Does increasing the energy barrier for an SN2 reaction increase or decrease the magnitude of the rate constant for the reaction?

Verified step by step guidance

1. Understand that the rate constant (k) for a reaction is related to the activation energy (Ea) by the Arrhenius equation: k = A * e^(-Ea/RT), where A is the pre-exponential factor, R is the gas constant, and T is the temperature in Kelvin.
2. Recognize that the energy barrier in a reaction is synonymous with the activation energy (Ea).
3. Consider that an increase in the energy barrier (Ea) means that the exponent in the Arrhenius equation becomes more negative.
4. Realize that a more negative exponent results in a smaller value for the exponential term, e^(-Ea/RT), thus decreasing the rate constant (k).
5. Conclude that increasing the energy barrier for an SN2 reaction decreases the magnitude of the rate constant for the reaction.

Key Concepts

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction is a type of nucleophilic substitution where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This reaction occurs in a single concerted step, meaning that bond formation and bond breaking happen simultaneously. The rate of an SN2 reaction depends on the concentration of both the nucleophile and the substrate.

Activation Energy and Rate Constant

Activation energy is the minimum energy required for a chemical reaction to occur. According to the Arrhenius equation, the rate constant (k) of a reaction is inversely related to the activation energy; as the energy barrier increases, the rate constant decreases. This relationship highlights how changes in energy barriers directly affect the speed of reactions.

Effect of Energy Barrier on Reaction Rate

Increasing the energy barrier for a reaction typically results in a slower reaction rate. In the context of an SN2 reaction, if the energy barrier is raised, fewer molecules will have sufficient energy to overcome this barrier, leading to a decrease in the rate constant. Thus, a higher energy barrier correlates with a lower likelihood of successful collisions between reactants.