College Chemistry : Standard Enthalpy and Entropy of Reactions

Study concepts, example questions & explanations for College Chemistry

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Example Questions

Example Question #1 : Standard Enthalpy And Entropy Of Reactions

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
Possible Answers:

Correct answer:


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.

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