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Chemical Thermodynamics and the Equilibrium State

Chemical Kinetics

The equilibrium position of a chemical reaction is determined by the free energy difference between the product and the reagent. Free energy is a state function, which means it depends on the state of the system only. In other words, the free energy difference between the product and the reagent will be the same regardless of whether the reaction is catalyzed.

Both thermodynamic issues and kinetic issues relate to the practicality of a reaction process. If equilibrium is not favorable or if the reaction rate is too slow, the reaction might not be feasible from a practical standpoint. Whether the barrier is thermodynamic or kinetic is an important consideration in laboratory problem solving. If the barrier is thermodynamic, for example, the chemist may seek to exploit LeChatelier's principle and adjust some condition (pressure, temperature, or concentration) to drive the equilibrium in the desired direction. If the barrier is the reaction rate, the chemist may seek a catalyst to increase the reaction rate.

The MCAT loves to present such a problem. From an MCAT writer's perspective, can you see how wonderfully kinetics vs. thermodynamic considerations can be made to fit into the multiple choice format? Look for it in your practice tests.

Chemical Kinetics

Reactions of Conjugated Species

The addition of hydrogen halide to conjugated dienes is the classic example involving the interplay of kinetics and thermodynamics in determining the nature and extent of reaction yield. Addition to conjugated dienes makes frequent appearance on the MCAT.

When hydrogen halide is reacted with a conjugated diene at low temperatures, direct addition (1,2 addition) is favored, but at room temperature and above, conjugate addition (1,4 addition) is favored. This occurs because the 1,2 product is kinetically favored but the 1,4 product is thermodynamically favored.

Why is one pathway kinetically favored and the other thermodynamically favored? The activation energy is greater for the conjugate addition pathway (1,4 addition), so it occurs more slowly than 1,2 addition. In other words, 1,2 addition is kinetically favored. However, the product of 1,4 addition contains an internal double bond, which is lower energy and the 1,2 adduct. Therefore, 1,4 addition is thermodynamically favored.

When the addition of hydrogen halide to conjugated diene is carried out under conditions such that the products may equilibrate (higher temperature), the product composition no longer reflects the relative rates of formation but tends to reflect relative stabilities. The 1,4 adduct is the major product. Under these conditions, if the 1,2 adduct does form, it is only temporary. The reagents reform by equilibrium and proceed to the 1,4 adduct.

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