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

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

Let's take a look at this exercise.

energy cycle using enthalpy change of formation question

Definition of Enthalpy Change of Formation

In order for us to draw the energy cycle, we need to understand enthalpy change of formation and be able to write out equations to show the enthalpy change of formation of these compounds.

Enthalpy change of formation is defined as the energy change when one mole of compound is formed from its constituent elements in standard states.

For the balanced equation the coefficient of the compound has to be fixed at one since enthalpy change of formation is with respect to per mole of compound:

energy cycle using enthalpy change of formation formation equations

Draw Energy Cycle

Next we can draw the energy cycle by writing out the main equation first:

energy cycle using enthalpy change of formation main equation

Since we are given the enthalpy change of formation of all the compounds, we can link this equation to elements in the standard state, which we can put below the main equation.

After balancing the elements, the energy cycle will look like this:

energy cycle using enthalpy change of formation add elements balanced

Now we can add arrows for each formation reaction.

First let's account for formation of SnO2. Since coefficient of SnO2 is 1, the enthalpy change of formation for SnO2 is unchanged.

energy cycle using enthalpy change of formation formation of SnO2

Next for formation of CO. Since coefficient of CO is 2, enthalpy change of formation for CO has to be doubled.

energy cycle using enthalpy change of formation formation of CO

Finally for formation of CO2. Since coefficient of CO2 is 2, enthalpy change of formation for CO2 also has to be doubled.

energy cycle using enthalpy change of formation formation of CO2

Apply Hess' Law

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

energy cycle using enthalpy change of formation show clockwise and counter clockwise arrow

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.

energy cycle using enthalpy change of formation hess law calculation

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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