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To determine the entropy changes for a chemical reaction; we have seen that the energy given off (or absorbed) by a reaction, and monitored by noting the change in temperature of the surroundings, can be used to determine the enthalpy of a reaction (example by using a calorimeter).
Changes in internal energy, that are not accompanied by a temperature change, might reflect changes in the entropy of the system. For eg., a change in entropy associated with the Haber process for the production of ammonia from nitrogen and hydrogen gas.
N₂(g) + 3H₂(g) --- 2NH₃(g)
At 298K as a standard temperature:
S°(NH₃) = 192.5 J/molK
S°(H₂) = 130.6 J/molK
S°(N₂) = 191.5 J/molK
Solution,
From the balanced equation we can write the equation for ΔS°(the change in the standard molar entropy for the reaction):
ΔS° = 2*S° (NH₃) - [S°(N₂) + (3*S°(H₂)]
ΔS° = 2* 192.5 - [191.5 + (3*130.6)]
ΔS° = -198.3 J/molK
It would appear that the process results in a decrease in entropy i.e. a decrease in disorder. This is expected because we are decreasing the number of gas molecules.
Learn more about the entropy change here:
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