o
 rxn
Enthalpy Change
The heat associated with a chemical reaction performed at constant pressure.
O
… means standard temperature and pressure
Enthalpy of Formation
o
 f
The heat associated with forming one mole of a substance from its elements, each in their standard
states.
Examples:
C( s, graphite)  2H 2( g )  1 CH 4( g )
1
H 2( g )  O2( g )  1 H 2 O( g )
2
1
H 2( g )  O2( g )  1 H 2 O(l )
2
Notice: always 1 as product coefficient.
Notice: particular states of matter are used.
Find values for these in: Appendix II of your textbook and http://webbook.nist.gov/chemistry
Notice: the zeros in the table correspond to the elements in their standard states.
Example: Estimate the enthalpy of formation of Silicon tetrafluoride (SiF4) using only the following
information:
SiH 4 ( g )  4 F2 ( g )  SiF4 ( g )  4 HF ( g )
  2631 kJ / mol
 f ( HF )  273 kJ / mol
 f ( SiH 4 )  34 kJ / mol
Solution: We can combine these reactions to net the desired reaction (using Hess’s Law):
SiH 4 ( g )  4 F2 ( g )  SiF4 ( g )  4 HF ( g )
  2631
1
1


4 HF ( g )  H 2 ( g )  F2 ( g )
2
2


Si ( s)  2 H 2 ( g )  SiH 4 ( g )
  4   273
Si ( s)  2 F2 ( g )  SiF4 ( g )
  1505 kJ / mol
  34
o
Example: Estimating the Enthalpy of reaction,  rxn
How much heat is associated with dissolving 6.00 grams of calcium chloride (CaCl2) in water?
CaCl 2 (s)  Ca
o
o
2

(aq)  2Cl (aq)
o
 rxn   f (products)   f (reactants)
 rxn   542.8  2   167.1   795.4
o
These are heats of formation
found in the Appendix of your
text. Units are kJ/mol
o
 rxn  81.6 kJ / mol
This is the heat released per mole of calcium chloride dissolved. The heat is released because the sign of
H is negative. To finish the question:
6.00 g CaCl 2 
mol CaCl 2
81.6 kJ

 4.41 kJ released
110.98 g CaCl 2 mol CaCl 2
Calorimeters
These are devices that measure heats of reaction.
Use insulated container.
Mix reactants.
Measure temperature change:
Temperature rise  H = - (exothermic)
Temperature fall  H = + (endothermic)
Example: Measuring the heat of a reaction
225 mg of iron (Fe) are combined with enough hydrochloric acid, HCl (aq), to make a solution volume of
100. mL. The temperature rises from 25.00 oC to 25.85 oC as a gas is released. Assuming the density of
the solution is 1.00 g/mL, what is the enthalpy change for the reaction in kJ/mol? The specific heat of
the solution is 4.18 J/goC.
q  (mass)  (C )  T
C is the specific heat of the solution
 4.18 J 
o
q  -100. g    o    0.85 C   355 J  0.355 kJ
g C 



q
 0.355 kJ 55.847 g Fe
o
 rxn 


  88.1 kJ / mol
moles 0.225 g Fe
mol Fe