Hess’s Law & Standard Enthalpies of Formation

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Presentation transcript:

Hess’s Law & Standard Enthalpies of Formation Chapter 5 part 3

Hess’s Law Enthalpy is a state function, therefore: In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

Since ΔH is a state function… It doesn’t matter the route. In any reaction the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. Therefore, one may sum up the enthalpies of a series of reaction just as one adds up a series of reactions. But…, if the reaction is reversed, so is the sign of the enthalpy. If the reaction is multiplied, so is the enthalpy.

Some ΔH’s cannot be measured in a Calorimeter: Example: C graphite  C diamond Yes this reaction is real!! This cannot be measured directly. Why???

But using Hess’s Law, it can be calculated: Take the heats of combustion for each: C graphite + O2(g) CO2 (g) ΔH = - 394 kj C diamond + O2(g) CO2 (g) ΔH = -396 kj If you reverse the second: CO2 (g)  C diamond + O2(g) ΔH = +396 kj

Hess’s Law of Summation CO2 (g)  C diamond + O2(g) ΔH = +396 kj C graphite + O2(g) CO2 (g) ΔH = -394 kj Equals C graphite  C diamond ΔH = +2 kj Is this reaction endothermic or exothermic?

Worksheet!!!

Standard Enthalpies of Formation