Click the Delta H is an Extensive Properly button, and analyze the relationship

Question

Click the Delta H is an Extensive Properly button, and analyze the relationship between the two reactions that are displayed. The reaction that was on the screen when you started and its derivative demonstrate that the change in enthalpy for a reaction, Delta H, is an extensive property. Using this property, calculate the change in enthalpy for Reaction 2. Reaction 1: C3H8(g) + 5O2(g) rightarrow 3CO2(g) + 4H20(g), Delta H1 = -2043 kJ Reaction 2: 4C3H8(g) + 20O2(g) - 12CO2(g) + 16H2O(g), Delta H2 =?

Explanation / Answer

An extensive property is one that is additive for independent, non-interacting systems. One such property is delta H, or the change in enthalpy. Take a look at reaction 1 vs. reaction 2; the only difference is that one molecule of propane (C3H8) is combusted in reaction 1, whereas 4 molecules of propane are combusted in reaction 2. We can think of reaction 2 as occurring in 4 different containers, each containing 1 molecule of propane. Since delta H is an extensive properties we need only add up the individual delta Hs for each container to find the total delta H. In this case delta H2 = -2043 kJ - 2043 kJ - 2043 kJ - 2043 kJ = 4*(-2043kJ) = -8172 kJ Thus It is easy to see that for any reaction A + B -> C + D, if we multiply the reaction by a constant k (kA + kB -> kC + kD), then the new standard total change in enthalpy can be arrived at by multiplying the delta H of original reaction by k.

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