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Unformatted text preview: ve off heat energy (ΔH < 0) are e xothe rmic. Combustion is obviously an exothermic chemical reaction.
Condensation of steam to liquid water is also exothermic, although water maintains a constant temperature during
the process. Processes that absorb heat energy (ΔH > 0) are e ndothe rmic. Sweat, liquid water that evaporates
www.webassig n.net/ebooks/wsug encheml1/lab_12/manual.html 1/7 4/15/13 Lab 12 - M easur ing Enthalpy Chang es from your skin, cools you by absorbing heat from your body.
ΔS, the change in e ntropy, is related to the
number of ways the energy of the system can be distributed. Entropy is commonly defined as disorder or
randomness. Entropy is high in gases, because the molecules are free to move about in all directions, fill the
space available, and adopt any orientation relative to each other. Entropy is low in solids, because the molecules
cannot move much relative to each other, and their orientation is fixed. With this information, one can see that
a chemical process will certainly be spontaneous if it is exothermic (ΔH < 0) and its entropy increases (ΔS > 0). If
the opposite holds (ΔH > 0 and ΔS < 0), the process cannot be spontaneous. What if both have the same sign?
If the process is exothermic, the process will be spontaneous below the temperature that satisfies the
condition ΔH = TΔS. It turns out that changes in entropy are small relative to changes in enthalpy in most
processes. It is usually not possible to tell whether entropy has increased or decreased in spontaneous
exothermic processes. The exceptions occur when gases are produced or consumed. Water will condense
spontaneously if the temperature is below 100°C; the process is sufficiently exothermic to offset the loss in
entropy that accompanies the phase change.
If the process is endothermic, the process will be spontaneous
above the temperature that satisfies the condition ΔH = TΔS. As mentioned above, entropy changes are small
relative to enthalpy changes when gases are n...
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