2.1 BOND ENTHALPIES

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Nice to see you back! I can tell you now that this topic is really nice and easy and if get a question on this topic in your exam, you're lucky! There is really nothing you shouldn't understand here. No more talking, let's just start.

In this topic it isall about making and breaking bonds. We are just going to start with some information that you should already know. Firstly, during all reactions bonds are broken on the left hand side of an equation (reactants), so that they can form new bonds on the right hand side of an equation (products). This makes sense doesn't it?

Now, I will tell you that in reactants when bonds are broken we need to supply energy for this bond to break, it needs power. On the other hand when a bond is created, it gives out the energy that is left. As everything is always balanced in equations, the exactly amount of energy that was supplied to break the bonds, is then released when bonds are produced.

So, where the bond enthalpies come in! Right now!

The goal of this topic is to find another way to calculate enthalpy change and this time we are going to consider the enthalpy (enthalpy- tells us how much heat was added or removed from a substance) of each covalent bond present.

The energy needed to break a covalent bond is called a bond enthalpy.

BOND ENTHALPY- enthalpy needed to break a covalent bond into its constituent (parts of a whole) parts.

Bond enthalpy of reactsnts is always positive because the bond is broken and energy absorbed. Endothermic process suggests a '+'.

Bond enthalpy of products is always negative because the bond is produced and energy is released. Exothermic process suggests a '-'.

In the exam, you will always be given bond enthalpy values if they ask you to find enthalpy change. However, what you need to know is that these values will be the average bond enthalpy.

AVERAGE BOND ENTHALPY- average value of the enthalply required to break a given type of covalent bond in the molecule of a gaseous species.

Although different bonds will have different bond enthalpies, each bond's energy needed to break it will also depend on the environment that they are in. As in the image above the O-H bond will have an influence on the C-H near it but not on the C-H bond that is farther away from it. This difference is small enough that in A-level chemistry you only use the average bond enthalpies.

You probably get used to this by now, I've got some examples for you.

e.g. As you can see you are given the equation and bond enthalpy values on the right. Now, you are asked to calculate the enthalpy change. There are four steps that I like to take when doing this type of questions.

1. First of all it is best to draw out all of the molecules. Just go through your equation and draw everything out. It helps because in this way you exactly know how many of each covalent bonds you've got.

2. Then calculate the bond enthalpy of all the reactants. The values will be positive because it is an endothermic process, all bonds are broken and energy is absorbed. Multiply each bond enthalpy by the number of this type of covalent bond.

3. Now do exactly the same thing to all of the products, but this time remember that it is an exothermic process, all bonds are created and energy is released therefore the values are negative. Again multiply them by the number of each type of covalent bond.

4. Next that you've got all of the calculations of products and reactants, you can find the enthalpy change (your final answer). In order to find this you add all of the bonds broken to all of the bonds created. Because your bonds ceated has a negative number you actually take it away from the bonds broken. So, in reality it is bonds broken - bonds created. At the end you can state that the reaction is exothermic because the final answer is negative. And you are DONE!!

I've got one more example to dispel your doubts.

This time, you only get the words and the table and you have to write the equation yourself, but I am sure you can do it. REMEMBER TO BALANCE THE EQUATION! Everything always has to be balanced because you can't create or destroy the elements.

e.g.

1. Draw out all of the molecules. If you are not sure how to draw any of the molecules just write the element of which there is the least, in this example nitrogen and then write the other element around, in this example hydrogen. To make sure, nitrogen can accept three electrons and therefore it can have three covalent bonds. This agrees to the drawn molecule in the image, it has three lines joioning an element to it.

2. Now calculate all broken bonds that are present. Taking time to count all of the bonds that are present. If that makes it easier for you circle the bonds that you already counted. In this way you will never forget to include any of the bonds. And trust me, this happend to me loads of times.

3. Exactly the same with all bonds created. But this time the numbers are negative becaue energy is released.

4. And your last step is to add them together (in theory you actually take them away from each other). Just so you know most of the reactions are exothermic therefore don't be surprised if even all the questions that you do, will have a negative final answer of enthalpy change.

That's all I've got for you today. If you would like to practice more questions, visit my yoututbe channel. Link is at the top of the page. Also, if you would like to a copy of the questions that I do on my youtube channel, email me, comment or leave a message in the box on the main page of this blog.

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!