Hess’s Law and Hof [aka heats of formation]
**technical difficulties abound with little delta symbols and superscripts and subscripts - do you know how to include symbols, superscripts and/or subscripts?



With Hess’s law – use each of the given equations, add to arrive at the target equation
-if the equation is reversed, the sign of H is also reversed (+ to – and/or – to +)
-if the equation is multiplied by a value, multiply H by the same value

One more thing, consider the equation written below:

4CO2 (g) + 6 H2O(g) -->2 C2H6 (g) + 7 O2 (g)            H = 849.0KJ

the molar enthalpy of CO2: HCO2 = 212.25 kJ/mol        [849kJ / 4 mol]

Any equation can be interpreted per mole of any substance in the equation

Heats of Formation Hof

-be able to write the formation reaction for any substance [produce one mole of product from elements in their standard state]

-when calculating H with heats of formation (data tbl 799-800) remember that it is:

                 (products) – (reactants)

For a small change a few elephant jokes. Ready or not, here they come. Q: How do elephants communicate? [On the elephone.]  Q: Why are elephants large, grey and wrinkled? [Have you ever tried ironing one?]  What time is it when an elephant sits on your fence? [Time to get a new fence.]