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This section elaborates on Hess's Law, highlighting that enthalpy (ΔH) is a state function. Regardless of whether a reaction occurs in one step or multiple steps, the overall change in enthalpy remains constant. This principle allows for the calculation of enthalpy changes for reactions that are difficult to measure directly, like the conversion of graphite to diamond. By applying Hess's Law, you can sum enthalpies from related reactions. Through an example involving carbon compounds, we explore how to evaluate whether a reaction is endothermic or exothermic.
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Hess’s Law Chapter 5 part 3
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. Hess’s Law
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?
