Understanding Bond Enthalpy: A Guide to Calculating Enthalpy of Reaction

we're going to be talking about bond enthalpy and how you can use it to calculate the enthalpy of reaction

bond enthalpy is the energy that it takes to break one mole of a

bond so one mole of a bond so different types of bonds will have different bond enthalpies

so as an example we can talk about a carbon hydrogen bond or a carbon hydrogen single bond

so this carbon is probably attached to some other stuff because carbons usually have more than one

single bond but we're going to ignore everything else attached to the carbon we're just

going to represent it as a big blob like popcorn maybe it's a protein it

could be it could be a sugar molecule it could be a lot of things but we're ignoring that blob

and one other thing i forgot to say earlier is that this is the energy it takes to

break one mole of a bond in the gas phase so it's a pretty specific

definition so in the case of our carbon-hydrogen bond

the bond enthalpy of this bond so if we break this bond let's do a sort of dotted line if we

break this bond we have to add energy and what we'll get as our products is

we'll get our popcorn and what happens is when we break this bond the two

electrons that originally made up the bond one of the electrons will go to the carbon

and the other electron will go to the hydrogen and we usually represent

single electrons like that using a single dot sort of like when you write lewis structures you can write lone

pairs with two dots so here's our carbon with one dot or one electron and our hydrogen with

one electron and these are both still in the gas phase so the delta h of this reaction

is the bond enthalpy which i will abbreviate as be so some important things to remember

about bond enthalpy are that bond enthalpy is always positive so it's always going to take energy

you're always going to have to add energy to break a bond

if we take the reverse of the bond if we take the reverse of the bond enthalpy

so another way to think about this is to to flip this reaction so if we take the

reverse of this reaction that means we're making a bond and

since we know that breaking a bond always takes energy that means making a bond

always releases energy so it will always be negative to

make a bond and that's another way of saying it will always release energy

and then the third thing that we're going to discuss about bond envelope is that you can use it

you can use bond enthalpy to estimate delta h of reaction and delta h of reaction is

or the enthalpy of reaction is something that chemists are often interested in we want to know if it's exothermic or

endothermic you might know that there's lots of other ways of calculating delta h of reaction such as

using hess's law or another way is using delta h of

formation and then there are other ways too so this is just another way that we can use

to calculate delta h of reaction using bond dental piece so we're going to go through an example

of that next so the example reaction is taking propine which is c3h4

gas and reacting it with hydrogen so hydrogen gas

to get propane c3h8 gas

and i don't know about you i'm pretty bad at looking at a chemical formula like this and knowing exactly

what the molecule looks like so i'm going to draw out the lewis structures so the lewis structure for propine

propine has three carbons and one triple

one carbon-carbon triple bond and then it has four hydrogens so that's propine

and we also have hydrogen gas and our product is propane so propane

has all single bonds so three so three carbons with single bonds

and eight hydrogens bound to the carbons so that's the reaction we are interested

in and what we want to know here is what is delta h of reaction and how can we calculate it using bond

enthalpies we said earlier that bond enthalpies a bond enthalpy is the energy it takes to break a bond so what we're

going to do next is look at our reaction in terms of what bonds are broken and what bonds are formed and this is a lot

easier to do using the lewis structures first let's talk about which bonds are broken

we if we compare our reactants and our products

we're breaking this carbon-carbon triple bond we're breaking this carbon-carbon triple bond and we're also going to

break this hydrogen hydrogen bond and one thing we forgot to do earlier which is super important is we actually need

to make sure our reaction is balanced and we have four plus two six hydrogens on our

reactant side and we have eight hydrogens on our product side that's not balanced so we actually need two

hydrogen molecules on the reactant side so let's draw one more in so yes we said we are breaking a

hydrogen hydrogen bond we're actually breaking two hydrogen hydrogen bonds it's important to keep track of how many

of each type of bond we're breaking because the bond enthalpy is per mole so if you have twice as many moles it'll

take twice as much energy to break to break all of those bonds and then we can look at the bonds that are formed so

we have not since we broke this carbon-carbon triple bond that means we

needed to make a new bond and the new bond we made in our product molecule is this carbon-carbon carbon

carbon single bond not only did we form a new single bond between these two carbons but now these

carbons are attached to a bunch of hydrogens so we made four new

carbon hydrogen bonds so let's write that out so that we can keep track of them when we do our final

calculation of delta h of reaction so if we just look at the bonds broken the bonds we broke

we have a carbon carbon triple bond and we have a couple hydrogen hydrogen bonds let's also just write down how many of

each we have because we'll need that for our calculation so we have one carbon-carbon

triple bond and we have two hydrogen hydrogen bonds that are broken and then

we can also look up their bond enthalpies which are in kilojoules per mole

bond enthalpies you can typically look up in your textbook or online and they usually come in a table of bond

enthalpies and so the units can be kilojoules per mole sometimes you'll also see calories

or kilocalories per mole i already looked up these bond enthalpy values so carbon-carbon triple bonds have a bond

enthalpy of 835 kilojoules per mole and hydrogen hydrogen bonds have a bond enthalpy of 800. sorry

436 kilojoules per mole and then next if we look at the bonds that are broken we have

a carbon carbon single bond and and we have

one of those bonds forming and the bond enthalpy for that which is also in terms of kilojoules per mole is

346. and last but not least we have the carbon hydrogen bonds that we're

forming and we have four of those and each of those the bond enthalpy is 413

kilojoules per mole so now we can take all of this information and put it together to

calculate delta h of reaction so delta h of reaction if we're thinking about it in terms of

bonds made and broken it's the total energy change during a reaction

and so it's just the energy it takes to break all of our bonds and the reactants

so to break this carbon-carbon triple bond and the two hydrogen hydrogen bonds plus the energy

it takes to make the bonds to make

new product bonds we said earlier that you always have to add energy to break bonds so bond

enthalpy is always positive so we know this part of our calculation should always be a positive number what that

means is that it always releases energy to make new bonds and when energy is released delta h

becomes more negative so this number here when we're talking about adding up the

energy it takes to make new bonds these should be negative numbers so now let's plug in the values we have for bond

enthalpy for all of these bonds that are made and broken in our reaction let's start with the bonds that are

broken so we have our carbon-carbon single bond that will require 835 kilojoules per mole

and we have only one of them and we also have to break two hydrogen hydrogen bonds so 2 times

436 kilojoules per mole which is the bond enthalpy of that bond so that's all of the bonds we break

now we have to add up the energy that's released when we make the new bonds so we have this carbon-carbon single bond

so that is 346 kilojoules per mole and that's negative

because that energy is released and then the last bond is the carbon-hydrogen bond

also negative because the energy is released and we have four of them and each of

them will release 413 kilojoules per mole so if we stick

all of this into our calculator to get our final answer what i got was that the delta h of reaction for this for this

hydrogenation reaction between propine and hydrogen gas is negative

291 kilojoules per mole we can see that this overall reaction

releases energy because delta h is negative so it's exothermic

and that's how you can use bond enthalpies to calculate delta h of reaction