Problem 16 - Chemistry A Guided Inquiry

Question

What one feature of a reaction coordinate diagram is indicative of kinetic control of a reaction? Explain your reasoning. i) \(\quad \Delta H^{\circ}<0\) ii) \(\quad \Delta H^{\circ}>0\) iii) Activation energy is large iv) Activation energy is small

Step-by-Step Solution

Answer

The 'Activation Energy is large' option is indicative of kinetic control of a reaction. It refers to the high energy barrier that must be overcome for a reaction to proceed, which is a central concept in kinetic control of reactions.

Step 1: Understand Kinetic control and its representation on reaction diagrams

Kinetic control is observed in reactions for which the rate is determined by the height of the energy barrier (activation energy) and not by the overall change in enthalpy (ΔH) of the reaction. Features indicative of kinetic control on reaction coordinate diagrams are those associated with the activation energy, which is represented by the height of the energy barrier between reactants and products.

Step 2: Evaluate the given options

Looking at the options: i) \(\Delta H^{\circ}<0\) ii) \(\Delta H^{\circ}>0\) iii) Activation energy is large iv) Activation energy is small. The first two options correspond to whether the reaction is exothermic (\(\Delta H^{\circ}<0\)) or endothermic (\(\Delta H^{\circ}>0\)) respectively, which is related to the thermodynamics of the reaction, not kinetics. The last two options, on the other hand, are directly related to the kinetics of the reaction and the concept of kinetic control.

Step 3: Choose the correct option

Option iii) 'Activation energy is large' is more likely indicative of kinetic control. It is because a reaction under kinetic control will be slower if the activation energy is high, limiting the speed at which the reaction can proceed, regardless of the potential thermodynamic favorability of the reaction.

Key Concepts

Reaction Coordinate Diagram

A reaction coordinate diagram is a powerful visual tool used to depict the energy changes that take place during a chemical reaction. It graphs the energy of the system along the 'reaction coordinate', which represents the progress of the reaction from reactants to products.

At the start of the diagram, you'll find the energy level of the reactants, and at the end, the energy level of the products. The highest point on this path represents the transition state, the point at which the system is no longer reactants, but not yet products either. The energy difference between the reactants and the transition state is called the activation energy, crucial for understanding reaction kinetics.

Activation Energy

Activation energy, denoted as E_a, is the minimum energy required for a reaction to proceed. It acts as an energy barrier that reactants must overcome to transform into products. In a reaction coordinate diagram, this is represented by the peak that lies between the reactants and the transition state.

Significance of Activation Energy

Activation energy is essential for understanding the kinetics of a reaction. A high activation energy indicates that the reactants require a substantial energy input to react, which typically translates to a slower reaction rate. Conversely, a low activation energy suggests that the reaction can occur more readily, often leading to a faster reaction rate. This concept is key to the idea of kinetic control, where the reaction rate is more influenced by the activation energy than by the change in enthalpy.

Enthalpy Change

Enthalpy change (ΔH) of a reaction reflects the total heat absorbed or released during the process at constant pressure. It's an indication of whether a reaction is exothermic (ΔH<0, releases heat) or endothermic (ΔH>0, absorbs heat).

In a reaction coordinate diagram, the vertical gap between the energy levels of the reactants and the products represents the enthalpy change. However, the magnitude of ΔH does not necessarily dictate the reaction rate. A reaction can be exothermic with a large, negative ΔH and still proceed slowly if the activation energy is high. This distinction is crucial for understanding the difference between kinetic and thermodynamic control of reactions, with the former being dictated by activation energy and the latter by enthalpy change.

Reaction Kinetics

Reaction kinetics is the study of the rates of chemical reactions and the factors affecting those rates. It looks at how quickly reactants are converted to products and which aspects of the reaction mechanism control this rate. Key factors include the concentration of reactants, temperature, the presence of a catalyst, and the activation energy.

Understanding Kinetic Control

Kinetic control implies that the rate-determining step is the one with the highest activation energy, often making it the slowest step in the reaction pathway. A high activation energy, as associated with kinetic control, means that even if a reaction is thermodynamically favorable (e.g., a large negative ΔH), it may proceed at a slow rate because the required energy to surpass the energy barrier is significant.