Enthalpy Calculations

Well, in order to do enthalpy calculations, you should know what enthalpy is. Enthalpy is the measure of total energy in a system. This symbol (ΔH), indicates the change in the reaction in KJ/mole. That indicates the change in potential energy during the reaction. To calculate that you use this formula: (energy of products - energy of reactants).

So... this lesson is kind of hard to explain unless with an example. So.. let's start with an exampleee =)

*When the KJ is on the right side of an equation, it becomes a negative number as it is an exothermic reaction. Vice versa, when the KJ is on the left side, it is positive as the reaction is endothermic.

Example 1 /

2 NO2(g) + O2(g) --> 2 NO2 + 131 KJ

well... we can see that in this reaction, the KJ/moles can be expressed in these ways :

- 131 KJ       or  -131KJ___     which can reduce to  -65.5 KJ__     
  1 mol O2         2 mol NO2                                      1 mol NO2                                                                         
    and again... -65.5 KJ__
                       1 mol NO2             

Now that you know these, it will be much easier to calculate the energy release, and how many moles needed to produce a certain amount of energy.

1. Ex/ Calculate the energy released when 0.55 moles of NO2 is produced.

So... back to mole conversions...again =.=" We set up the equation...

0.55 moles NO2   x   -65.5 KJ__    =   -36KJ  
                               1 mol NO2   


So.. remember to do your sig figs, and because it is a negative number, it is releasing energy, thus it is a exothermic reaction.

2. Ex/ How many moles of O2 are needed to produce 1523KJ of energy?

1523 KJ   x  1 mol O2   =  - 11.63 moles = 11.63 moles O2
                    -131 KJ

* Moles cannot be a negative number, so when this case repeats itself, drop the negative sign.

SEE! isn't it easy? Yay .... well ya think u so smart? Why don'tcha try doing these practice problems first:

http://www.docstoc.com/docs/19490471/Enthalpy-Worksheet
http://spiepho.sbc.edu/worksheets/ ( click on the 2nd Chapter 6)

Well...If you still need help, i guess you can watch these videos:









YAY. all is well? :) ok good good...good stuff x)