Why does the calculated values of enthalpy change different from the reference values? Suggest two reasons.
Average bond enthalpies (note the word "average") are calculated in a different way from formation enthalpies. Formation enthalpies are well-defined and precise, meaning that two different people could measure them the same way and get the same answer. Average bond enthalpies are averages over many different types of bonds.
For example, there is only one way to make CH4
, but there are many different molecules with C−H bonds. The enthalpy of dissociation of a C−H bond in CH4 is different from that in C2H6, and in C6H6, and really in any other molecule you can think of. To find average bond enthalpies, a selection of a large number of those bonds in many different molecules is taken, and the enthalpy of dissociation for each is measured. Then, the average (mean) is recorded. If you look at some different charts of bond enthalpies, you will notice two things: First, there are some numbers marked "exact" - H−H, Cl−Cl, F−F
, for example. This is because there is only one molecule with each of those types of bonds. The other thing you will notice is that depending on which chart you are using, many of the numbers will be slightly different. This is because there is no universal, unchanging standard describing which molecules are used to determine each bond - it depends upon what the people making the chart decided to use.
Because of this difference, when making predictions, average bond enthalpies are less accurate than formation enthalpies. In practice, they are used for quick estimations, or as a last resort when formation enthalpies are not available.
When I taught chemistry, I would have my students measure reaction enthalpies in lab, then calculate them using formation enthalpy tables and average bond dissociation enthalpy tables. The goal was to show them that average bond enthalpies can be reasonably close for rough estimates, but that formation enthalpies are much better in practice. It also showed them that accurately measuring reaction enthalpy in the lab is difficult!
Well, let me put on my clown nose and try to bring some humor to this question!
1. Maybe the calculated values of enthalpy heard a really funny joke and just couldn't keep it together, causing them to change uncontrollably. "Enthalpy: Can't stop laughing!"
2. Or, perhaps the calculated values of enthalpy are just rebellious souls who refuse to follow the rules and standards set by the reference values. They like to do their own thing, you know? "Enthalpy: The rebellious renegade!"
Remember, these are just light-hearted suggestions to provide a little laughter. The real reasons for differences between calculated and reference values of enthalpy could be due to errors in measurements, approximation methods, or differences in experimental conditions.
The calculated values of enthalpy may differ from the reference values for two main reasons:
1. Idealized assumptions: Calculated values of enthalpy are often based on idealized assumptions and simplified models that may not fully capture the complexities of the real system. These assumptions can lead to deviations between the calculated values and the reference values, which are typically measured or obtained from more accurate experimental data.
2. Approximations and limitations: The calculations used to determine enthalpy values often involve approximations and assumptions due to mathematical constraints or lack of complete information. These approximations can introduce errors or uncertainties, leading to disparities between the calculated and reference values. Additionally, physical limitations of the measuring equipment or laboratory conditions can also contribute to variations between the calculated and reference values.
The calculated values of enthalpy may differ from the reference values due to a few reasons. Here are two common explanations for this discrepancy:
1. approximations and simplifications: Enthalpy calculations often involve making certain assumptions and approximations to simplify the calculations. These simplifications can introduce errors and lead to discrepancies between the calculated and reference values. For example, assumptions such as ideal gas behavior or neglecting certain effects like non-ideal mixing can affect the accuracy of the calculated enthalpy values.
2. Experimental limitations: Reference values of enthalpy are typically obtained through experimental measurements. However, experimental techniques can have inherent limitations, such as measurement errors, uncertainties in the reference data, or incomplete data sets. These limitations can contribute to the discrepancies between the calculated and reference enthalpy values.
It is important to note that the accuracy of calculated enthalpy values depends on the quality of the thermodynamic models used and the input data provided. Improving these aspects and minimizing assumptions can help reduce the differences between calculated and reference values of enthalpy.