2.1 ENTHALPY CHANGE

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Hi, we are going to start with thermochemistry for this unit. There is some kind of change in every reaction and thermochemistry is the study of energy change that happens in a reaction. I think today, we are going to start with some very important definitions, whithout whose this unit would be difficult to understand. So, let's get staright to the information:

The little 'triangle' shows a change in condition which for this unit will be energy (heat energy).

'H' represents the stored energy (for this unit it will be heat) content of a system.

'θ' show that standard conditions are being used

When we combine the triangle and H we get enthalpy change ( change in heat content).

You can't measure the change in enthalphy directly, so as when you find a change in something, you take your final answer and take it away from the initial answer. Thi is exactly what happens here. You take the heat content of products and take away the heat content of reactants to get the change in heat content.

Also, when we talk about standard conditions, this is what it means:

- 298K ( 25 degrees Celcius);

- 1atm (101000Pa)

- standard states of elements

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In this unit energy changes are in form of heat and this can be classified as either exothermic or endothermic.

EXOTHERMIC --> in this type, heat energy is given out to the surroundings.

This means that there is more heat energy at the beginning than there is at the end because some of the energy from the reaction goes away to the surroundings. If we just go with out logic and put some random numbers in, to just visualise it, we can very easily remember the pattern. As we know from before, in order to calculate the enthalpy change, we have to do products - reactants. The heat content at the end is lower than at the beginning, so if you put in numbers to visualise it, you get 6-9= -3. A bigger number is taken away from a smaller number and we get a negative number. This is exactly how exothermic reactions work. H(products) < H(reactants), so the change in H= negative.

The diagram shows what I already discussed but as a sketch.

e.g. the example I put shows you how to draw a diagram and how to describe everything that is going on in the reaction, on this diagram.

ENDOTHERMIC --> in this type of reaction, heat energy is absorbed.

This is the exact opposite as to exothermic reaction. In this instance there is more energy at the end than there is at the begninnig because it is being absorbed from the surroundings. As before we can use numbers to visualise our point, so 9-6= 3. This time however, we have greater heat content at the end (products) and smaller heat content at the beginning (reactants) and we get a positive number. This is eaxctly what happens in endothermic reactions, you get a positive enthalpy change. H(products) > H(reactants), therefore, change in H= '+'. Again the diagram is there to just show you what is happening in the reaction.

e.g. I drew everything on the diagram for you to understand and learn how to draw these types of diagrams, when they ask you about it in the exam.

I do really hope that this makes sense. It may seem slightly confusing but don't overthink it. Just try to put in easy numbers and you will quickly understand why exothermic is negative and endothermic is positive. Just a quick tip at the end to remember the terms. 'Exo' similar to exit (exit of heat) and then you instantly know the other one must be absorbtion.

As we talk about all this heat energy being absorbed and given away, you must know that energy cannot be ever created or destroyed and this is called conservation of energy.

CONSERVATION OF ENERGY --> energy cannot be created or destroyed, it can only be transferred from one form to another.

So, in exothermic reactions, heat energy leaves the system and enters the surroundings. Similarly, in endothermic reactions, heat energy leaves surrounding and enters the system. The energy is not destroyed neither created in these reactions, it is just transferred from one place to another.

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I have faith that you are keeping up with me. The next part that we are going to have a look at is something I culdn't understand for a long time, but now that I do, I hope I can pass it onto you in a way that you will understand as well. And what I mean are standard enthalpy changes. Fortunately for us, there are only three enthalpy changes that you have to be familiar with. Let's start with the first one:

STANDARD ENTHALPY CHANGE OF FORMATION --> it is change in energy when one mole of a substance is formed from its constituent (parts of a whole) elements.

What this means is you put the product on the right hand side of the euqation and then you write all the 'constituent' parts that make this product on the left hand side (reactants). When you do that and you are sure there is only one mole of a substance as product, the change in energy content between the beginning and the end of reaction is the enthalpy change of formation.

An example that I found for you is with magnesium oxide. We need to add magnesium and oxygen to make magnesium oxide but what you need to remember is to always have one mole of product, so only half of oxygen is needed (can't just put O without the little two in reactants, because oxygen always goes in pairs).

What's also important to know is that all of the elements that are in their standard state have an enthalpy change of formation of 0, as in the example with hydrogen that I put on the image above.

STANDARD ENTHALPY CHANGE OF COMBUSTION--> this is the enthalpy when one mole of a substance is completely combusted in oxygen under standard conditions.

All this sentence means is that whatever substance they give, oxygen is added to it as it would during a combustion and then carbon dioxide and water is produced. That is literally all that you have to know about the enthalpy change of combustion. Of course, you need to be able to write the equation down and balance it properly. And if you don't know to balance equations, visit my other post where I explain it.

As always I've got an example for you using ethene. You put ethene as your reactant (remeber only one mole) together with oxygen (remember they go in pairs) and it forms carbon dioxide and water. Next, you need to balance the equation, keeping in mind that there can only be one mole of ethene.

And the last but not least,

STANDARD ENTHALPY CHANGE OF REACTION-->this enthalpy is the one in which number of moles of reactants react as specified in the equation.

You don't need to make sure that the number of moles is one in this instance but you have to know that the entahlpy chnage of reaction changes as the number of moles changes, so be careful not to mix them.

In order to calculate the enthalpy change of reaction, you require enthalpy change of formation of each substance (of course the ones that are in their standard states are 0). All you want to do now, is take the enthalpy chage of formation of products and take away the enthalpy change of formation of reactants from it to get the enthalpy change of reaction. Anyways, you've got this equation written down on the picture above.

Don't worry, if this question comes up on your exam, you will be provided either with all the enthalpy changes of formation and you will have to find the enthalpy change of reaction, OR you will have the enthalpy change of formation and all but one enthalpy change of formation and you will be asked to find that missing one.

No examples here, but if you want some, go to my youtube channel where I post a video with past paper questions for every single topic that I do. This is all for this topic, I hope you enjoyed or at least got through it :)

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!