Section 15.4 Calculating Enthalpy Change
• Apply Hess’s law to calculate the enthalpy change for
a reaction.
• Explain the basis for the table of standard enthalpies
of formation.
• Calculate ∆Hrxn using thermochemical equations.
• Determine the enthalpy change for a reaction using
standard enthalpies of formation data.
Hess's Law
• Hess’s law states that if you can add two or more
thermochemical equations to produce a final
equation for a reaction, then the sum of the
enthalpy changes for the individual reactions is the
enthalpy change for the final reaction.
• Sometimes it is impossible or impractical to
measure the ∆H of a reaction by using a
calorimeter, so Hess’s law allows you to
calculate unknown ∆H values using known
reactions and their experimentally determined
∆H values
Hess's Law (cont.)
Standard Enthalpy (Heat) of Formation
• The standard enthalpy (heat) of formation
is defined as the change in enthalpy that
accompanies the formation of one mole of
the compound in its standard state from its
elements in their standard states.
• The standard state of a substance means
the normal physical state of the
substance at 1 atm and 298 K (25°C).
For example, in their standard states, iron
is a solid, mercury is a liquid, and oxygen
is a diatomic gas.
• Example: A typical standard heat of
formation reaction is the formation of one
mole of SO3 from its elements.
• S(s) + 3 /2 O 2 (g) → S O 3 (g) ∆ Hf°=396kJ The product of this equation is SO3
Standard Enthalpy (Heat) of Formation (cont.)
• Elements in their standard states have a
• The formation of compounds are placed
above or below elements in their standard
states.
• Standard enthalpies of formation of many
compounds have been measured
experimentally. For example, consider the
equation for the formation of nitrogen dioxide.
1 /2N2(g) +O2(g) → NO2(g) ∆Hf° = +33.2 kJ
The elements nitrogen and oxygen are
diatomic gases in their standard states, so
their standard enthalpies of formation are
zero. When nitrogen and oxygen gases react
to form one mole of nitrogen dioxide, the
experimentally determined ∆H for the reaction
is +33.2 kJ.
• That means that 33.2 kJ of energy is
absorbed in this endothermic reaction. The
energy content of the product NO2 is
33.2 kJ greater than the energy content of
the reactants. On a scale on which ∆H f°
of reactants is 0.0 kJ, ∆ Hf° of NO2 is
+33.2 kJ.
Standard Enthalpy (Heat) of Formation (cont.)
Standard Enthalpy (Heat) of Formation (cont.)
• Standard enthalpies of formation can be
used to calculate the enthalpies for many
reactions under standard conditions by
using Hess’s law.
• The summation equation
Standard Enthalpy (Heat) of Formation (cont.)
Q1/Explain what is meant by Hess’s law
and how it is used to determine ∆ H°.
Hess’s law says that if two or more
equations add up to an overall equation,
the ∆H°rxn of the overall equation is the
sum of the ∆H° values of the equations
that were combined
• Q2/Describe how the elements in their
standard states are defined on the scale of
standard enthalpies of formations.
• They are assigned enthalpies of formation
of zero.
Section 15.4 Assessment
What is the enthalpy of oxygen in its
standard state?
A. 0.00 kJ
B. 15.99 kJ
C. 100.0 kJ
D. 8.00 kJ
A.
B.
C.
D.
A
B
C
D
Section 15.4 Assessment
Two or more thermochemical reactions
can be summed to determine the overall
enthalpy changes based on what law?
A. Boyle’s law
B. Hess’s law
C. Gay-Lussac’s law
D. law of conservation of energy
A.
B.
C.
D.
A
B
C
D
The standard enthalpy of an element in its
standard state is ____.
A. 0.00 calories
B. 0.00°C
C. 0.00 kilocalories
D. 0.00 kJ
A.
B.
C.
D.
A
B
C
D
End of sec 15.4