1.1 What is Chemistry?

http://www.ck12.org Chapter 17. Thermochemistry

When performing enthalpy calculations, a few points need to be remembered:

• ∆Hfvalues are generally given in kJ/mol, so we need to multiply each value by the total number of moles of
  each component in the reaction.


• The standard enthalpy of formation for any element in its most stable form at 25°C is zero.


• The state of the compound must be noted so the correct enthalpy value can be used. For example,∆Hf is
  -285.8 kJ/mol for liquid water and -241.8 kJ/mol for water vapor.


• Keep close track of the signs of each value, since many∆Hfvalues are negative.

The easiest way to explain this process is through an example problem. Let’s consider the energy changes that occur
during the combustion of methane gas:

Example 17.2

Calculate the enthalpy of reaction for the following process:

CH 4 (g)+2O 2 (g)→CO 2 (g)+2H 2 O(l)

You will need to make use of the following data:

• ∆Hffor CH 4 (g) = -74.87 kJ/mol


• ∆Hffor CO 2 (g) = -393.5 kJ/mol


• ∆Hffor H 2 O(l) = -285.8 kJ/mol

Answer:

We are given∆Hfvalues for every reaction component except for O 2 (g). Because O 2 (g) is the standard form of
elemental oxygen, it has a∆Hfvalue of zero. Now use the following equation:

∆Hrxn=Σn∆Hf(products)−Σn∆Hf(reactants)

= (− 393 .5 kJ/mol+ 2 (− 285 .8 kJ/mol))−(− 74 .87 kJ/mol+0 kJ/mol)

=− 965 .1 kJ/mol−(− 74 .87 kJ/mol)

=− 965 .1 kJ/mol+ 74 .87 kJ/mol

=− 890 .3 kJ/mol

The enthalpy change that takes course during this reaction is highly negative, indicating a strongly exothermic
reaction. This is consistent with what we know about the combustion of methane, the primary component in natural
gas.

Lesson Summary

• The first law of thermodynamics tells us that energy can be converted from one form to another but cannot be
  created or destroyed.


• When discussing energy changes for reactions that are run at a constant pressure, it is generally simpler to
  measure and tabulate changes in enthalpy (H), which can be thought of as the heat content of a system at
  constant pressure.


• The standard enthalpy of formation (∆Hf) for a substance is the enthalpy change that would be necessary to
  form that substance from its elements in their standard states at 25°C.


• The enthalpy of a reaction can be calculated from∆Hfvalues for its various components using the following
  equation:∆Hreaction=Σn∆Hproducts−Σn∆Hreactants.