Mr. Chapman
Chemistry 30
Norquay School
Standard Enthalpies of Formation and Hess’ Law
We know that chemical reactions actually occur in _______, and that by adding up the steps
and enthalpy changes of a particular chemical reaction, we can determine its ______________.
Scientists, however, do not calculate _____________ for all of the reactions in the world. This
would be a huge, unending task that could never be finished.
Instead of doing this, scientists record and use enthalpy changes for only one type of reaction –
______________________________________________________________________________
The following is a description of what it means for a substance to be in its standard state:
For example, in their standard states:
Iron is a
Mercury is a
Oxygen is a diatomic
Examine the following reactions:
Mr. Chapman
Chemistry 30
Norquay School
Definition:
Standard enthalpy of formation:
Exercise:
Predict the product of the following formation reactions, then use the table R-11 on page 975
to determine the ΔH°f of the compound. Also, tell if the formation is exothermic / endothermic.
1. Pb(s) + Cl2(g) 
ΔH°f =
2. Mn(s) + S(s) 
ΔH°f =
3. H2(g) + ½ O2(g) 
ΔH°f =
Why Do We Care About Standard Enthalpies of Formation?
As mentioned before, the enthalpy change of some chemical reactions is impossible to
determine by _________________ (such as measuring temperature change). Using _________,
we can calculate the enthalpy changes in many chemical reactions by adding up the
_______________________________________
On a loose leaf, copy down the following example from the overhead using standard formation
equations to determine an enthalpy change.
** Note: the standard heat of formation of an element in its standard state is zero. Because
of this, we ignore elements in their standard states when calculating an enthalpy change.
Mr. Chapman
Chemistry 30
Norquay School
The stepwise procedure that we just used to calculate the enthalpy change showed how
standard heats of formation can be used to produce the desired equation and its enthalpy
change. The procedure can be summed up using the following formula:
Summation Equation:
H overall
=
H of products
-
H of reactants
After we complete practice problem #35 on page 541 in class, try to find the overall enthalpy
change of the following reactions using the summation equation and the table of standard
heats of formation given on the next page.
C2H4 (g)
+
Cl2
4 NH3 (g)
+
5 O2 (g)
C2H3Cl (g)
4NO (g)
+
+
HCl (g)
H=?
6 H2O (g)
H=?
Mr. Chapman
Chemistry 30
Norquay School
Selected Heats of Formation
** Heats of formation of elements is 0 kJ/mol
Substance
Heat of Formation
H f (kJ/mol)
Substance
Heat of
Formation
H f (kJ/mol)
CO2(g)
- 393.5
HCl ( g )
- 92.3
CO (g)
- 110.5
H2O ( l )
- 285.8
CH4 (g)
- 74.4
H2O ( g )
- 242.0
C2H4 (g)
+52.5
H2O2 ( g )
- 187.8
C2H6 (g)
- 83.8
SO2 ( g )
- 296.8
C3H8 (g)
- 104.7
SO3 ( g )
- 395.7
C4H10 (g)
- 125.6
NO ( g )
+90.2
C8H18 (l)
- 250.1
NH4Cl (s)
- 314.4
CH3OH ( l )
- 239.1
NO2 ( g )
+33.2
C2H5OH ( l )
- 235.2
NH3 (g)
- 45.9
C2H3Cl (g)
+37.3
H2S ( g )
- 20.6
H2SO4 (l)
- 814.0
HNO3 (l)
- 174.1