State that a reversible reaction in a closed system is at equilibrium when: (a) the rate of the forward reaction is equal to the rate of the reverse reaction (b) the concentrations of reactants and products are no longer changing

What is a Reversible Reaction?

A reversible reaction is one that can run in both directions: the reactants turn into products, and the products can turn back into reactants. It is written with a double arrow:

\$A + B \rightleftharpoons C + D\$ ⚖️

When Is the System at Equilibrium?

In a closed system, equilibrium is reached when two conditions are met:

1. Rate Condition: The rate of the forward reaction equals the rate of the reverse reaction.

   \$r{\text{forward}} = r{\text{reverse}}\$ 🔄

2. Concentration Condition: The concentrations of reactants and products stop changing.

   \$\frac{d[A]}{dt} = \frac{d[B]}{dt} = \frac{d[C]}{dt} = \frac{d[D]}{dt} = 0\$

Analogy: The Traffic Jam

Imagine a one‑way street that suddenly turns into a two‑way street. Cars (reactants) move forward, but at some point, traffic starts moving back in the opposite direction (products).

When the number of cars going forward equals the number going back, and the traffic density on each side stays constant, the road is at equilibrium – a traffic jam that doesn’t change over time. 🚗🚙

Example: Haber Process

The synthesis of ammonia is a classic reversible reaction:

\$N2(g) + 3H2(g) \rightleftharpoons 2NH_3(g)\$ ⚗️

The equilibrium constant expression is:

\$Kc = \frac{[NH3]^2}{[N2][H2]^3}\$

If the reaction starts with 1 mol N₂, 3 mol H₂ and no NH₃, the system will adjust until the rate of NH₃ formation equals the rate of NH₃ decomposition, and the concentrations settle to constant values.

Exam Tips 📚

• Always write the balanced equation with the correct stoichiometric coefficients before forming the equilibrium expression.
• Remember that the equilibrium constant, \$K_c\$, is dimensionless; you can ignore units in the expression.
• When given initial concentrations and asked to find the equilibrium concentration, set up an ICE table (Initial, Change, Equilibrium).
• Be careful with the signs: a positive change for a reactant means it’s being consumed (negative in the ICE table).
• Check that your final answer is physically reasonable (e.g., concentrations cannot be negative).

Quick Recap

• Reversible reaction: \$A + B \rightleftharpoons C + D\$

• Equilibrium when:

\$r{\text{forward}} = r{\text{reverse}}\$ and

concentrations are constant

• Use the equilibrium constant expression to relate concentrations at equilibrium.

• Practice ICE tables and remember the units of \$K_c\$ are dimensionless.