Question

If $\mathrm{A}\to \mathrm{B}$ is exothermic, how does the activation energy for the forward reaction compare with the activation energy for the reverse reaction $(\mathrm{A}\leftarrow \mathrm{B})$ ?

Short answer

In an exothermic reaction (A → B), the activation energy for the forward reaction (A → B) is higher than the activation energy for the reverse reaction (A ← B). This is due to the negative change in enthalpy (∆H), indicating that the energy of the products is lower than that of the reactants, resulting in a lower energy barrier for the reverse reaction. Mathematically, this relationship is represented as $Ea,reverse=Ea,forward+∆H$, where ∆H is negative for an exothermic reaction.

Step-by-step solution

Understand Activation Energy

Activation energy is the minimum energy required for a chemical reaction to proceed. It is the difference in energy between the reactants and the transition state of the reaction. In other words, it is the energy barrier that the reactants need to overcome to transform into products.

Exothermic Reactions

An exothermic reaction is a chemical reaction that releases energy in the form of heat or light. In this case, the energy of the products is lower than the energy of the reactants, resulting in a negative change in enthalpy (∆H).

Activation Energy for Forward and Reverse Reactions

For a given reaction, the activation energy for the forward reaction (Ea, forward) is the energy difference between the reactants and the transition state. The activation energy for the reverse reaction (Ea, reverse) is the energy difference between the products and the transition state. When the reaction is exothermic, the change in enthalpy (∆H) is negative. This means that the energy of the products is lower than the energy of the reactants. Thus, the activation energy for the forward reaction will be higher than the activation energy for the reverse reaction. Mathematically, this can be represented by the equation: Ea, reverse = Ea, forward + ∆H Since ∆H is negative for an exothermic reaction, Ea, reverse will be smaller than Ea, forward.

Conclusion

In an exothermic reaction (A → B), the activation energy for the forward reaction (A → B) is higher than the activation energy for the reverse reaction (A ← B). This is because the energy of the products is lower than the energy of the reactants, leading to a lower energy barrier for the reverse reaction.

Key concepts

Exothermic Reaction

An exothermic reaction is a type of chemical process where energy is released to the surroundings, usually in the form of heat or light. The defining characteristic of this kind of reaction is that the total energy stored in the chemical bonds of the reactants is higher than that in the products. Put simply, after the reaction occurs, excess energy is 'left over' and released.

This release of energy can often be observed as a rise in temperature in the surroundings. For example, the combustion of gasoline in car engines or a log burning in a fireplace are common exothermic reactions. They not only produce heat but serve practical purposes as well—powering vehicles and warming spaces, respectively.

In terms of a reaction profile diagram, exothermic reactions demonstrate a downward slope from reactants to products. This visual representation is helpful as it shows us that the reactants sit at a higher energy level before the reaction occurs and that they 'fall' to a lower energy state, signifying the release of energy. Importantly, exothermic reactions are spontaneous under the right conditions since they lead to a decrease in the system's energy.

Chemical Reaction

A chemical reaction involves the transformation of one set of chemical substances (reactants) into another (products). During this process, the molecular structure of the substances changes, leading to new properties and compositions. Chemical reactions are fundamental to countless biological and technological processes, from the digestion of food in our bodies to the creation of products like plastics and pharmaceuticals.

Different types of chemical reactions include synthesis, decomposition, single replacement, and double replacement. Crucially, each has unique characteristics and rules about how atoms rearrange during the process. For a reaction to occur, however, reactants must collide with sufficient energy to overcome the activation energy barrier. This initial input of energy is necessary to break existing bonds so that new ones can form, leading to the final product.

Understanding the nature of chemical reactions is crucial for fields like chemistry, environmental science, medicine, and engineering. As such, educators aim to simplify these concepts, using analogies like 'energy hills' that reactants must 'climb' to help students visualize and grasp the processes involved.

Enthalpy Change

The enthalpy change (olinebreak olinebreak olinebreak ΔH) is a measure of the heat content in a chemical system and reflects the energy exchanged with the environment during a chemical reaction under constant pressure. This change can either be positive or negative, corresponding to endothermic (energy absorbed) or exothermic (energy released) reactions, respectively.

In the context of the original exercise, the enthalpy change is negative for an exothermic reaction, indicating that the products have lower energy than the reactants. The concept of enthalpy is central to thermochemistry and is a key factor in determining whether a reaction is energetically favorable. It's important to note that a negative enthalpy change does not automatically mean a reaction will proceed; the system must also overcome the activation energy barrier.

olinebreak ΔH is also used in calculating the energy efficiency of processes and in designing chemical reactions for various applications. In educational settings, the enthalpy concept is often demonstrated using calorimetry experiments, where temperature changes induced by reactions allow students to calculate olinebreak ΔH, linking theoretical knowledge to practical observations.