Reversible Reactions and Chemical Equilibrium
Equilibrium is a dynamic balance of opposing reaction rates, not a stopped reaction.
1. Reversibility in a closed system
For $aA+bB\rightleftharpoons cC+dD$, the forward rate is initially greatest when reactants dominate. As products accumulate, the reverse rate rises. In a closed system at constant temperature the two rates can become equal. Concentrations then stay constant even though particles keep reacting.
2. Equilibrium constant and reaction quotient
For the homogeneous reaction above, $K_c=\frac{[C]^c[D]^d}{[A]^a[B]^b}$. Exponents come from the balanced equation. Pure solids and pure liquids are omitted because their effective concentrations are constant. A large $K_c$ favors products; a small $K_c$ favors reactants. The reaction quotient $Q_c$ has the same form but uses the current, not necessarily equilibrium, concentrations.
| Comparison | Prediction |
|---|---|
| $Q_c<K_c$ | Net forward reaction |
| $Q_c=K_c$ | Equilibrium |
| $Q_c>K_c$ | Net reverse reaction |
3. Le Chatelier reasoning
- Write the balanced equation and count gaseous moles.