Enthalpy and Energy Calculations

remember the MOLE map?



Click to enlarge

 Well, we are going to add something to it.

=  KJ   

 1mol 

------------------------------->

MOLES                 ENERGY

<-------------------------------------

= 1mol

  KJ

So what is Enthalpy?

It is the total heat, or ENERGY contained in the system.

And what is ΔH?

It is the energy change of the reaction, expressed in kilojoules (KJ) per mole of one chemical.

For example:

4Na + 1O2  → 2Na2O + 520KJ

Calculate the energy released when 0.41mol of Na2O is produced.

*Think back to the mole map. Incorporate the MOLES ↔ ENERGY formula into the mole map.

To find the energy, we use the formula  KJ/1mol  because the mole is given.

                                                             

0.41mol Na2O X     -520KJ     =  -106.6KJ  =  110KJ energy released

                              2mol Na2O                             or  -110KJ

                                                            *DON'T FORGET SIG FIGS!

Now calculate how many grams of O2 would be needed to produce 1100KJ of energy.

We are now calculating  from energy to moles to grams.

formula:  Energy X 1mol   X  MMg

                                   KJ         1mol

-1100KJ  X  1mol O2   =  2.1mol O2 

                          -520KJ

2.1mol O2 X     16g     =  33.6g  =  34g O2

                     1mol O2 

34g O2 would be needed to produce 1100KJ of energy.