This document discusses standard enthalpies of formation (ΔHf°) and how to calculate the standard enthalpy of reaction (ΔH°) using ΔHf° values. Standard enthalpies of formation are defined as the enthalpy change for the formation of one mole of a compound from its constituent elements in their standard states. ΔHf° values are tabulated at 298 K and 1 atm and can be looked up in tables. The standard enthalpy of reaction can be calculated using the equation ΔH° = ΣΔHf°(products) - ΣΔHf°(reactants). An example reaction of N2O4(g) → 2NO2(g

1. Standard Enthalpies of
Formation and Calculating
Enthalpy of Reaction ()
By Shawn P. Shields, Ph.D.
This work is licensed by Shawn P. Shields-Maxwell under a
Creative Commons Attribution-NonCommercial-ShareAlike

2. Standard Enthalpy of Formation ()
Standard Enthalpies of formation () are
defined as the enthalpy change for the
formation of one mole of a given
compound from its constituent elements
in their standard states.


3. Standard Enthalpy of Formation ()
Examples:
½N2(g) + O2(g) → NO2(g)
Ca(s) + ½O2(g) → CaO(s)
C(graphite) + 2Br2(l) → CBr4(s)


4. Standard Enthalpy of Formation ()
Standard Enthalpies of formation () are
tabulated at 298 K (usually) and 1 atm.
They are compiled in huge tables of
thermodynamic quantities.


5. Examples of Standard Enthalpies of
Formation () in a Table
An example is given below.
Here is the column that provides values (in kJ/mol).


6. Standard Enthalpies of Formation () for
Elements in their Standard States
Standard Enthalpies of formation () for pure
elements in their standard states are assigned
zero. (
Examples:


7. Calculating () Using Std Enthalpies of
Formation ()
We can calculate (i.e., ) from these tabulated
enthalpies of formation ()
For the reaction
aA + bB → cC + dD


8. Calculating () Using Std Enthalpies of
Formation ()
Another way to write the equation:
For the reaction aA + bB → cC + dD

9. Example: Calculate () Using Std Enthalpies
of Formation ()
For the reaction N2O4(g) → 2NO2(g)
The following data was found in the table:
Calculate using these values for the given reaction.


10. Example: Calculate () Using Std Enthalpies
of Formation ()
For the reaction N2O4(g) → 2NO2(g)
The following data was found in the table:
Use the equation:


11. Example: Calculate () Using Std Enthalpies
of Formation ()
For the reaction N2O4(g) → 2NO2(g)
The following data was found in the table:
Fill in the appropriate quantities:
Endo or exothermic?
More examples coming soon…


12. What You Should Be Able to Do (so
far)
Describe what standard enthalpies of formation () are and
how they are acquired.
Describe how standard enthalpies of formation () can be
used to calculate the standard enthalpy of reaction () .
Be able to use standard enthalpies of formation () to
calculate the std enthalpy of reaction (), often called ().
Determine whether enthalpy of reaction () is exo- or
endothermic from the sign of ∆H.
