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AP®︎/College Chemistry
Course: AP®︎/College Chemistry > Unit 6
Lesson 8: Bond enthalpiesBond enthalpies
The enthalpy of a bond is the enthalpy change that occurs when 1 mole of a particular bond is broken in the gas phase. Since energy is required to break a chemical bond, bond enthalpies are always reported as positive values. For any chemical reaction, the estimated change in enthalpy is the sum of the bond enthalpies of the bonds broken minus the sum of the bond enthalpies of the bonds formed. Created by Jay.
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- At, why would the bond enthalpies be different? Does it have anything to do with bond hybridization? 2:19(1 vote)
- Bond enthalpies are essentially a measure of how strong a covalent bond is. The higher the bond enthalpy the more energy is required to break that bond and hence the stronger the bond. Carbon-carbon double bonds are stronger than carbon-carbon single bonds and therefore have higher bond enthalpy values.
Bond hybridization describes more the geometry around the central atom and the distance between the bonding atoms. But double bonds are stronger than single bonds mostly because there is more orbital overlap and electron density between the two atoms in double bonds compared to single bonds. Greater orbital overlap creates stronger chemical bonds.
Hope that helps.(3 votes)
- Isn't the equation sum of products - sum of reactants? Why is it flipped in the video. I thought bonds broken = reactants and bonds made = products. I am very confused.(2 votes)
- Atshouldn't the bond enthalpies of bonds formed be negative? 6:54(2 votes)
- Jay subtracts the enthalpies of the formed bonds from the enthalpies the broken bonds at. He just didn't do the do all the calculations in one step. 7:09(1 vote)
- Hey bro bro bro so uhm like that uhm so I cannot understand why bond enthalpies of bond formed is positive number. You said that the forming the bond releases energy bro. It is kind of little bit confusing bro bro man man bro man bro. Thank you bro!(0 votes)
- Bond enthalpy is defined as the amount of energy required to break a certain bond. Since breaking bonds requires an input of energy, the sign is positive.
We can use bond enthalpies do determine the change in enthalpy of an entire reaction. We use the positive bond enthalpy values for the reactants, since they are having their bonds broken, and negative bond enthalpy values, since they are forming bonds which now releases energy.
Hope that helps.(2 votes)
Video transcript
- [Instructor] Bond enthalpy
is the change in enthalpy or delta H for breaking a particular bond in one mole of a gaseous substance. If we think about the
diatomic chlorine molecule, so Cl2, down here is a
little picture of Cl2, each of the green spheres
is a chlorine atom and they're bonded together
by a single covalent bond. It would take energy to break this bond in diatomic chlorine gas and turn diatomic chlorine gas, Cl2 in to two individual chlorine atoms. So we're going from Cl2 in
the gaseous state to 2Cl. Bond enthalpy can be
symbolized by the letters BE. So the bond enthalpy of the
chlorine-chlorine single bond is equal to +242 kilojoules per mole. And what this means is, if we have one mole of
chlorine-chlorine bonds, it takes +242 kilojoules of
energy to break those bonds. Bond enthalpies are always positive because it takes energy to break bonds. Another name for bond enthalpy
is bond dissociation energy. So you might see this symbolized as BDE or just simply the letter D. Bond enthalpies are often found in the appendices of chemistry textbooks. For example, we just saw the
chlorine-chlorine single bond, the bond enthalpy is
242 kilojoules per mole. Whereas to break a
carbon-carbon single bond takes 348 kilojoules of energy per mole of carbon-carbon single bonds. A carbon-carbon double
bond has a bond enthalpy of 614 kilojoules per mole. Since the carbon-carbon
double bond is stronger than a carbon-carbon single bond, it takes more energy to
break the double bond. And that's why the
carbon-carbon double bond has a high higher bond enthalpy. So the higher the value
for the bond enthalpy, the stronger the bond. Notice that these are
average bond enthalpies. So the average bond enthalpy for a carbon-carbon single bond is around 348 kilojoules per mole. You might see slightly
different values for this, depending on which
textbook you're looking in, but they're all pretty
close to the same value. The reason why these are
average bond enthalpies is because if we look at two
different molecules down here, this is ethane on the left
and propane on the right, if we break a carbon-carbon
single bond in ethane, the bond enthalpy is slightly different from breaking a carbon-carbon
single bond in propane. And that's why we use
average bond enthalpies. We've already seen that it
takes energy to break bonds. So to break the
chlorine-chlorine single bond in diatomic chlorine gas takes
+242 kilojoules per mole. If it takes energy to break bonds, that means energy is
given off when bonds form. So when two individual chlorine
gas atoms come together to form a chlorine-chlorine bond, so let's go into highlight that in here. So this bond is forming,
energy is given off. The magnitude of energy is
still 242 kilojoules per mole, however, now we have this
negative sign in here to indicate the energy is
given off when bonds form. Bond enthalpies can be used to estimate enthalpies of reactions. So to find the change in the enthalpy for a chemical reaction, you take the sum of the bond enthalpies
of the bonds broken. And from that you subtract the sum of the bond enthalpies
of the bonds formed. The minus sign is in there because energy is given off when bonds form. A good way to remember this equation is to remember that B comes
before F in the alphabet. So B before F therefore it's bonds broken minus bonds formed. Let's use bond enthalpies to estimate the enthalpy of reaction for
the following reaction here, methane with chlorine gas
to form chloro methane and hydrogen chloride gas. It's often helpful to draw dot structures for these kinds of problems. If we look at the methane dot structure, we would need to break one
carbon-hydrogen single bond in order to get to our products. We would also need to break a
chlorine-chlorine single bond. Next, one of the chlorine
goes over to the CH3 to form CH3Cl. So therefore we are forming one carbon-chlorine single bonds, and the other Cl goes with the hydrogen. So we also need to form one
hydrogen-chlorine single bond. The next step is to
sum the bond enthalpies of the bonds broken. So let's think about this. For our reactants we're breaking bonds. So we have one mole of methane reacting with one mole of chlorine. And since we're breaking one
carbon-hydrogen single bond for every one molecule of methane, since we have one mole
of methane molecules, we're breaking one mole of
carbon-hydrogen single bonds. Therefore we can write down here, one mole of carbon-hydrogen bonds, and the bond enthalpy for a
carbon-hydrogen single bond is 413 kilojoules per mole. Since there's one
chlorine-chlorine single bond for every Cl2 molecule, and we have one mole
of chlorine molecules, we're breaking one mole of
chlorine-chlorine single bonds. So to this we're gonna add one mole of chlorine-chlorine single bonds. And the bond enthalpy for a
chlorine-chlorine single bond is 242 kilojoules per mole. Moles cancel out and we get that the sum of the bond enthalpies of the bonds broken is equal to 655 kilojoules. Next, we need to sum the bond enthalpies of the bonds formed. So we're forming one
mole of chloro methane and one mole of hydrogen chloride gas. And since we're forming one
carbon-chlorine single bond for every molecule of chloro methane, since we're forming one
mole of chloral methane, we're reforming one mole of
carbon-chlorine single bonds. So let's write down here,
we're forming one mole of carbon-chlorine bonds
and the bond enthalpy for a carbon-chlorine single bond is equal to 328 kilojoules per mole. And since we form one
hydrogen-chlorine single bond for every molecule of hydrogen chloride, since we're making one
mole of hydrogen chloride, we're forming one mole of
hydrogen-chlorine single bonds. So to this we add one mole of hydrogen-chlorine single
bonds and the bond enthalpy for a hydrogen-chlorine single bond is 431 kilojoules per mole. Moles cancel and we get that
the sum of the bond enthalpies of the bonds formed is
equal to 759 kilojoules. Next, we're ready to find
the change in enthalpy for our chemical reaction. The sum of the enthalpy
of the bonds broken, we found that was equal to 655 kilojoules. And from that we subtract the
sum of the bond enthalpies of the bonds formed, which
we found was 759 kilojoules. So 655 minus 759 gives -104 kilojoules. Sometimes we see kilojoules
or kilojoules per mole or kilojoules per mole of reaction. Kilojoules per mole of reaction just means how the balanced equation is written. And let's see how we can look at the units to get kilojoules per mole of reaction when we do the calculations. If you go back to breaking the carbon hydrogen bond over here, we've seen there's one mole
of carbon-hydrogen bonds that we need to break for
how the equation is written. Therefore, we can write
a conversion factor of one mole of carbon-hydrogen bonds per one mole of reaction as it's written. And then we multiply
that by the bond enthalpy as 413 kilojoules per mole
for a carbon-hydrogen bond, this cancels out moles
of carbon-hydrogen bonds and this gives us kilojoules
per mole of reaction as our units. So it's more time consuming
to write it this way but we could do that for all of our different bond enthalpies to get kilojoules per mole of reaction for the units for our final answer. When everything is under
standard conditions, we need to add a superscript of note. So this would be the
standard change in enthalpy for a chemical reaction. So for the value we just calculated, - 140 kilojoules per mole of reaction, this is under standard of conditions. So this is actually the
standard change in enthalpy for this chemical reaction. Remember that bond
enthalpies are only averages. And so this value that we
calculated is only an estimate for the standard change in enthalpy for this chemical reaction. A more accurate way of
finding this standard change in enthalpy for a chemical reaction is to use standard
enthalpies of formation. And when you use standard
enthalpies of formation to find the standard change in enthalpy for this particular chemical reaction, you get -99.8 kilojoules
per mole of reaction. So -104 is pretty close to -99.8.