2.1 ENTHALPY CHANGE CALCULATION

IF YOU WOULD LIKE A COPY OF PAST PAPER QUESTIONS FOR THIS TOPIC OR ANY OTHER TOPIC PLEASE EMAIL ME (space is provided on the main page of my blog)

IF YOU WOULD LIKE TO GO THROUGH THE QUESTIONS WITH ME, PLEASE VISIT MY YOUTUBE CHANNEL

Hi everyone! Today's post is about enthalpy change calculation. If you wonder what this is, it is the equation you use when you want to find enthalpy change undirectly, for example from an experiment where you measure temperature change of a substance. I am sure you will do these experiments in your class in school and if not you can find them in WJEC Chemistry for AS-level revision book and watch them on youtube. So. once you look through these experiments, you will realise that the aim of these eperiments is to find a temperature change in order to put it into equation q=mcΔT.

As you can see from the equation, using the temperature change from an experiment (for example usign a calorimeter) you can't find enthalpy change straight away. However, you can get there by firstly calculating heat transferred to surroundings. In order to do that you have to measure the mass in cm cubed of the solution (remember to always use the mass of solution, not the solid because solution is the one that is changing the temperature) and the temperature change of the solution ( it doesn't matter if it is Kelvin or Celcius because the change will always be the same). Then, all you need to do it just multiply these two values by specific heat capacity (which is 4.18) to get the heat transferred to the surroundings.

Next, you can calculate the enthalpy change. I told you, you can't get straight to enthalpy change, you need to have two steps. And this is the second one. You take your heat transferred and divide it by number of moles that reacted. There are two very important points that you need to remember here. Firstly, always use the substance that there is less of to calculate the number of moles (n=m/Mr or n=cv) and secondly, always put a minus sign in front of the heat transferred. This is because there is an increase in temperature (solution is heated up) so the reaction is exothermic (heat released) and must have a minus sign in front.

So, after all of this, I want you to know that there are different types of reactions that they may give and ask you to calculate the enthalpy change however, the steps are pretty the same. I have three examples prepared for you with different situations in each. But for now I will show you the steps I always take to get my answer.

There is not much that I can say about the steps but just to show them to you in examples. Here is the first one:

1. Find the temperature change of solution (copper sulphate) after magnesium has been added. Both the initial and final temperatures are provided to you, so just take them away from each other.

2. Calculate heat transferred using mass of the solution which is 100cm cubed of copper sulphate, specific heat capacity and the temperature change from above.

3. Find the number of moles. In this example you are provided with concentration and volume of copper sulphate so you can find number of moles by doing n=cv.

4. You can now find the enthalpy change because you've got all the values you need to use the second equation. Remember about the minus in front!

5. Last step, however this is not always required. Look out in what units the examiner wants you to state the answer. If it is in kJ, you need to do this step as well. (I usually do it anyway just because the number looks much nicer this way).

1. There is nothing you need to do here because the change in temperature is given to you.

2. As before calculate heat transferred. In this example you have two solutions that are added together, so all together the mass is 100cm cubed.

3. We've got the concentration and volume, so moles can be found. Of course you only use concentration and volume of one of the substances. This is why there is 50cm cubed in the equation and because concentration is in mol per dm cubed, you need to divide 50 by 1000 to turn it into dm cubed.

4. Calculate enthalpy change putting a minus in front.

5. Turn it into kJ.

1. This time the temperature fell, so be careful to put a minus in front of the temperature change.

2. Next, calculate heat transferred. It should have a minus in front because the change in temperature was a minus number. Also, don't be tempted to put 7.8 as mass. It is always the mass of solution, so in this example 50cm cubed.

3. n= m/Mr. We have mass and we can find the Mr of ammonium nitrate to find number of moles. So that you know, if you are provided with all the values for either of the equations, calculate both and the smaller number of moles is the one you want to use. The higher number might mean that the substance was in excess.

4. Enthalpy change now. As you probably know by now, you have to put a minus in front of the heat transferred but because it is already a minus we get a plus overall (minus and a minus is a plus).

5. Change it into kJ.

That's all for you now. If you want to practice some more questions, visit my youtube channel and remember to let me know if you want a copy of past paper questions that I put together.

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!