Based on the following reaction profile, how many intermediates are formed in the reaction A⟶C? How many transition states are there? Which step, A⟶B or B⟶C, is the faster? For the reaction A⟶C, is Δ𝐸 positive, negative, or zero? [Section 14.5]
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insert step 1> Identify the number of peaks and valleys in the reaction profile. Each peak represents a transition state, and each valley represents an intermediate.
insert step 2> Count the number of valleys to determine the number of intermediates formed in the reaction A⟶C.
insert step 3> Count the number of peaks to determine the number of transition states in the reaction A⟶C.
insert step 4> Compare the activation energies of the steps A⟶B and B⟶C. The step with the lower activation energy is the faster step.
insert step 5> Determine the overall change in energy (Δ𝐸) for the reaction A⟶C by comparing the energy of the reactants (A) and the products (C). If the energy of C is lower than A, Δ𝐸 is negative; if higher, Δ𝐸 is positive; if the same, Δ𝐸 is zero.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Reaction Profile
A reaction profile is a graphical representation of the energy changes that occur during a chemical reaction. It typically shows the energy of reactants, products, intermediates, and transition states as the reaction progresses. Understanding the profile helps identify the energy barriers and the overall energy change (ΔE) of the reaction, which is crucial for determining the number of intermediates and transition states.
Intermediates are species that are formed during the reaction but are not present in the final products; they exist at local energy minima on the reaction profile. Transition states, on the other hand, are high-energy states that occur at the peaks of the energy profile, representing the point of maximum energy along the reaction pathway. The number of each can be determined by analyzing the reaction profile.
Activation energy is the minimum energy required for a reaction to occur, influencing the rate of the reaction. The step with the lower activation energy is typically the faster step. By comparing the energy barriers of the steps A⟶B and B⟶C on the reaction profile, one can determine which step is faster and infer the sign of ΔE based on the overall energy difference between reactants and products.
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