How to Draw Energy Cycle involving Enthalpy Change of Combustion - Energetics

In this video we want to learn how to draw the energy cycle given enthalpy changes of combustion of compounds and calculate the enthalpy change of a reaction using Hess' Law.

Let's take a look at this exercise.

1. Draw Energy Cycle

First we have to write out the equation that represents enthalpy change of formation for propanol.
Remember formation is forming 1 mole of a compound from its elements in their standard states.

Since the products of combustion of carbon, H2 gas and propanol are all CO2 and H2O, we can put them together at the bottom of the energy cycle and balance the equations.

For energy cycle involving combustion, we need to add O2 gas to force out the combustion reactions so that we can make use of the enthalpy change of combustion for calculation.

Now we can add arrows for each combustion reaction.

First let's account for combustion of C3H8O. Enthalpy change of combustion is with respect to per mole of substance reacted with excess oxygen. Since coefficient of C3H8O is 1, enthalpy change of combustion for C3H8O remains as it is.

Next for combustion of C. Since coefficient of C is 3, enthalpy change of combustion for C has to be multiplied by 3.

Finally for combustion of H2. Since coefficient of H2 is 4, enthalpy change of combustion for H2 has to be multiplied by 4.

2. Apply Hess' Law

Once the energy cycle is complete, we can determine which of these arrows are clockwise and counter-clockwise.

Finally we can apply Hess' Law by putting all the clockwise arrows on one side of the equation and all the counter-clockwise arrows on the other side of the equation to solve for the unknown enthalpy change.

For the detailed step-by-step discussion on how to draw this energy cycle, check out this video!

Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore

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