DG and Direction of Reactions
To see how DG values can be used to predict how reactions will go, consider the four cases you encountered in the Entropy Module:
1. DH < 0, DS > 0:
This is an exothermic reaction with an increase in entropy. Thus DSuniv > 0 and DG < 0. The Second Law of Thermodynamics says that such a reaction is product-favored, so a DG less than zero also means a product-favored reaction.
Example: 2CH4(g) + 3O2(g) 2CO2(g) + 2H2O(g)
2. DH > 0, DS < 0
This is an endothermic reaction with a decrease in entropy. Thus DSuniv < 0 and DG > 0. The Second Law of Thermodynamics says that such a reaction is reactant-favored, so a DG greater than zero also means a reaction is reactant-favored.
Example: 6CO2(g) + 6H2O(g) C6H12O6(s) + 6O2(g)
The minus sign in the definition of DG was given so that a negative values of DG corresponded to a product-favored process and a positive value to a reactant-favored process. Thus, a product-favored reaction will go on its own, without outside intervention, and can often be made to do useful work in the process. Thus, it is like an exothermic reaction with a negative value of DE or DH. A reaction with a negative DG is called exergonic to emphasize this. Conversely, a reaction with a positive value of DG is reactant-favored and requires the input of energy to go. Such a reaction is called endergonic .
3. DH > 0, DS > 0
This is an endothermic reaction with a positive entropy change. This sort of reaction is reactant-favored at low temperatures and product-favored at high temperatures. DG predicts the same thing, since at low temperatures the DH term of DH - TDS will dominate and DG will be greater than zero. Conversely, at high temperatures, the TDS term will dominate and DG will be less than zero.
Example: CaCO3(s) CaO(s) + CO2(g)
4. DH < 0, DS < 0
This is an exothermic reaction with a negative entropy change. This sort of reaction is product-favored at low temperatures and reactant-favored at high temperatures. DG predicts the same thing, since at low temperatures the DH term of DH - TDS will dominate and DG will be less than zero. Conversely, at high temperatures, the TDS term will dominate and DG will be greater than zero.
Example: N2(g) + 3H2(g) 2NH3(g)
Select positive or negative signs for DH and DS to see what sort of reaction results. Click on the mouse icon at left to clear the radio buttons and text.
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