Hf° is the change in
enthalpy associated with the formation of a compound from the
elements. Everything is in its standard
stateStandard enthalpies of formation.
Hf° is the change in
enthalpy associated with the formation of a compound from the
elements. Everything is in its standard
state
The heat of formation of NO2 is
From a table of Hf°
values reactions can be put together using Hess's Law.
Determine the H for the following
reaction
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Construct each compound from its elements and the
sum the Hf
°.
The reactions are...
Use the Hf ° reactions
to make the reaction of interest.
Since MgO is a reactant the MgO formtation reaction must be turned around. Since the reaction is turned around the DH must also be turned around; i.e., the additive inverse is used.
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You may have noticed that we do not have to write down all of the reactions.
You may have noticed that the Hf°
of the reactants is always subtracted from the Hf°
of the products.
H =
Hf°prod
- Hf°reactants
Calculate H for the following reaction
using Hf ° values.
construct each compound from its elements and the
sum the Hf°'s
H = Hf°prod
- Hf°reactants
H = 6(-242 kJ) + 2(135.2 kJ) -
2(-75 kJ) - 3(0 kJ) - 2(-46 kJ) = -940 kJ
