All College Chemistry Resources
Example Question #4 : Thermochemistry And Changes Of State
Use average bond enthalpies to estimate the enthalpy change, in kilojoules, of the combustion of one mole of hexane to form liquid water and carbon dioxide gas.
|Type of Bond||Average bond enthalpy|
|Carbon-hydrogen single bond||414|
|Carbon-oxygen double bond||736|
|Oxygen-oxygen double bond||498|
|Oxygen-hydrogen single bond||464|
|Carbon-carbon single bond||347|
Start by writing the balanced equation for the combustion of hexane:
Recall the following equation:
Remember that energy is required to break bonds, so should be positive. Energy is also released when bonds are made so should be negative.
Next, draw out the Lewis structures of each molecule to figure out the number of bonds made or broken.
For the hexane, there are bonds that must be broken, and bonds that must be broken.
For each oxygen, there is only 1 oxygen double bond to break. However, we will need to multiply this number by its stoichiometric coefficient.
For each carbon dioxide, there are double bonds between carbon and oxygen to break. For the given equation, there are total of these double bonds because we have moles of carbon dioxide.
For each water, there are single bonds between hydrogen and oxygen. For the given equation, there are a total of of these bonds because we have moles of water.
Now, use the given information regarding to average bond enthalpies to find the change in enthalpy for the reaction.