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The change in entropy is
denoted DS. It's computed in exactly the same fashion
as the change
in enthalpy, DH. Simply look up the standard molar
entropies of the products and reactants and subtract the reactants from the
products. As an example, for the generic reaction
- aA + bB -> cC + dD
- DS = (S DSo products)
-(S DSo reactants)
- DS = (c*DSc0 +d*DSd0) - (a*DSa0 + b*DSb0)
Be careful to check that
the standard molar entropy you look up is for the correct phase of that
material: the entropy of a gas is higher than the entropy of a solid, for
example.
Example: What is the change in entropy for the reaction
- 2H2(g) + O2(g) -> 2H2O(g)
Solution: Look up the standard molar entropies for each of the
products and reactants
| Compound |
DS0 (J/mol*K) |
| H2(g) |
130.6 |
| O2(g) |
205.0 |
| H2O(g) |
188.7 |
Next, simply subtract the entropies
of the reactants from those of the products, making sure to include the
stochiometric coefficients
- DS = (2*DSH2O0) - (2*DSH20 + 1*DSO20)
- DS = (2* 188.7) - (2*130.6 + 1*205.0)
- DS = -88.6 J/mol*K
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