This post is the substance of General Posturasdeyogafaciles.comistry Lecture 23.With this lecture we are going to continue our discussion of Enthalpy and introduce how to calculate it using Heats of Formation and Hess's Law.

More on Enthalpy

According to the previous lecture, Enthalpy is a measure of the heat gained or lost by a system under constant pressure.Additionally, it is a state function, which means that its value depends on the current state of something.Therefore, Enthalpy changes with the starting and ending amounts of heat and has no effect on how the process is done between these two points.That is why we say the value is "independent of course"

Enthalpy's other properties include:

The property Enthalpy is extensive.Amounts of reactants consumed determine the magnitude of *H.When the reactants are doubled, the enthalpy also doubles.

A posturasdeyogafaciles.comical reaction reversed results in the same enthalpy, but in the opposite direction.

An enthalpy change depends on the state of the reactants and products.State (i.e. g, l, s or aq) must be specified.

A problem using an enthalpy value is as follows:

When 4.5 grams of methane are burned in a constant pressure system, what is the amount of heat energy released?

Calculate the moles of methane in 4.5 grams per mole/16 grams each: 0.2815% mole CH4 = 4.5 grams per mole

Multiply the mole quantities by the Enthalpy per mole: 0.28125 * -802 kJ = -225.56 kJ or -2.3e2 kJ

It is important to note that the units on the Enthalpy value in the reaction are only kJ and not kJ/mol.In balance posturasdeyogafaciles.comical reactions, the per mole unit is omitted often but is understood to be there even if it is not written.From the reaction above, you can see that 1 mole of Methane or 2 moles of Oxygen or 1 mole of Carbon Dioxide correspond to the enthalpy value for the reaction.

Enthalpy Values

Enthalpy values for many substances have already been determined experimentally, and are readily available in tables of physical constants.Generally, the values are taken at a "standard state.".All substances in solution are in this state at 1 atm pressure and at a specified temperature, usually 25oC and 1M concentration.

Scientists can compare results based on the thermodynamic standard state

The standard enthalpy of reaction is *HoRxn - enthalpy change at 1 atm and 298.15 K.

In addition to enthalpies, there are many other types:

Physical change's attributes

The Enthalpy of Formation is particularly useful.An enthalpy of formation (*HoF) indicates the heat change that results when a mole of a compound is produced from its elements (in their most stable and natural state at one atmosphere of pressure).In its most stable form, any element has a zero enthalpy of formation.


A *HoF of zero is associated with elements in their most stable and natural elemental forms, while a *HoF value is associated with forms that are not stable or require further processing.


Standard Enthalpy of Reaction

Standard enthalpy of reaction (HORxn) is the result of a reaction occurring at 1 atm.One method of calculating the Enthalpy of Reaction in Calorimetry is already described.Among the other methods we will learn later are:

Standard Heats of Formation vs. Heats of Reaction

Heat of Reaction Calculation using Hess" Law

The calculation of the Heat of Reaction from Standard Heats of Formation is based on these equations:


From the Heats of Formation of each molecule in the reaction, the Heat of Reaction can be calculated.Based on the diagram above, the *HoRxn value is calculated as the sum of the moles of the products times their *HoF values minus the sum of the moles of the reactants times their ΔHoF values.

.The Enthalpy value is a state function.No matter how you get there, it doesn't matter where you start or end.

Here is an example of what you would do when completing such a calculation:


If you combine two or more equations to produce a new equation, you must add the *H's to determine the ΔH for the new equation.

Before we begin, here are some rules:

You have to multiply the number of moles needed by the same number for an equation to find the number of moles required for a reaction.

1) If you turn a reaction around so a molecule is on the correct side for the reaction, you change the sign of the ΔH value for that reaction.

"Which is the target reaction - S (s) + 3/2 O2 (g) + SO3 (g) + H?

Sulfur is located in Equation 1 of both reactions we were given, so we just copy it as is.As you can see, the second equation contains the correct amount of Sulfur Trioxide.Because there are two terms in the equation, we must divide it in half to get it into the correct form:

Addition is the next step after division.Essentially, anything that appears on both sides of the equation can be cancelled in equal amounts:

The formula is: (1 SO2 (g) + 1/2 O2 (g) + 1/3 SO3 (g) H2 = -99.1 kJ

No rewrites were found

Now, you know 3 ways to calculate the heat of a reaction:


The 4th method, using bond energy values, will be taught in about a month, once we have learned how to draw Lewis structures.