5.2 Hess’s Law The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual.

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5.2 Hess’s Law The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps You should be able to “add” equations together to get the products you want and then add the ΔH° values to get the ΔH° of the reaction

Hess’s Law H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested. However, we can estimate H using published H values and the properties of enthalpy.

Hess’s Law Hess’s law: If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps. Because H is a state function, the total enthalpy change depends only on the initial state (reactants) and the final state (products) of the reaction.

Energy Level Diagram Basically, a way to visually represent what is happening in an enthalpy cycle

Does Hess’s Law sound like any other law of the universe you’ve heard Does Hess’s Law sound like any other law of the universe you’ve heard? How is it different?

Just Like Algebra X + Y = 36 Y = Z + X 2Y = Z + 36

(example p.226)

1. Focus on compounds that only appear in that step. Find H°reaction of C2H4 + 6F2  2CF4 + 4HF If given: H2 + F2  2HF H°= -537 kJ 2C + 2H2  C2H4 H°= +52 kJ CF4  C + 2F2 H°= +680 kJ 1. Focus on compounds that only appear in that step. 2. Manipulate equations to match reaction of interest, AND do the SAME to H°

3. Rewrite equations and H° to reflect changes and check work. Find H°reaction of C2H4 + 6F2  2CF4 + 4HF If given: H°= H°= H°= 3. Rewrite equations and H° to reflect changes and check work. 4. Carry out Hess’s Law and find the sum of H°reaction

Enthalpies of Formation An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms. Section 12 of data booklet If something is already in its elemental form: Hf = 0 Eg.: O2 Hf = 0

Total enthalpy change of a reaction is the difference between the sum of heat of formation of products and sum of heat of formation of reactants.