In Lesson 1, you used a calorimeter to infer the enthalpy of a reaction. However, it is not always possible to infer the enthalpy change of a reaction using a calorimeter. For example, the slow process of corrosion results in temperature changes too small to be studied using calorimetry. In such cases, you can utilize Hess' Law to predict the enthalpy change for a reaction.

Based on thermodynamic principles, Hess' Law states that the addition of chemical equations yields a net chemical equation with an enthalpy change equal to the sum of the enthalpy changes of the steps. Hess' Law allows us to combine standard enthalpy change values of known reactions in order to solve for enthalpy changes of unknown reactions. Consider the example shown below:

Provided below is the information concerning a theoretical chemical system. Assume that the net reaction cannot be conducted in a calorimeter therefore its Δ_rH cannot be inferred from calorimetric data. However, the Δ_rH values for Reactions I, II, and III are known. Can we use the given known Reactions I, II, and III to predict Δ_rH for the net reaction? In fact, we can! 

1. First, write down the net reaction. Sometimes the net reaction is already provided for you, as it is in this case. If you are required to write the net reaction on your own, make sure it is balanced!
   AB + CD → AC + BD
   
   
2. Next, manipulate Reactions I, II, and III such that they will add up to yield the net equation.

   
   To do this, compare the net reaction with Reactions I, II, and III. If a substance is located on the reactant side of the net reaction, it must also appear in the same amount on the reactant side of the given equation. If it does not, reverse the given reaction and multiply it by whole number or a fraction, such that the number of moles of the substance in the known equation matches the number of moles of the substance in the net equation. If you reverse the reaction, you must also reverse the sign of the enthalpy change.

   
   Similarly, if a substance is located on the product side of the net reaction, it must also appear in the same amount on the product side of the given equation. If it does not, reverse the given reaction and multiply it by whole number or a fraction, such that the number of moles of the substance in the given equation matches the number of moles of the substance in the net equation.  If you reverse the reaction, you must also reverse the sign of the enthalpy change.

   
   AB is a reactant in the first reaction. AB is also a reactant in the net reaction. There is one mole of AB reactant in the net reaction and one mole in the given reaction. Therefore leave Reaction I as it is.

   
   In Reaction II, there is one mole of CD reactant and one mole of AC product.  This matches the net reaction, so leave Reaction II as is.

   
   There is one mole of BD reactant in the third given reaction; however, there is one mole of BD product in the net reaction. For this reason, reverse Reaction III, including its Δ_rH value. BD then becomes a product in Reaction III.

   Reaction I: AB → A + B ΔrH = +75 kJ Reaction II: A + CD → AC + D ΔrH = -50 kJ Reaction III: B + D → B D ΔrH = +125 kJ Notice that the sign has changed.

3. Add the above three reactions to get the net reaction. Cancel entities that appear on both sides of the arrows in equal amounts. Add up the amounts for terms that are on the same side of the reactions – all the reactants and all the products. Next, add the standard enthalpies for the manipulated known reactions, paying attention to the negative and positive signs.
   
   
   
   

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