5.2 Hesss 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 H H values to get the H H of the reaction Hesss 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. Hesss Law Hesss 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) Find Hreaction of C2H4 + 6F2 2CF4 + 4HF product reactant
If given: 2x 2x H2 + F2 2HF H= -537 kJ Flip
direction 2C + 2H2 C2H4 product 2x reactant Flip direction
CF4 C + 2F2 Flip direction H= +52 kJ 2x Flip H= +680 kJ
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 Find Hreaction of C2H4 + 6F2 2CF4 + 4HF If given:
3. Rewrite equations and H to reflect changes and check work. 4. Carry out Hesss Law and find the sum of Hreaction 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.
REACTION MUST BE BALANCED! Step 1: Usually, you can find these in data booklet section 12. And wait: why isnt oxygen gas listed?
-105 5 x (0) 3 x (-394) 4 x (-286) = (-2326) (-105) = -2221 kJ mol-1
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