Reversible reactions Knowledge-Based Study

Study notes

1. What Is a Reversible Reaction?

   In many everyday chemical processes, such as combustion, reactants are permanently converted into products. These are known as irreversible reactions and typically proceed in only one direction until at least one reactant is entirely consumed.

   In contrast, a reversible reaction is a chemical change where the products can react together to reform the original reactants under the same set of conditions. Rather than using a single forward arrow ($ightarrow$), reversible reactions are represented by a double arrow:

   $$\text{Reactants} ightleftharpoons \text{Products}$$

   • Forward reaction: Reactants are converted into products (reading left to right).
   • Reverse reaction: Products are converted back into reactants (reading right to left).

   The Path to Dynamic Equilibrium

   When a reversible reaction takes place in a closed system—where no matter can enter or leave—it eventually establishes a state called dynamic equilibrium.

   Consider what happens from the start of the reaction:

   1. Initially: The concentration of reactants is at its highest, so the rate of the forward reaction is at its maximum. With no products present initially, the rate of the reverse reaction is zero.
   2. As the reaction proceeds: Reactants are consumed, causing the forward reaction rate to decrease. Simultaneously, product molecules accumulate, causing the reverse reaction rate to increase.
   3. At equilibrium: The rate of the forward reaction becomes exactly equal to the rate of the reverse reaction.

   The term dynamic highlights that the chemical processes do not stop; both forward and reverse transformations continue at identical speeds.

   Observing the Equilibrium State

   At dynamic equilibrium, there is a clear distinction between what can be observed and what occurs at the molecular level:

   • Microscopic level: Reactant molecules continuously collide to form products, while product molecules simultaneously break down or collide to reform reactants.
   • Macroscopic level: Because the forward and reverse rates are equal, there is no net change in observable properties such as color intensity, pressure, temperature, or total concentration of any species.

   For example, in the synthesis of hydrogen iodide:

   $$\text{H}_2(g) + \text{I}_2(g) ightleftharpoons 2\text{HI}(g)$$

   Once equilibrium is achieved, the characteristic purple color of gaseous iodine remains constant because iodine is produced at the exact same rate it is consumed.

   Essential Rules and Common Pitfalls

   Understanding reversible reactions requires avoiding several common misconceptions:

   • Equal Rates vs. Equal Concentrations: At equilibrium, the rates of the forward and reverse reactions are strictly equal. However, the concentrations of reactants and products are generally not equal; they simply remain constant over time.
   • Requirement of a Closed System: If products are permitted to escape (such as an open container releasing a gas), dynamic equilibrium cannot be maintained because the reverse reaction cannot occur at the necessary rate.
   • Equilibrium Can Be Approached from Either Direction: Whether starting purely with reactants or purely with products, a closed reversible system at a given temperature will eventually establish the same equilibrium state.