this methanol, we have -239.2.

[00:17:04]So, -239.2 * 1.

[00:17:09]So, that ladies and gentlemen gives us -239.2 kilojoules per mole.

[00:17:17]Then, we can now calculate our standard heat or our heat of reaction, sorry.

[00:17:21]Let's go to standard. So, the heat of reaction is products minus reactants.

[00:17:26]So, the product is -394.43 minus minus two. Okay.

[00:17:45]We get negative 155.3.

[00:17:50]So, this is simply kilojoules to give in kilojoules per mole.

[00:17:54]So, why? Because it's one mole of in particular reference, it's methanol.

[00:17:59]It's going to be this guy.

[00:18:03]So, that is our heat of reaction. So, we've used heat of formation to calculate the heat of reaction. Now, let's talk about Hess's law.

[00:18:11]Let me increase some of the So, Hess's law.

[00:18:35]So, Hess's law is based on the fact that enthalpy is a state function.

[00:18:42]The fact that enthalpy is a state function.

[00:18:46]So, it's based on the fact that enthalpy is a state function, which is a state property. Okay?

[00:18:53]So, what it mean What it means by state property is that this property only depends on the initial and final state.

[00:19:01]It only depends on the initial and final state.

[00:19:04]It It It doesn't matter how you arrived at that state. Okay?

[00:19:08]So, sometimes, um then my Okay, money is a state property.

[00:19:13]It's a state property because it doesn't matter how you make the money. Okay, it's it doesn't matter where it started from and where you end it.

[00:19:21]So, same enthalpy is a state function.

[00:19:24]It only depends on the initial and the final state. So, for example, you can have a reaction that might have maybe let's say for example an enthalpy change, the enthalpy of reaction.

[00:19:37]But then, you can go here you or you can just find a particular point where you can reach then travel like this then you discover that okay, here I'll have my first change, here I'll have my second change, then here I'll have my third change.

[00:20:00]Okay? Then you're going to conclude that oh, the heat of reaction hence is going to be the summation of the first heat of reaction plus the second heat plus the third. So, this is a very important property of enthalpy.

[00:20:17]So, it's a state function. So, this here this path here can be the sum of all these paths. So, enthalpy is a state function doesn't depend on how you arrive at that particular point. What matters is that the final and the initial state are the same. So, thermodynamics equations are simply chemical include an enthalpy change. So, the chemical equations with an enthalpy Okay?

[00:20:52]These are chemical equations with an enthalpy change.

[00:20:56]So, let's say for example the the equations we were solving. Okay?

[00:21:02]You you have let's say for example carbon dioxide plus uh plus hydrogen, this is carbon monoxide.

[00:21:12]So, plus hydrogen reacts to produce carbon dioxide. Okay?

[00:21:18]So, we have this, let's say.

[00:21:21]For the reaction, then they tell us this is 298.15 kJ/mol. So, this is a thermodynamic equation. Why? Because it's an equation that involves enthalpy. Now, there are certain properties of a thermodynamic equation.

[00:21:37]So, property this equation, then the sign of the enthalpy will also flip. So, that means if carbon dioxide becomes a reactant instead of being the product, then we have this situation, then the enthalpy of reaction will flip. So, it will become negative 298.15.

[00:22:00]That's the first property.

[00:22:02]Okay? The second property is that if I multiply this by a constant, two.

[00:22:15]Okay?

[00:22:16]So, here to form water, but I've just simplified it. So, if I multiply that by two, it means I have 2 CO plus two hydrogen, right?

[00:22:25]Then I also have two carbon dioxide, but remember, this guy will also be multiplied twice.

[00:22:31]So, that is 298.

[00:22:34]um 15 * 2, this guy will become five 96.3.

[00:22:43]Okay, so that Those are the most two important properties of um of thermodynamic equations.

[00:22:51]Then we can use these properties together with Hess's law to calculate the heat of reaction of a reaction.

[00:23:14]>> So, here's the question. So, I'm saying Hess's law is based on the principle that enthalpy is a state function. State the meaning of a state function. So, I already said that a state function is simply a property which only depends on um the final and the initial state and not on how the property arrived at that state.

[00:23:35]So, what matters is the final and the initial state. So, that is a state property.

[00:23:39]So, the standard heat of reaction of the following chemical reactions were determined experimentally as shown. So, we have the heat of reactions here.

[00:23:49]Then, apply Hess's law to determine the heat of reaction for this reaction. Okay?

[00:23:56]So, what do you need? So, the first thing you have to do is to write down all the equations you've been given. So, you have two water, okay, being broken down to produce um two hydrogen plus oxygen, then you have positive 572. So, I'll forget about the signs for now.

[00:24:18]So, carbon dioxide is carbon plus oxygen, then you get 390 394.

[00:24:30]Then, you have two C2 H2, okay, plus five oxygen to produce plus water.

[00:24:43]Then, this is negative 2,600.

[00:24:46]Negative 2,600.

[00:24:49]Okay.

[00:24:50]So, the first thing you need to look for is your final equation, how it looks like. It's very important to see how it looks like. So, it has this guy that's carbon, that has also hydrogen.

[00:25:01]So, you ask yourself, which elements or compounds or molecules are in these three equations, but they are not supposed to be in the final equation?

[00:25:11]Okay? So, is water supposed to be in the final equation? No. So, we don't want water in the final equation.

[00:25:16]Um is hydrogen in the final equation?

[00:25:19]Well, yes.

[00:25:20]Is oxygen in the final equation? No, it's not. Carbon dioxide? No, no.

[00:25:25]Carbon? Yes. Oxygen? No.

[00:25:27]So, you you you get to have an idea of what you should you should just remove.

[00:25:30]Okay? You know that, okay, water I should cancel. Water I should cancel oxygen. I don't want carbon dioxide.

[00:25:38]Okay? So, once you have an idea of what you don't want, it's easy for you to cancel them, to find ways in which you can cancel them.

[00:25:45]Okay, let's say I want to cancel oxygen first.

[00:25:47]So, I know that, okay, here there's oxygen, here there's oxygen.

[00:25:50]But this oxygen and this oxygen are on the same side. So, if I add them, I'll get two.

[00:25:54]I want to cancel them. So, I'll say, okay, let me flip the second equation.

[00:25:59]So, that the sign changes as well as oxygen goes on the other side.

[00:26:04]So, that means I have Okay, so I've flipped it.

[00:26:10]Then I'll just keep the second equation the first equation the way it is with two water here producing two hydrogen plus oxygen. Then this remains 572.

[00:26:23]Then I'll add this equation this equation that I add them.

[00:26:26]So, since I'm adding, this is in a different side as this one. So, it means the oxygen here can cancel.

[00:26:33]Okay?

[00:26:35]So, you can just say, goodbye to oxygen because oxygen is canceled here.

[00:26:41]Okay?

[00:26:44]But would this be a very good idea?

[00:26:46]I think the answer is no, because look, if I cancel the oxygen here too early, >> [snorts] >> if can see the oxygen [clears throat] quite early.

[00:26:55]Then I'm going to remain with this oxygen which will remain and cancelled.

[00:26:58]So, not a very good approach.

[00:27:02]So, this is I said that cancelling first should be something bigger, okay. So, then we cancel water first. So, let me just get the first equation and the second equation the way they are. Then I can cancel water, right?

[00:27:20]So, I'll get the first equation the way it is.

[00:27:29]two the way it is.

[00:27:43]And um this is -2600.

[00:27:50]Okay.

[00:27:51]Then I'll add them.

[00:27:53]Then you see the water here is two units to it to cancel, but look, there's oxygen and it's on different sides. So, one of them will cancel. Then I remain with four here.

[00:28:03]So, we have two C2 H2 plus four oxygen.

[00:28:10]Then we have two hydrogen plus um plus this guy.

[00:28:16]Four carbon dioxide.

[00:28:17]Then you can add these two equations.

[00:28:25]You you get something like -2028.

[00:28:31]Okay.

[00:28:32]Now, it means this has started having a language, like carbon dioxide. I don't want carbon dioxide.

[00:28:46]So, then an option is that look, this here therefore has also been used. So, I remain with only one option.

[00:29:00]So, it means I'll multiply the second equation by four.

[00:29:03]Okay, I'll multiply it by four.

[00:29:05]So, if I multiply by You also have four oxygens.

[00:29:14]Then if I multiply 3 and 2 * 4, then I'm getting 1576.

[00:29:23]These are like dates.

[00:29:25]1576 So, then I have that. Then I add them.

[00:29:32]I know my friend oxygen will go away.

[00:29:34]Then this guy Okay, brilliant. Then carbon dioxide will also go. This guy will also go.

[00:29:41]Okay, then I remain with two of C2H2 on this side.

[00:29:48]Then I have two hydrogen plus four carbon.

[00:29:54]Then the difference would be negative 452. So, we do really have the equation the way we wanted it, right? But not the numbers because it's 1 1 1 2 1.

[00:30:08]So, I will divide here by two. Leaving this side but you know whatever you do to the equation, you also do to the enthalpy change. So, if I divide by two throughout, let me just get some space on top.

[00:30:23]So, if I divide by two by two throughout, it means I'll have um C2H2 plus um oh, is that equals to hydrogen. This guy will go plus carbon.

[00:30:42]Let me arrange it in the way it's supposed to be arranged. So, it's C2H2.

[00:30:47]Then, we have two carbon plus hydrogen.

[00:30:51]Then, what is the enthalpy?

[00:30:54]So, the enthalpy is this divided by two.

[00:30:57]That gives us minus 226, okay, kilojoules per mole.

[00:31:03]So, that is how you find the enthalpy using Hess's law.