Bond dissociation energy, also referred to as bond-breaking energy, is the amount of energy required to break a single bond between two atoms. This concept is important in chemistry as it dictates how atoms arrange themselves into compounds and molecules.
When discussing bonds, usually the term bond strength is used. This is the amount of force that it takes to pull two atoms apart from each other. The difference between these two terms is that bond strength refers to the pulling force only, not the amount of energy required to do so.
Bond dissociation energies can be calculated using various theories and equations in chemistry. These calculations vary depending on the element that bonds are being broken within, but most of them use at least one of three basic theories: electron affinity theory, lattice theory, and thermal theory.
This article will discuss how to calculate the bond dissociation energy for breaking all of the bonds in one mole of methane, CH4.
Calculate the bond dissociation energy for H-H bonds
In order to calculate the bond dissociation energy for hydrogen bonds, you must first understand what a hydrogen bond is. A hydrogen bond is a weak chemical bond between hydrogen atoms that are attached to separate molecules.
These molecules are called ions, and the separating molecule is named the solvent. Solvents can be gases, liquids, or even crystals. Ion pairs are formed when a solvent molecule accepts a single electron from a different molecule. This makes it into an ion and creates a charge difference between the two molecules.
Because of this charge difference, the two ions can attract each other and form a weak chemical bond. This bond is referred to as a hydrogen bond due to the presence of a hydrogen atom in the solvent molecule and the attachment of one on the ion pair forming it.
To calculate the dissociation energy of all of these bonds, you must use an equation that takes into account all of these individual bonds.
Calculate the bond dissociation energy for C-C bonds
Now that you can calculate the total bond dissociation energy for carbon, you can do the same for all the other atoms! You can also apply this to ionic bonds, not just covalent ones.
Bond dissociation energy is the amount of energy required to break a bond between two atoms. This is specific to each bond, depending on the type of atom pairs.
The difference in bond strength between carbon and hydrogen bonds is small, only about 2-3 kJ/mol. This makes calculating the total dissociation energy for CH4 very close compared to CD2H6, so it is worth calculating both values to get an average value.
The Bond Dissociation Energy (BE) for one mole of CH4 (100g) is approximately: 65 kJ/mol.
Add up all of the bond dissociation energies
So, now you have all of your values for the bond dissociation energies. You calculated the bond dissociation energy for each of the four bonds in methane, CH4. Now you need to add all of these values up to get the total bond dissociation energy for methane, CH4.
The total bond dissociation energy is 52 kcal/mol. This means that it would take about 52 kilocalories to break apart one mole of methane, CH4. That is a pretty large number!
This number is very useful when thinking about how much heat is released when methane oxidizes. When one mole of methane oxidizes, one mole of oxygen is produced. There are four bonds broken in this process, so the average bond dissociation energy for oxygen is 52 kcal/mol as well.
Divide by the number of bonds broken
The final step in calculating the bond dissociation energy is to divide the total energy by the number of bonds broken in methane. You calculated this above, but we will repeat it here.
Total bond dissociation energy = 2 × 8.8 kcal/mol – (4 × 2) = 6.4 kcal/mol
There you have it! The bond dissociation energy for the breaking of all the bonds in a mole of methane is 6.4 kilocalories per mole. Now you know how much energy is required to break all the C-H bonds in one molecule of methane!
It is important to note that this number does not represent the exact amount of energy released when methane reacts with something else. It only represents the amount of energy required to break all the C-H bonds in methane, which is what this question asked you to find.
Get an average bond dissociation energy
Once you have your individual bond dissociation energies, you need to calculate the average bond dissociation energy for all of the bonds in a molecule. You do this by calculating the mean, or average, of all the individual bond dissociation energies.
To illustrate this concept, imagine you had ten friends and you calculated how much money it would take for each friend to buy you a present. The range of how much each friend would spend on you would be from $100 to $1,000. To get an average gift price, you would simply calculate the mean, or average, of all of those numbers.
The same concept applies to atomic bonds: to get an average bond dissociation energy, you calculate the mean of all of the individual ones. This will give you a more accurate picture of how many bonds will break when calculating thermal displacement.
Use this average to calculate the breaking of a methane molecule
Next, you need to calculate the average bond dissociation energy of a single methane molecule. To do this, you need to find the average of the potentials of all the possible configurations of a single methane molecule.
You can do this by using linear combinations, also known as combination derivatives. Combination derivatives are linear combinations of the potentials that have zero as the value for one of the variables and one for all of the other variables.
There are two combination derivatives you need to find for methane: one for all four potentials with zero as the value for R-hat and one for all four potentials with V as the value for R-hat. Then, you average these two to get your answer!
Calculate these values and you have your average dissociation energy per bond in methane.
Use this process to break down other molecules with multiple bonds
Once you have learned how to calculate the bond dissociation energy for single, double, and triple bonds, you can use this knowledge to break down any molecule with multiple bonds.
First, find the total bond energy by adding up the single, double, and triple bond energies. Then divide this number by the number of bonds the molecule has to get an average bond energy per bond.
Then you can use the same steps as above to calculate the average potential energy per atom and calculate the total potential energy of dissociation. Divide one from the other to get your fraction of atoms that will dissociate.
This is just one way to learn how to calculate chemical dissociation energies.
Consider how temperature affects this process
When thermodynamics comes into play, the process of dissociation becomes more complicated. The reason for this is that temperature affects the rate at which all processes occur.
At room temperature, the bonds in methane are very strong, so it takes a significant amount of energy to separate the atoms that make up one molecule of methane. However, as you increase the temperature, the molecules begin to vibrate more rapidly.
These vibrations make it easier to break down the bonds between the atoms that make up methane, so it dissociates more quickly. At higher temperatures, the average kinetic energy of the molecules also increases. This also helps break down the bonds between atoms in methane.
So as temperature increases, dissociation occurs more rapidly.
Leave a Reply