How do I calculate the standard enthalpy of formation?

Step 1 - Find Enthalpy Equations from the Chemistry Data Booklet

Each of these equations were found using the chemistry data booklet on page 4 and 5.  For example, looking up calcium oxide provides the ΔfHo and then the reactants are those elements that make up calcium oxide.

Step 2 - Rearrange the Enthalpy Equations

In the net enthalpy equation (in the question), calcium oxide and water are both on the reactants side, which means those two equations (from Step 1) would have to be reversed.  The calcium hydroxide is a product in the net enthalpy equation, which is why it needs stay as it is in Step 1.

Step 3 - Add the Enthalpy Equations

Notice that when an equation is reversed, so it its ΔfHo as well.

Step 4 - Sketching the Chemical Potential Energy Diagram

At this point, check to see if this diagram is needed.   In this example, it is not.

Step 5 - Apply Hess' Law

From the previous lesson, you learned that Hess' Law is the following equation:

Notice that the net enthalpy change is equal to the enthalpy of formation for the product minus the enthalpies of formation of the reactants.  Therefore,

Step 6 - Replace the ΔfHo with nΔfHmo

Substituting the definition ΔfHo = nΔfHmo and combining terms results in the following formula, where ΣnΔfPHmo is the standard enthalpy change of all the products (P), and ΣnΔfRHmo is the standard enthalpy change of all the reactants (R).

Therefore, according to Hess’ law and empirically determined molar enthalpies of formation, the standard enthalpy change for the slaking of lime is reported as follows:

Video Example

The video on the right recaps what you have learned in the last 3 lessons.  It then applies them to solve an enthalpy question.   Near the end of the video it covers possible sources of error in calorimeters.

Self Check

Complete “Section 11.5” questions 2 on page 514 of the textbook

In the previous Self-Check activity you used the standard molar enthalpies of formation for compounds to predict the reaction enthalpy. Can this process be reversed?

 

If given a reaction enthalpy, most likely determined by experiment, a standard molar enthalpy of formation for a species in the reaction can be determined as long as the standard molar enthalpies of formation for the other species are known.

 

Self Check 2

Calculate the standard molar enthalpy of formation for hexane, C6H14(l). Assume that hexane’s standard molar enthalpy of combustion is −4162.9 kJ/mol.
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