Calculating Reaction Enthalpy Using Bond Enthalpies

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Raymond Le 2G
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Joined: Fri Sep 25, 2015 3:00 am

Calculating Reaction Enthalpy Using Bond Enthalpies

Postby Raymond Le 2G » Mon Jan 11, 2016 9:13 pm

Why is it that when calculating the enthalpy of formation is it "(Sum of Bond Enthalpies of Reactants)" - "(Sum of Bond Enthalpies of Products)" as opposed to the other way around, like it is when calculating the reaction enthalpy when using enthalpy of formation?

Ronald Yang 2F
Posts: 86
Joined: Fri Sep 25, 2015 3:00 am

Re: Calculating Reaction Enthalpy Using Bond Enthalpies

Postby Ronald Yang 2F » Mon Jan 11, 2016 9:42 pm

Bond enthalpies refer to enthalpies for the breaking of certain bonds. When you calculate the enthalpy change for a reaction, you must add up the enthalpy changes due to the breaking of bonds in reactants and the forming of bonds in products. So, when we look at the reactants, we simply just add the bond enthalpies because the bonds within the reactants are being broken (breaking bonds is an endothermic reaction, so there's no need to change the sign of the bond enthalpies, since they are already given in positive values). However, when we look at the products, we must find the enthalpy changes for the forming of bonds, and since bond enthalpies give the energy needed to break certain bonds, we can find the enthalpy change for bond formation (opposite to breaking bonds) by adding up the bond enthalpies of bonds that are formed and making this sum negative, as forming bonds is an exothermic reaction, so a negative sign is placed in front of these enthalpies. The key thing to note for this method of calculating the change in enthalpy of a reaction is that bond enthalpy refers to the energy required to break a specific bond, so typically positive (for the forming of bonds, that same energy is released from the bond instead of absorbed, so we use the negative form of that bond enthalpy value.)

Same idea applies to the calculation of the standard reaction enthalpy with standard enthalpies of formation of the reactants and products. The standard enthalpy of formation of a compound refers to the standard enthalpy that results in combining elements into a compound through the formation of bonds(exothermic reaction, so standard enthalpies of formation are negative). To calculate the standard reaction enthalpy, we need to know the sum of the standard enthalpies of formation for the reactants and the standard enthalpies of formation for the products. Again, we are breaking the bonds of reactants and forming the bonds of products. Thus, the sum of the standard enthalpies of formation for the reactants must be positive (breaking bonds is endothermic, so positive deltaH) and the sum of the standard enthalpies of formation for the products must be negative (forming bonds is exothermic, so negative deltaH). Since the standard enthalpy of formation of a compound is negative, we don't have to change the sign for the sum of the standard enthalpies of formation for the products, since it's already negative. That's why we subtract the sum of the standard enthalpies of formation for the reactants in this method.

Sorry for the long answer! Please feel free to ask more questions if any of this doesn't make sense!


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