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This video. Were gonna talk about how to estimate the enthalpy of a reaction reaction using average bond dissociation energies. So lets work on this question.
Hydrogen gas with chlorine gas to produce hydrochloric acid estimate. The enthalpy of formation of one mole of hcl using the average bond dissociation energies listed in a table below so the first thing you need to do is write a balanced chemical equation. So hydrogen gas reacts with chlorine gas to produce hydrochloric acid.
Now its a balancing we need to put it to however we want to estimate the enthalpy information of one mole of hcl. So therefore we need to put a 1 in front of hydrochloric acid and to balance. The equations we need to put 1 2 in front of hydrogen gas and chlorine now.
What you need to understand is that it takes energy to break a bond. If you want to separate the hydrogen atoms. Within a molecule you need to put energy to break them apart.
And so anytime. Youre breaking a bond. Its an endothermic process.
Energy is absorbed likewise. The reverse is true anytime. Livan is formed.
Its an exothermic process. Energy is released. So it turns out that to calculate the enthalpy of this reaction or at least to estimate.
It its going to be the sum of all the bond energies of the reactants because to get the reaction. Going you need to break the bonds of the reactants now. This is positive because breaking the bond is endothermic and then its li minus the sum of all the bond energies of the products now this is negative.
Because when a bond forms to produce the product energies released. So thats the equation that we have so for the reactants. We have a hydrogen molecule.
So we got to break the hydrogen bond and we also have to break a chlorine bond. Now were going to multiply.
It by the coefficient. So thats just 1 2. Now for the products and we only need to break one hydrochloric acid bond.
Now the energy thats required to break an a hydrogen bond. Its a 432. So this is going to be 1 2.
Times. 432 and to break a chlorine chlorine bond. Its 239 to break an hdl bond.
Its a 427 so all you got to do is plug. These values in your calculator. And youre gonna get the answer so lets see what its going to be.
So the answer that i got for this problem is negative 915. Kilojoules per mole. So thats how you can estimate.
The enthalpy of this reaction. And so thats it for this problem. Now lets work on another example estimate the enthalpy of combustion of methane using the average bond dissociation energies listed below so we need to write the combustion reaction for methane methane is going to react with oxygen gas and in any combustion reaction youre going to get carbon dioxide and water now were going to assume that were dealing with one mole of methane.
Because thats the best number to choose if were not given any other number so therefore we need to put a 1 in front of co2. We have 4 hydrogen atoms on the left. So were gonna put a 2 in front of h2o and everything is not balanced.
Yet. We have a total of 4 oxygen atoms on the right side. So we got to put a 2 in front of it soon.
So. Now. The reaction is balanced so just like before were gonna use the same equation.
The enthalpy of the reaction is going to be the sum of the bond energies are the reactants minus the sum of the bond energies of the products. So thats not going to change now in this problem.
We need to draw the lewis structures of these molecules. So methane. For example looks like this there are 4 carbon hydrogen bonds.
So for the reactant side. Im gonna put. Four ch bonds now oh looks like this this is the lewis structure of oxygen gas.
It contains one bond. But that bond is a double bond and we have two oxygen molecules. So.
This is gonna be two o2 molecules and then minus the bond energies for the reactants carbon dioxide looks like this so within a single co2 molecule there are two carbon oxygen double bonds now water h2o looks like this each water molecule has two o h bonds and theres two water molecules so therefore theres a total of four o h bonds so once you determine the number of bonds that are found in every molecule in this reaction you can now do the calculations. So im gonna be plays ch with the value that corresponds to it and so thats four thirteen. This is going to be four times four thirteen then the oxygen oxygen double bond has an energy value of 495 and then minus the carbon.
Oxygen value which is 7 45 and then the oh h value which is 467 so im gonna perform the calculations one step at a time for this problem so four times four 13 thats 1652 and then 2 times 495. Thats 990 next. We got two times 745.
Which is fourteen ninety and then four times 467. Which is 1868 so if we add 1652 and 990. This is going to be two thousand six hundred and forty two so thats how many kilojoules per mole that we to put in to break all the bonds of the reactants and then 1490 1868 thats 3358 so we have to put in two thousand six hundred and forty two kilojoules per mole of energy into the system in order to break the bonds of all of the reactants and were going to get back 3358 kilojoules per mole of energy as the bonds of the products continue to form and so the net amount that were getting from the system is negative seven hundred and sixteen kilojoules per mole and so thats how you can estimate the enthalpy of a reaction using the average bond dissociation energies lets work on one more example estimate the enthalpy of the reaction when one mole of methane reacts with chlorine gas to produce carbon tetrachloride and hydrochloric acid.
So lets write. A reaction. So we have methane reacting with cl.
And thats kind of produce ccl. 4. And hcl.
So were dealing with only one mole of methane to balance the reaction. We need to put a 2 actually rather well first we need to put a 4 in front of hcl to have 4. Hydrogen atoms.
Now we have a total of 8 chlorine. Atoms on the right side.
So we need a 4 in front of cl. 2. Now.
The reaction is balanced. So now we can ask them. 8.
The enthalpy of the reaction using the fact that the enthalpy of reaction is the sum of all the bond energies of the reactants minus. The sum of all the bond energies of the products. So on a reactant side of the equation.
We have methane. Which consists of 4 carbon hydrogen. Bonds.
And chlorine has one single bar. Between the two chlorine atoms and we have four chlorine atoms on the left side. Now for the products.
We have four carbon chlorine bonds and also for hydrogen chlorine bonds. So now just plug in the values. So lets replace eh with four hundred and thirteen and the chlorine chlorine bond is 239.
And then the carbon chlorine bond. Thats 339 and the hydrogen chlorine bond is 427. So just like the last problem.
Were going to do this one step at a time so four times four 13. Thats 1652 and four times. 239 is 956 four times three 39.
Its 1356 and four times four 27. Thats 1708 so 1652 plus nine 56. Thats 2608 and 13 56.
Plus 1708 his 3064 so in this reaction. It takes this amount of energy to break all of the bonds of the reactants and then when the bonds of the products are produced 3064 kilojoules of heat energy per mole will be released. So the difference between those two values will give us the enthalpy of the reaction.
Which is negative four hundred and fifty six kilojoules per mole and thats it for this video. .
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