0.9 Energetics of chemical reactions  (Page 4/7)

By studying many chemical reactions in this way, we discover that this result, known as Hess' Law , is general.

Hess' law

The heat of any reaction is equal to the sum of the heats of reaction for any set of reactions which in sumare equivalent to the overall reaction.

(Although we have not considered the restriction, applicability of this law requires that all reactionsconsidered proceed under similar conditions: we will consider all reactions to occur at constant pressure.)

A pictorial view of Hess' Law as applied to the heat of is illustrative. In , the reactants $C\left(s\right)+2H_{2}O\left(g\right)$ are placed together in a box, representing the state of the materials involved in the reaction prior to the reaction. The products $C{O}_2\left(g\right)+2H_2\left(g\right)$ are placed together in a second box representing the state of the materials involved after the reaction. The reaction arrowconnecting these boxes is labeled with the heat of this reaction. Now we take these same materials and place them in a third boxcontaining $C\left(s\right)$ , $O_2\left(g\right)$ , and $2H_2\left(g\right)$ . This box is connected to the reactant and product boxes withreaction arrows, labeled by the heats of reaction in and .

This picture of Hess' Law reveals that the heat of reaction along the "path" directly connecting the reactantstate to the product state is exactly equal to the total heat of reaction along the alternative "path" connecting reactants toproducts via the intermediate state containing $C\left(s\right)$ , $O_2\left(g\right)$ , and $2H_2\left(g\right)$ . A consequence of our observation of Hess' Law is therefore that thenet heat evolved or absorbed during a reaction is independent of the path connecting the reactant to product. (This statement isagain subject to our restriction that all reactions in the alternative path must occur under constant pressureconditions.)

A slightly different view of results from beginning at the reactant box and following a complete circuit through the otherboxes leading back to the reactant box, summing the net heats of reaction as we go. We discover that the net heat transferred (againprovided that all reactions occur under constant pressure) is exactly zero. This is a statement of the conservation of energy:the energy in the reactant state does not depend upon the processes which produced that state. Therefore, we cannot extract any energyfrom the reactants by a process which simply recreates the reactants. Were this not the case, we could endlessly produceunlimited quantities of energy by following the circuitous path which continually reproduces the initial reactants.

By this reasoning, we can define an energy function whose value for the reactants is independent of how thereactant state was prepared. Likewise, the value of this energy function in the product state is independent of how the productsare prepared. We choose this function, $H$ , so that the change in the function, $\Delta (H)=H_{\mathrm{products}}-H_{\mathrm{reactants}}$ , is equal to the heat of reaction $q$ under constant pressure conditions. $H$ , which we call the enthalpy , is a state function , since its value depends only on the state of the materials under consideration, that is, the temperature,pressure and composition of these materials.