T - Enthalpy and Thermochemical Equations (Lesson)

Enthalpy and Thermochemical Equations

Enthalpy

In the previous lesson, it was shown that the total internal energy for a system can be considered the sum of both heat and work exchanged between the system and the surroundings. Because most reactions take place in open containers where no pressure build up is possible (or where no work can be done), the energy exchanged is primarily in the form of heat. When considering chemical reactions, the term enthalpy (H) is often used to describe the total heat content of a system. Not to be confused with q, which is the amount of heat exchanged between the system and surroundings, H is the total heat content. As it turns out this value cannot be known; however, the enthalpy change for a system can be experimentally determined:

LaTeX: \Delta H=H_{final}-H_{initial}ΔH=HfinalHinitial

This thermodynamic parameter follows the same sign conventions as discussed previously. If a chemical reaction gives off heat it is referred to as exothermic and is noted with a negative sign (LaTeX: -\DeltaΔH). Conversely, if a chemical reaction absorbs heat, then it is referred to as endothermic and is noted with a positive sign (LaTeX: +\Delta+ΔH).  Take for example the thermite reaction illustrated below:

As can be imagined from the visible flames being produced, this is a very exothermic process. To help illustrate the changes in enthalpy associated with chemical processes, it is common to rely upon a thermochemical equation to show both the substances involved in the chemical reaction as well as the magnitude of the energy change in the form of a LaTeX: \DeltaΔH value. The thermochemical reaction for the thermite reaction is shown below as an example; this reaction releases so much heat that the elemental iron that is produced is converted into molten metal.

Fe2O3 (s) + 2 Al (s)  LaTeX: \longrightarrow  Al2O3 (s) + 2 Fe (s)    LaTeX: \DeltaΔH = -851.5 kJ

Energy Diagrams

In addition to the LaTeX: \DeltaΔH value being displayed in the thermochemical equation, it is also helpful to see a visual representation of this energy change. This is precisely what energy diagrams do. An energy diagram for the thermite reaction is shown below:

Notice a few things regarding this diagram. First, it is noticeable that there are no units along the y-axis. As discussed previously, it is not possible to know the exact enthalpy content of a substance; rather, enthalpy changes are the only measurable quantities. Therefore, the enthalpy content of the reactants and products is unknown, but it is possible to determine that 851.5 kJ of energy was released from the system into the surroundings. This is also evidenced by showing that the system has decreased in energy content as shown by the lower energy level of the products compared to the reactants. Secondly, it should be noted that the x-axis is labeled "reaction coordinate." This is not to be confused with time, but it should be thought of as the degree to which the reaction has progressed.

An additional energy diagram (below) has been drawn for a process that is endothermic as opposed to the exothermic thermite reaction. The reaction described is:

6 CO2 (g) + 6 H2O (g)  LaTeX: \longrightarrow  C6H12O6 (s) + 6 O2 (g)

Notice on this diagram that the products have an increased amount of energy compared to the reactants indicating that energy is entering the system from the surroundings. Additionally, the LaTeX: \DeltaΔH value is positive indicating heat was absorbed by the system. 

Enthalpy as a Stoichiometric Quantity

Returning once again to the thermite reaction, it can also be seen that this equation provides additional information other than simply informing about the quantity of energy released during this process. The change in enthalpy value can actually be used to write relationships between individual reaction components and the energy change, much in the same way that mole-to-mole ratios are used to create relationships between two different substances. Using the thermite equation, the following relationships can be established

Fe2O3 (s) + 2 Al (s)  LaTeX: \longrightarrow  Al2O3 (s) + 2 Fe (s)    LaTeX: \DeltaΔH = -851.5 kJ

  • The reaction of 1 mole of Fe2O3 will result in the release of 851.5 kJ of energy
  • The reaction of 2 moles of Al will result in the release of 851.5 kJ of energy
  • The production of 1 moles of Al2O3 is accompanied by a release of 851.5 kJ of energy
  • The production of 2 moles of Fe is accompanied by a release of 851.5 kJ of energy

But what happens if these precise amounts of substances are not used/produced?  As an example, how can the amount of energy released to the surroundings be determined beginning with 15.3 g of Al instead of 2 moles? Again, we can use the enthalpy value from the thermochemical equation as a conversion factor:

LaTeX: 15.3gAl(\frac{1 mol Al}{26.98 g Al})(\frac{-851.5 kJ}{2 mol Al})=-241kJ15.3gAl(1molAl26.98gAl)(851.5kJ2molAl)=241kJ

Notice how the energy/mole relationship was used which related the 2 moles of Al from the balanced equation to the -851.5 kJ of enthalpy change.

You Try It!

In the following self-assessment activity, complete enthalpy and thermochemical equation problems. Click on the plus sign to check your answer!

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION