2.1 HESS'S LAW

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Hi everyone! In today's post we are going to revise Hess's law. Do you know what it menas? Or are you completely new to it? It doesn't matter because I am going to explain everything you need to know to be able to answer questions about Hess's law. As always, you will find past paper questions on my youtube channel.

If you visited my previous post, you know all about enthalpy changes: enthalpy change of combustion, formation and reaction. Today's post will show you how to calculate enthalpy change if we can't find it directly.

You need to use Hess's law when you can't find the enthalpy change directly. For example benzene is not formed from carbon and hydrogen combined together directly. The reaction doesn't go straight from reactants to products. It has more than one route. I will start you off with the definitions of Hess's law.

HESS'S LAW: the total enthalpy change for a reaction is independent of the route taken from the reactants to the products.

Right then, all this definition means is that reactions don't always go in a straight line, sometimes a different route has to be taken, where an intermediate is present in the middle. For example our benzene, carbon together with hydrogen form benzene (route 1- directly) however, you can't find enthalpy change of this because it is actually more complicated to form benzene than just add hydrogen and carbon together. It takes a different route to form benzene with an intermediate in the middle (route 2 - indirectly). This may be much easier to calculate the enthalpy change. But the point is that thanks to Hess's law, no matter which route we take to calculate enthalpy change, the total enthalpy change will always be the same.

To visualise Hess's law, we can draw cycles.

You have to know that there are two ways to calculate enthalpy change using Hess's law and you need to be able to draw cycles for both of them. Let's start with the first one:

USING ENTHALPY CHNAGE OF COMBUSTION VALUES

Ok, so let's start with some facts first. When we are given combustion values and are asked to find the enthalpy change, we have to know couple of rules, exactly two! The first of them is that in order to calculate enthalpy change we have to do enthalpy change of combustion (reactants)- enthalpy change of combustion (products). I found a quick way of remembering this, which actually helped me when I was revising for my A-level chemistry. This is a shortened version of the equation, CRP which stands for combustion, reactants, products. The second rule is when drawing the cycle the arrow has to pointed down but don't worry if you don't understand what I mean right now, it will be much easier to show you once we get onto the examples.

e.g.

In this example we are asked to calculate the enthalpy of formation of enthanol given the following enthalpies of combustion. The first thing that we have to look for with this type of questions is what enthalpy changes they are providing us with. In this instance it is the enthalpy change of combustion. What you should have in mind right now is the CRP and arrows down. Just write it down somewhere, just so you remember and start drawing your cycle. So, as on the image above, write out the equation for the reaction. Now, because we use combustion values, underneath the equation write down the common products of combustion (this will always be the same if you are provided with combustion values: water and carbon dioxide and the arrows will always go down to the combustion products). The next thing you have to do is make sure you have the same amount of each element (either look at the reactants and bottom line or products and bottom line). In this instance, we have 2 carbons in reactants and therefore we put two carbons in the bottom line (the 2 in front of carbon dioxide), we also have 6 hydrogens in reactants and therefore we put 6 hydrogens in the bottom line (the 3 in front of water). In this way our oxygens also match, so the equation is balanced.

After pur cycle is fully drawn, we put in the enthalpy change of combustion values into our cycle. Whenever in the original equation the substance has a number in front, we have to multiply our enthalpy change of combustion value by this number. This simply just means that there are more than one of this substance. Next, we use our CRP to write out the calculation equation, which is reactants- products. We sub in alll of the numbers and put them into our calculator (remember about brackets, order of calculation) and we get our final answer. It's not that bad, isn't it?! :)

The second way, a question may ask us to use Hess's law, is:

USING ENTHALPY CHANGE OF FORMATION VALUES

This time our quick and easy way of remembering how to calculate enthalpy change using enthalpy change of formation values is FPR and arrows up. And then of course we have our cycle, which shows you exactly what is going on. I don't know about you, but I think that with chemistry it is best to just show an example and then everything starts to be clearer. So let's do one.

e.g. as always we've got the equation written and the enthalpy change of formation values right below it. Once you figure out if it is CRP or FPR, you can start drawing your cycle. In this case this is a FPR because we have the formation values above. So, what you want to do is just write the equation down and then underneath it write all of the elements that are included in the equation. In this case we have carbon, hydrogen and oxygen. Elements won't just diappear from the equation, all the elements that are in reactants will also be in products, although they may be in a different compound or form. Next, you just have to balance them, exactly as in the previous example with CRP.

The arrows in this example go up. Write the enthalpy change of formation values beside the arrows and multiply them where applicable. In order to calculate the enthalpy change for this reaction you need to do enthalpy change of formation (products) - enthalpy change of formation (reactants). Put in the numbers shown in the image above and use a calculator to find your final answer.

The way I memorize it if the arrow should go up or down is that in formation the elements at the bottom make up something that is above them, so the arrow is pointing upwards. In combustion the substances that are at the top make the combustion products at the bottom, so the arrow goes down.

Here I have one more question for you as an explained example:

At the top, we have an equation as alway and you also have enthalpy change of formation values next to it (in the exam they will tell you whether the values stated are combustion or formation, I just forgot to write it down). It's formation so FPR and arrows up. Underneath the equation we write the elements that are present in the equation and balance it. Furthermore, you can write the enthalpy change of formation values next to the arrows but this is not neccessary if you get used to the questions. Next, all you need to do, is just take away reactants from products and find your final answer.

I promise you, this is not that bad. Try couple of questions and you will find that, it is quite easy. I have prepared a video with past paper questions for you on my youtube channel, so your're very welcome to visit. Remember if you would like a copy of past paper questions that I've put together email me, or use the box on the main page of this blog or just leave a commnet.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!