GOLDEN STARS MONTESSORI ACADEMYChemical reactions can be classified into two main types: irreversible and reversible. Irreversible reactions proceed in one direction only, usually until one of the reactants is completely used up. For example, burning wood produces ash and smoke; you cannot turn ash and smoke back into wood.
Reversible reactions, on the other hand, can proceed in both the forward and reverse directions simultaneously. This means that products can react to reform the original reactants. We use a double arrow (⇌) to show that a reaction is reversible. Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (Haber-Bosch process for ammonia production)
In a reversible reaction, when reactants are mixed, the forward reaction (reactants forming products) starts. As products form, the reverse reaction (products forming reactants) also begins. Initially, the forward reaction is faster. However, as reactants are consumed, its rate decreases. At the same time, as products accumulate, the rate of the reverse reaction increases. Eventually, a point is reached where the rate of the forward reaction becomes exactly equal to the rate of the reverse reaction. At this point, the system is said to be in dynamic equilibrium.
Dynamic means the reactions are still ongoing; molecules are constantly converting between reactants and products. Equilibrium means there is no net change in the concentrations of reactants and products over time. They remain constant, even though individual molecules are still reacting.
This graph illustrates how the rates of forward and reverse reactions become equal at equilibrium.
Characteristics of Dynamic Equilibrium:
The equilibrium constant (Kc) is a value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to the power of its stoichiometric coefficient in the balanced chemical equation. It tells us the extent to which a reaction proceeds to completion.
For a general reversible reaction: aA + bB ⇌ cC + dD where a, b, c, and d are the stoichiometric coefficients, and A, B, C, D are the chemical species.
The expression for the equilibrium constant, Kc, is: Kc = ([C]ᶜ [D]ᵈ) / ([A]ᵃ [B]ᵇ)
General form of the equilibrium constant expression for a reversible reaction.
Let's look at an example to calculate Kc.
Example 1: Consider the reaction: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) At a certain temperature, at equilibrium, the concentrations were found to be: [PCl₅] = 0.20 mol/dm³ [PCl₃] = 0.50 mol/dm³ [Cl₂] = 0.50 mol/dm³
Calculate Kc for this reaction.
Solution:
Write the Kc expression: Kc = ([PCl₃][Cl₂]) / [PCl₅]
Substitute the equilibrium concentrations: Kc = (0.50 mol/dm³ * 0.50 mol/dm³) / 0.20 mol/dm³ Kc = 0.25 / 0.20 Kc = 1.25
(Note: For simplicity at this level, we often focus on the numerical value of Kc, though it does have units which depend on the stoichiometry of the reaction).
A crucial real-world application of chemical equilibrium is the Haber-Bosch process, which synthesizes ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂): N₂(g) + 3H₂(g) ⇌ 2NH₃(g). This process, conducted under specific high pressure and temperature conditions, is vital for producing artificial fertilizers, which are essential for feeding the global population by enhancing crop yields. Understanding and manipulating this equilibrium allows industries to maximize ammonia production efficiently.
For the reaction 2NO₂(g) ⇌ N₂O₄(g), at equilibrium, the concentration of NO₂ is 0.06 M and N₂O₄ is 0.18 M. Calculate the equilibrium constant, Kc. What does this Kc value suggest about the relative amounts of product and reactant at equilibrium? Home Practice Idea: Think about how the water level in a bucket with a small hole in the bottom might reach a "dynamic equilibrium" if water is constantly being poured in at the same rate it leaks out. How is this similar to a chemical reaction at equilibrium?