Enthalpy Calculations

This is where The Mole makes a guest appearance... and then stays!

As we mentioned before, ΔH is the energy change in the reaction in KJ/mol . 

Fun example (not) :

Measurement of enthalpies of combustion.

This is basically a method identical to that alcohol's investigation!

• Known mass of water placed in beaker (150g) 
• Temperature noted (23 ºC) 
• Final temperature noted (43ºC) 
• Mass of ethanol needed to obtain heat increase (0.9g)

Therefore 

Heat gained by water in calorimeter = m x c x DT 

= 150g x 4.2 J g^-1 K^-1 x 20ºC 

= 12,600 J Heat produced by burning 0.9g of ethanol = 12.6kJ

A simple calorimeter.

Since 0.9g is 0.9/46 mol ethanol:

Heat produced by burning 1 mol of ethanol = 12.6 kJ / (109/46) kJ mol^-1= 644kJ
Therefore DH_c [CH₃CH₂OH] = -644kJ mol^-1

^...

^{In simpler terms, it is how much energy is produced or absorbed in a chemical reaction per 1 mole of the substance.}  

^Example:

^{If you had 3 moles of Carbon, and the energy produced per one mole is -478 KJ. Then how much energy is produced in total?}

^{-478 KJ  X   3 moles =  -1434 KJ}

^{1 mole} 

^{Notice how the energy is negative; because it was produced, not absorbed!}

^{Example 2:}

^{If you had 349 g of oxygen, then how much energy is absorbed per one mole of oxygen? (energy from the reaction is +234 KJ/mol)}

^{(gotta do some mole calculations here)}

^{grams ---> moles ---> energy}

^{349 g  X   1 mole    X   234  KJ   =   5104 KJ} 

              ^{16.0  g         1 mole}