Reversible and irreversible reactions. Chemical equilibrium

A reaction that goes both forwards and backwards at the same time is called reversible and is written with the sign ⇌ (for example N₂ + 3H₂ ⇌ 2NH₃). A reaction goes practically to the end (is irreversible) if a product leaves the system as a precipitate or gas, if a weak electrolyte (for example water) forms, or if a large amount of energy is released. When a reversible reaction starts in a closed vessel the forward rate is high and the reverse rate is near zero; as product accumulates the reverse rate grows and the forward rate falls. The state in which they become equal is called chemical equilibrium: v(forward) = v(reverse). Equilibrium is dynamic: the reactions do not stop, but the concentrations stay constant. For aA + bB ⇌ cC + dD, K = [C]^c·[D]^d / ([A]^a·[B]^b), where [ ] are equilibrium concentrations; K equals k₁/k₂ and depends only on temperature, not on concentrations. A large K means many products at equilibrium, a small K means many starting substances. Pure solids are left out of the K expression.