difference in methods of calculating enthalpy

[Ella Figueroa 2H]

When given a problem and asked to solve for the enthalpy (energy) what tools can you use to determine which method is the best to use? I also wanted to clarify that for every single method the table of pre-determined enthalpy values is necessary.

[Abhay Mahajan 3D]

When given a problem and asked to solve for the enthalpy (energy) what tools can you use to determine which method is the best to use? I also wanted to clarify that for every single method the table of pre-determined enthalpy values is necessary.

There are three main methods we have been taught in order to calculate enthalpy. To determine which method to use, it really depends on how the information is presented. Sure, you may need to be given enthalpy values to answer the problem but it can be presented in different forms.

In the case where you are given the enthalpies for entire reactions, you would use Hess’s method of adding and manipulating the enthalpy values to find the enthalpy of the reaction in question (method 1). When you are given bond enthalpy values and you know which bonds break and are formed within a reaction, you can use those bond enthalpies to determine the enthalpy of the reaction (method 2). Lastly, when given the enthalpies of the products and of the reactants, you can simply subtract the enthalpy of formation of products from that of the reactants to find the enthalpy of the reaction because enthalpy is a state function (method 3).

Hope I answered your question.

[Alexa Le 3F]

Hi, I remember Dr. Lavelle stating in the lecture that we would need to know all of the different methods to calculate enthalpy, and the method we use will be determined by the type of information we are given in the question. The first method we used was called Hess's method, in which we are given the change in enthalpy for two different reactions. We would then try to calculate the total enthalpy change for the combination of the two reactions. This would be used when you are actually given the enthalpy values. The second method is using bond enthalpies to calculate the change in enthalpy of the reaction. In this method, you would need to know that for the reactants, bonds are being broken and energy is required, so it would be a positive value. The products would have bonds formed and energy being released, so it would be a negative value. You would look at the specific bonds that are broken between molecules, and calculate the change in enthalpy from there by looking them up from a table of values. Finally, the 3rd method is utilizing the idea that enthalpy is a state function and can thus be added or subtracted. In this method, you would subtract the enthalpy of product formation and the enthalpy of the reactant formation. Hope that helps!