## 8.2 Reversible Reactions and Dynamic Equilibrium This section explores the nature of chemical reactions that can proceed in both forward and reverse directions, leading to a state of dynamic equilibrium where the concentrations of reactants and products remain constant. ### Reversible Reactions A **reversible reaction** is a chemical reaction where the products, once formed, can react with each other to re-form the original reactants. These reactions do not proceed to completion, even with stoichiometric amounts of reactants, because they occur simultaneously in both the forward and backward directions. - The direction from reactants to products is the **forward reaction**. - The direction from products back to reactants is the **reverse reaction**. - A double arrow ($\rightleftharpoons$) is used in chemical equations to indicate that a reaction is reversible. **Examples of Reversible Reactions:** $$ \mathrm{N}_{2(\mathrm{g})}+3\mathrm{H}_{2(\mathrm{g})} \rightleftharpoons 2\mathrm{NH}_{3(\mathrm{g})} $$ $$ 2\mathrm{NO}_{2(\mathrm{g})} \rightleftharpoons \mathrm{N}_{2}\mathrm{O}_{4(\mathrm{g})} $$ $$ 2\mathrm{NO}_{(\mathrm{g})}+\mathrm{Cl}_{2(\mathrm{g})} \rightleftharpoons 2\mathrm{NOCl}_{(\mathrm{g})} $$ $$ \mathrm{PCl}_{5(\mathrm{g})} \rightleftharpoons \mathrm{PCl}_{3(\mathrm{g})}+\mathrm{Cl}_{2(\mathrm{g})} $$ ### Achieving Dynamic Equilibrium Consider the reaction between steam ($\mathrm{H_2O}$) and carbon monoxide ($\mathrm{CO}$) to understand how equilibrium is established. $$ \mathrm{H}_{2}\mathrm{O}_{(\mathrm{g})}+\mathrm{CO}_{(\mathrm{g})} \rightleftharpoons \mathrm{H}_{2(\mathrm{g})}+\mathrm{CO}_{2(\mathrm{g})} $$ **1. The Forward Reaction** - When steam and carbon monoxide are first mixed, their concentrations are at a maximum. - The initial rate of the forward reaction is at its highest because the frequency of collisions between reactant molecules is greatest. - As the reaction proceeds, the concentrations of $\mathrm{H_2O}$ and $\mathrm{CO}$ decrease, causing the rate of the forward reaction to slow down. $$ \mathrm{H}_{2}\mathrm{O}_{(\mathrm{g})}+\mathrm{CO}_{(\mathrm{g})} \longrightarrow \mathrm{H}_{2(\mathrm{g})}+\mathrm{CO}_{2(\mathrm{g})} $$ **2. The Reverse Reaction** - As the products, hydrogen ($\mathrm{H_2}$) and carbon dioxide ($\mathrm{CO_2}$), accumulate, they begin to collide and react to form the original reactants. - This reverse reaction starts at a rate of zero and gradually increases its speed as the product concentrations rise. $$ \mathrm{H}_{2(\mathrm{g})}+\mathrm{CO}_{2(\mathrm{g})} \longrightarrow \mathrm{H}_{2}\mathrm{O}_{(\mathrm{g})}+\mathrm{CO}_{(\mathrm{g})} $$ **3. The State of Equilibrium** - Eventually, a point is reached where the rate of the forward reaction becomes equal to the rate of the reverse reaction. - At this stage, the reaction is said to be in **chemical equilibrium**. - The concentrations of all reactants and products become constant and no further *macroscopic* changes are observed. > **Chemical Equilibrium:** The state of a reversible reaction at which the composition of the reaction mixture does not change is called the state of chemical equilibrium. The graph below illustrates how the concentrations of reactants and products change over time until equilibrium is reached. ### The Nature of Dynamic Equilibrium Although the overall concentrations appear constant at equilibrium, the reaction has *not* stopped. At the microscopic level, there is intense activity: - Individual reactant molecules continue to combine to form products. - Individual product molecules continue to combine to re-form reactants. This state is called **dynamic equilibrium** because the system is dynamic (molecules are constantly reacting), but it is balanced (the forward and reverse reaction rates are equal). There is no net change in the overall composition of the mixture. See also for the distinction between observable changes and molecular-level activity. ### Possible Questions and Answers **Q:** What is the key characteristic of a reversible reaction? **A:** It can proceed in both the forward (reactants to products) and reverse (products to reactants) directions. **Q:** What does the double arrow ($\rightleftharpoons$) signify in a chemical equation? **A:** It indicates that the reaction is reversible and has reached a state of equilibrium. **Q:** What is meant by "dynamic equilibrium"? **A:** It is a state where the rate of the forward reaction is equal to the rate of the reverse reaction. At the molecular level, reactions are still occurring, but there is no net change in the concentrations of reactants and products.