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

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: reactant product 2 x H2 + F2  2HF 2 x H°= -537 kJ Flip direction 2C + 2H2  C2H4 H°= +52 kJ Flip direction product 2 x CF4  C + 2F2 2 x H°= +680 kJ reactant Flip direction Flip direction 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: 2H2 + 2F2  4HF H°= -1074 kJ C2H42C + 2H2 H°= -52 kJ 2C + 4F2 2CF4 H°= -1360 kJ H°= -2486 kJ 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.

Step 2: Also, these are not always balanced for you Step 2: Also, these are not always balanced for you. REACTION MUST BE BALANCED! Step 1: Usually, you can find these in data booklet section 12. And wait: why isn’t oxygen gas listed? -105 5 x (0) 3 x (-394) 4 x (-286) = (-2326) – (-105) = -2221 kJ mol-1