Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach you what you need to know about kinetic control versus thermodynamic control for kinetics. Watch this video to get all the mcat study tips you need to do well on this section of the exam!
We might assume that chemical reactants will naturally produce only one product, but that isn’t the case. In some situations, reactions can proceed down two or more pathways forming two distinct products. This may sound like a headache for chemists, but there are certain advantages to having multiple reaction pathways. Chemists can manipulate the temperature of the reaction environment to favor one product at one temperature, and another product at a different temperature, allowing for a more diverse reaction output.
Kinetic and Thermodynamic Control
When a reaction has two competing pathways, one product is the thermodynamic product and the other is the kinetic product. The thermodynamic product is more stable but requires a greater activation energy in order to be produced. The kinetic product is less stable but has a lower activation energy.
Consider the reaction coordinate diagram for this two-product reaction. Since the thermodynamic product requires a greater activation energy than the kinetic product, the transition state of the thermodynamic product will be located higher up on the diagram. Based on the formula for the reaction rate constant, activation energy is inversely proportional to reaction rate. Having less activation energy, the kinetic product will be formed faster than the thermodynamic product. However, while the thermodynamic product is formed more slowly, it is much lower in energy than the kinetic product. When a molecule is lower in energy, it’s more stable and less likely to revert back into reactants. In summary, we can say that the kinetic product forms more quickly, but the thermodynamic product is more stable and longer lasting.
Manipulating Reaction Conditions
Recall that the kinetic product has a lower energy of activation to form, so formation of this product is relatively favorable under conditions of low energy. The thermodynamic product has a higher energy of activation, so formation of this product is relatively favorable under conditions of high energy. We can favor formation of the kinetic product by reducing the energy of the reaction system, which can be accomplished by reducing reaction temperature. Conversely, by running a reaction at warmer temperatures, more energy is added into the system and allowing the thermodynamic product’s high-energy transition state to be overcome.
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