The values for the molar bond enthalpy for each component in a compound can be found in the table of standard molar enthalpies of formation. This table is a compilation of all known compounds, and it is updated periodically as new compounds are discovered.
By finding the difference between the total molar bond enthalpy of a reaction and the sum of the individual molar bond enthalpies of the products and reactants, you can estimate the value of the reaction standard entropy change, ΔS°rxn.
This value can then be used in the general reaction energy equation, Δh°rxn = ΔE°rxn – TΔS°rxn, to estimate the temperature dependence of a reaction.
Find the enthalpy of formation for each molecule
The next step is to find the enthalpy of formation for each molecule. You can do this by looking it up in a table or online, or calculating it based on the reactant and product molecules.
Enthalpy of formation is the heat absorbed or released when forming a compound from its pure elements. It is sometimes called intrinsic formation energy, and it is measured in kilojoules per mole (kJ/mol).
Online tables will usually list values for both enthalpy of formation and entropy of formation. Entropy of formation is similar to temperature, in that a higher value means the substance changes state more slowly.
Both of these values can be used to estimate the enthalpy of reaction between two compounds. However, you must make sure that the two compounds have the same chemical formula, or are at least close.
Find the standard enthalpy of reaction
Once you have the standard state Gibbs energy of reaction, you can find the molar bond enthalpy of reaction. The standard enthalpy of reaction is the change in Gibbs energy of reaction per molecule of the product.
This means that if you have 1 mol of product, then the standard enthalpy of reaction is the change in Gibbs energy of reaction per mol of product.
You can use this value to estimate the magnitude of Δh°rxn, the heat of reaction. The difference between these two values is how much heat is given off or taken in during the chemical reaction.
The more positive the standard enthalpy of reaction, the more heat is given off during the chemical reaction. The more negative it is, the more heat is taken in. Both are measured in units of Joules per molecule(s) (J/mol).
Divide each Δh°f by the number of moles of gas produced
Now that you have the values for Δh°rxn, you can use this to find the approximate value of Δh°f for the reaction.
You will need to know how many moles of each gas are produced in the reaction, however. You can find this number in the table of molecular weights.
Then, divide each Δh°f by the number of moles of gas produced and you will have your answer!
You should get very close to zero as an answer, which means your answer is accurate! The small difference might be due to rounding errors when calculating some numbers.
Make sure to check your work several times to ensure that you have an accurate value for Δh°rxn.
Take the sum of each Δh°rxn and divide by 2
As mentioned before, the total change in the enthalpy of a reaction is equal to the sum of the changes in enthalpy for each step of the reaction. You can calculate the total change in enthalpy by multiplying the standard temperature and pressure values by the sum of all individual Δh°rxn.
You can use this same method to estimate the value of Δh°rxn for the whole reaction if you have tabulated molar bond enthalpies. By taking the sum of each Δh°rxn and dividing by 2, you will get an approximate value for Δh°rxn for the whole reaction.
This is a very useful thing to know when you do not have access to computers or calculators that can calculate enthalpies. It is also helpful if you are doing chemistry on your own without a lab set-up.
Take the mean value for Δh°rxn
A final point to note is that the standard enthalpy of reaction is usually reported as the change in enthalpy per mole of product produced. However, as we have seen here, the definition can be extended to include mass ratios.
Therefore, you can take the mean value of Δh°rxn by taking the mean value of Δh°rxn over all products. This gives you a more general definition of the standard enthalpy of reaction that can be used with any reaction.
How to Use Enthalpy in Chemical Reactions: Wrap Up
We have covered a lot of material on enthalpy and how it applies to chemical reactions. We began with a basic explanation of what enthalpy is and how it relates to temperature. Then we discussed how to calculate enthalpy and how it relates to other variables in a chemical reaction. Next, we learned how to use enthalpy in chemical reactions and some applications for it. Finally, we reviewed some general tips for using enthalxy in your chemistry work.
